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QP34  .B83  1 905     A  text-book  of  human 


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Presentea  by 

I,  DR.  WILLIAM  J.  OILS  1^ 

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OILS  FELLOWSHIP 

in  Bioloyc&l  Chemistry 


Digitized  by  the  Internet  Archive 

in  2010  with  funding  from 
Columbia  University  Libraries 


http://www.archive.org/details/textbookofhumanpOObrub 


A 

TEXT-BOOK  OF  HUMAN  PHYSIOLOGY. 


BRUBAKER 


A 


TEXT-BOOK 


OF 


HUMAN  PHYSIOLOGY. 


INCLUDING  A  SECTION  ON 


PHYSIOLOGIC  APPARATUS. 


BY 

ALBERT  P.   BRUBAKER,  A.M.,  M.D., 

PROFESSOR   OF    PHYSIOLOGY    AND    HYGIENE    IN    THE   JEFFERSON   MEDICAL   COLLKGE  ;      PROFESSOR 

OF     PHYSIOLOGY     IN    THE     PENNSYLVANIA     COLLEGE    OF    DENTAL    SURGERY  ; 

LECTURER  ON    PHYSIOLOGY    AND    HYGIENE    IN    THE    DREXEL 

INSTITUTE   OF   ART,    SCIENCE,   AND    INDUSTRY. 


Secon&  lEMtion.     1Revise&  an^  }£nlaroe^. 
Mitb  Colored  ipiatcs  an&  356  IFUustrations. 


PHILADELPHIA  : 

P.  BLAKISTON'S  SON   &   CO., 

I0I2    WALNUT    STREET. 
1905. 


/7  ^^^ 


Copyright,   1905,  by  P.  Blakiston's  Son  &  Co. 


PKESS    or   WM.    f.    FELL    COMPA^ 
PMILADELPHI 


K. 


TO 

HENRY  C.  CHAPMAN,  M.D., 

PROFESSOR  OF   INSTITUTES  OF    MEDICINE   AND    MEDICAL    JURISPRUDENCE   IN 
THE   JEFFERSON    MEDICAL  COLLEGE, 

IN  GRATEFUL  RECOGNITION  OF  THE  MANY  KINDNESSES  RECEIVED  FROM 

HIM,  DURING  A  PERIOD  OF  TWENTY-FIVE  YEARS,  THIS 

VOLUME  IS  RESPECTFULLY  DEDICATED  BY 

THE  AUTHOR. 


PREFACE  TO  SECOND   EDITION. 


In  the  preparation  of  a  second  edition  the  opportunity  has  been 
taken  to  eliminate  inaccuracies,  to  revise  paragraphs  with  a  view 
of  removing  obscurities  and  to  insert  additional  material  which  it  is 
believed  will  render  the  consideration  of  certain  topics  more  complete 
and  accurate.  The  additional  statements  will  be  found  in  the 
sections  relating  to  the  chemistry  of  the  proteids,  the  chemistry  of 
digestion,  the  movements  of  the  intestines,  the  production  of  lymph, 
the  nerve  mechanism  of  the  heart,  and  the  physiology  of  vision. 
These  insertions  have  increased  the  size  of  the  volume  by  about 
seventeen  pages. 

I  wish  to  express  my  thanks  for  the  generous  reception  of  this 
text-book  by  both  teachers  and  students  and  trust  that  it  may 
continue  to  merit  their  approval. 

A.  P.  B. 


PREFACE. 


The  object  in  view  in  the  preparation  of  this  volume  was  the 
selection  and  presentation  of  the  more  important  facts  of  physiology, 
in  a  form  which  it  is  believed  will  be  helpful  to  students  and  to 
practitioners  of  medicine.  Inasmuch  as  the  majority  of  students  in 
a  medical  college  are  preparing  for  the  practical  duties  of  professional 
life,  such  facts  have  been  selected  as  will  not  only  elucidate  the 
normal  functions  of  the  tissues  and  organs  of  the  body,  but  which 
will  be  of  assistance  in  understanding  their  abnormal  manifestations 
as  they  present  themselves  in  hospital  and  private  work.  Both  in 
the  selection  of  facts  and  in  the  method  of  presentation  the  author 
has  been  guided  by  an  experience  gained  during  twenty  years  of 
active  teaching. 

The  description  of  physiologic  apparatus  and  the  methods  of 
investigation,  other  than  those  having  a  clinical  interest,  have  been 
largely  excluded  from  the  text,  for  the  reason  that  both  are  more 
appropriately  considered  in  works  devoted  to  laboratory  methods 
and  laboratory  instruction,  and  for  the  further  reason  that  the  student 
receives  this  information  while  engaged  in  the  practical  study  of 
physiology  in  the  laboratory,  now  an  established  feature  in  the 
curriculum  of  the  majority  of  medical  colleges.  For  those  who  have 
not  had  laboratory  opportunities  a  brief  account  of  some  essential 
forms  of  apparatus  and  the  purposes  for  which  they  are  intended 
will  be  found  in  an  appendix. 

I  wish  to  acknowledge  my  indebtedness  to  Professor  Colin  C. 
Stewart  for  many  valuable  suggestions  in  the  preparation  of  different 
sections  of  the  volume;  to  Dr.  Carl  Weiland  for  assistance  in  the 
chapter  on  vision;  to  Dr.  Joseph  P.  Bolton  for  excellent  suggestions 
on  questions  relating  to  physiologic  chemistry. 


TABLE  OF  CONTENTS. 


CHAPTER  I.  PAGE 

Introduction, 17 

CHAPTER  II. 
Chemic  Composition  of  the  Human  Body, 24 

CH.\PTER  III. 
Physiology  of  the  Cell, 43 

CHAPTER  IV. 
Histology  of  the  Epithelial  and  Connective  Tissues, 49 

CHAPTER  V. 
The  Physiology  of  the  Skeleton, 60 

CHAPTER  VI. 
General  Physiology  of  Muscle-tissue, 65 

CHAPTER  VII. 
The  General  Physiology^  of  Nerve-tissue, 105 

CHAPTER  VIII. 
Foods, 136 

CHAPTER  IX. 
Digestion, 154 

CHAPTER  X. 
Absorption, 221 

CHAPTER  XI. 
The  Blood, 238 

CHAPTER  XII. 
The  Circulation  of  the  Blood, 272 

CHAPTER  XIII. 
Respiration, 350 

CHAPTER  XIV. 
Animal   HE-i^T, 401 

CHAPTER  XV. 
Secretion, 411 


xii  TABLE    OF   CONTENTS. 

CHAPTER  XVI.  PAGE 

Excretion, 43  6 

CHAPTER  XVII. 
The  Central  Organs  of  the  Nerve  System  and  their  Nerves, 456 

CHAPTER  XVIII. 

The  Medulla  Oblongata;   the  Isthmus  of  the  Encephalon;   the  Basal 

Ganglia, 483 

CHAPTER  XIX. 
The    Cerebrum, 502 

CHAPTER  XX. 
The    Cerebellum, 530 

CHAPTER  XXI. 
The  Cranial  Nerves, 538 

CHAPTER  XXII. 
The  Sympathetic  Nerve  System, 577 

CHAPTER  XXIII. 
Phonation;    Articulate  Speech, 588 

CHAPTER  XXIV. 
The  Special  Senses, 601 

CHAPTER  XXV. 
The  Sense  of  Sight, 614 

CHAPTER  XXVI. 
The  Sense  of  Hearing, 653 

CHAPTER  XXVII. 
Reproduction, 666 

APPENDIX. 
Physiologic  Apparatus,  687 


Index, 709 


TEXT-BOOK  OF  PHYSIOLOGY. 


CHAPTER  I. 
INTRODUCTION. 

An  animal  organism  in  the  living  condition  exhibits  a  series  of 
phenomena  which  relate  to  growth,  movement,  mentality,  and  re- 
production. During  the  period  preceding  birth,  as  well  as  during 
the  period  included  between  birth  and  adult  hfe,  the  individual 
grows  in  size  and  complexity  from  the  introduction  and  assimilation 
of  material  from  without.  Throughout  its  life  the  animal  exhibits 
a  series  of  movements,  in  virtue  of  which  it  not  only  changes  the 
relation  of  one  part  of  its  body  to  another,  but  also  changes  its  posi- 
tion relatively  to  its  environment.  If,  in  the  execution  of  these 
movements,  the  parts  are  directed  to  the  overcoming  of  opposing 
forces,  such  as  gravity,  friction,  cohesion,  elasticity,  etc.,  the  animal 
may  be  said  to  be  doing  work.  The  result  of  normal  growth  is  the 
attainment  of  a  physical  development  that  will  enable  the  animal, 
and,  more  especially,  man,  to  perform  the  work  necessitated  by  the 
nature  of  its  environment  and  the  character  of  its  organization.  In 
man,  and  probably  in  lower  animals  as  well,  mentality  manifests 
itself  as  intellect,  feehng,  and  voUtion.  At  a  definite  period  in  the 
life  of  the  animal  it  reproduces  itself,  in  consequence  of  which  the 
species  to  which  it  belongs  is  perpetuated. 

The  study  of  the  phenomena  of  growth,  movement,  mentality, 
and  reproduction  constitutes  the  science  of  animal  physiology. 
But  as  these  general  activities  are  the  resultant  of  and  dependent 
on  the  special  activities  of  the  individual  structures  of  which  an 
animal  body  is  composed,  physiology  in  its  more  restricted  and 
generally  accepted  sense  is  the  science  which  investigates  the  actions 
or  functions  of  the  individual  organs  and  tissues  of  the  body  and 
the  physical  and  chemic  conditions  which  underlie  and  determine 
them. 

This  may  naturally  be  divided  into: 
I.  Special  physiology,  the  object  of  which  is  a  study  of  the  ^^tal 
phenomena  or  functions  exhibited  by  the  organs  of  any  individual 
animal. 

17 


i8  TEXT-BOOK  OF  PHYSIOLOGY. 

2.  Comparative  physiology,  the  object  of  which  is  a  comparison  of 
the  vital  phenomena  or  functions  exhibited  by  the  organs  of 
two  or  more  animals  of  different  species,  with  a  view  to  un- 
folding their  points  of  resemblance  or  dissimilarity. 

Human  physiology  is  that  department  of  physiologic  science 
which  has  for  its  object  the  study  of  the  functions  of  the  organs  of 
the  human  body  in  a  state  of  health. 

Inasmuch  as  the  study  of  function,  or  physiology,  is  associated 
with  and  dependent  on  a  knowledge  of  structure,  or  anatomy,  it  is 
essential  that  the  student  should  have  a  general  acquaintance  not 
only  with  the  structure  of  man,  but  with  that  of  typical  forms  of 
lower  animal  life  as  well. 

If  the  body  of  any  animal  be  dissected,  it  will  be  found  to  be 
composed  of  a  number  of  well-defined  structures,  such  as  heart, 
lungs,  stomach,  brain,  eye,  etc.,  to  which  the  term  organ  was  originally 
applied,  for  the  reason  that  they  were  supposed  to  be  instruments 
capable  of  performing  some  important  act  or  function  in  the  general 
activities  of  the  body.  Though  the  term  organ  is  usually  employed 
to  designate  the  larger  and  more  famihar  structures  just  mentioned, 
it  is  equally  applicable  to  a  large  number  of  other  structures  which, 
though  possibly  less  obvious,  are  equally  important  in  maintaining 
the  hfe  of  the  individual — e.  g.,  bones,  muscles,  nerves,  skin,  teeth, 
glands,  blood-vessels,  etc.  Indeed,  any  complexly  organized  struc- 
ture capable  of  performing  some  function  may  be  described  as  an 
organ.  A  description  of  the  various  organs  which  make  up  the  body 
of  an  animal,  their  external  form,  their  internal  arrangement,  their 
relations  to  one  another,  constitutes  the  science  of  animal  anatomy. 

This  may  naturally  be  divided  into: 

1.  Special  anatomy,  the  object  of  which  is  the  investigation  of  the 

construction,    form,    and    arrangement    of   the   organs   of   any 
individual  animal. 

2.  Comparative  anatomy,  the  object  of  which  is  a  comparison  of  the 

organs  of  two  or  more  animals  of  different  species,  with  a  view 
to  determining  their  points  of  resemblance  or  dissimilarity. 
If  the  organs,  however,  are  subjected  to  a  further  analysis,  they 
can  be  resolved  into  simple  structures,  apparently  homogeneous,  to 
which  the  name  tissue  has  been  given — e.  g.,  epithelial,  connective, 
muscle,  and  nerve  tissue.  When  the  tissues  are  subjected  to  a 
microscopic  analysis,  it  is  found  that  they  are  not  homogeneous 
in  structure,  but  composed  of  still  simpler  elements,  termed  cells 
and  fibers.  The  investigation  of  the  internal  structure  of  the  organs, 
the  physical  properties  and  structure  of  the  tissues,  as  well  as  the 
structure  of  their  component  elements,  the  cells  and  fibers,  con- 
stitutes a  department  of    anatomic  science  known  as  histology, 


INTRODUCTION.  19 

or  as  it  is  prosecuted  largely  with  the   microscope,   microscopic 
anatomy. 

Human  anatomy  is  that  department  of  anatomic  science  which 
has  for  its  object  the  investigation  of  the  construction  of  the  human 
body. 


GENERAL  STRUCTURE  OF  THE  ANIMAL  BODY. 

The  body  of  every  animal,  from  fish  to  man,  may  be  divided 
into — 

1.  An  axial  portion,  consisting  of  the  head,  neck,  and  trunk;  and — 

2.  An  appendicular  portion,  consisting  of  the  anterior  and  posterior 

limbs  or  extremities. 

The  axial  portion  of  all  mammals,  to  which  class  man  zoologi- 
cally belongs,  as  well  as  of  all  birds,  reptiles,  amphibians,  and  os- 
seous fish,  is  characterized  by  the  presence  of  a  bony,  segmented 
axis,  which  extends  in  a  longitudinal  direction  from  before  backward, 
and  which  is  known  as  the  vertebral  column  or  backbone.  In  virtue 
of  the  existence  of  this  column  all  the  classes  of  animals  just  men- 
tioned form  one  great  division  of  the  animal  kingdom,  the  Vertehrata. 

Each  segment,  or  vertebra,  of  this  axis  consists  of — 

1.  xA.  solid  portion,  known  as  the  body  or  centrum,  and 

2.  A  bony  arch  arising  from  the  dorsal  aspect  and  surmounted  by 

a  spine-Hke  process. 

At  the  anterior  extremity  of  the  body  of  the  animal  the  vertebrae 
are  variously  modified  and  expanded,  and,  with  the  addition  of  new 
elements,  form  the  skull;  at  the  posterior  extremity  they  rapidly 
diminish  in  size,  and  terminate  in  man  in  a  short,  tail-Uke  process. 
In  many  animals,  however,  the  vertebral  column  extends  for  a 
considerable  distance  beyond  the  trunk  into  the  tail.  The  vertebral 
column  may  be  regarded  as  the  foundation  element  in  the  plan  of 
organization  of  all  the  higher  animals  and  the  center  around  which 
the  rest  of  the  body  is  developed  and  arranged  with  a  certain  degree 
of  conformity.  In  all  vertebrate  animals  the  bodies  of  the  segments 
of  the  vertebral  column  form  a  partition  which  serves  to  divide 
the  trunk  of  the  body  into  two  cavities — viz.,  the  dorsal  and  the 
ventral.     (See  Fig.  i.) 

The  dorsal  cavity  is  found  not  only  in  the  trunk,  but  also  in 
the  head.  Its  walls  are  formed  partly  by  the  arches  which  arise 
from  the  posterior  or  dorsal  surface  of  the  vertebras  and  partly  by 
the  bones  of  the  skull.  If  a  longitudinal  section  be  made  through 
the  center  of  the  vertebral  column,  and  including  the  head,  the 
dorsal  cavity  will  be  observed  running  through  its  entire  extent. 
Though  for  the  most  part  it  is  quite  narrow,  at  the  anterior  ex- 


TEXT-BOOK  OF  PHYSIOLOGY 


tremity  it  is  enlarged  and  forms 


Fig.  I. — Diagrammatic  Longitud- 
inal Section  of  the  Body. 
V,  V.  Bodies  of  the  vertebrae 
which  divide  the  body  into  the 
dorsal  and  ventral  cavities,  a,  a'. 
The  dorsal  cavity.  C,  p'.  The 
abdominal  and  thoracic  divisions 
of  the  ventral  cavity,  separated 
from  each  other  by  a  trans- 
verse muscular  partition,  the 
diaphragm  d.  B.  The  brain. 
Sp.  C.  The  spinal  cord.  e.  The 
esophagus.  S.  The  stomach, 
from  vv^hich  continues  the  intes- 
tine to  the  opening  at  the  poste- 
rior portion  of  the  body.  /.  The 
hver.  p.  The  pancreas,  k.  The 
kidney,  o.  The  bladder.  /'.  The 
lungs,     h.  The  heart. 


the  cavity  of  the  skulL  This  cavity 
is  hned  by  a  membranous  canal, 
the  neural  canal,  in  which  are  con- 
tained the  brain  and  the  neural  or 
spinal  cord.  Through  openings  in 
the  sides  of  the  dorsal  cavity  nerves 
pass  out  which  connect  the  brain 
and  spinal  cord  with  all  the  struc- 
tures of  the  body. 

The  ventral  cavity  is  confined 
mainly  to  the  trunk  of  the  body. 
Its  walls  are  formed  by  muscles 
and  skin,  strengthened  in  most 
animals  by  bony  arches,  the  ribs. 
Within  the  ventral  cavity  is  con- 
tained a  musculo-membranous  tube 
or  canal  known  as  the  ahmentary 
or  food  canal,  which  begins  at  the 
mouth  on  the  ventral  side  of  the 
head,  and,  after  passing  through 
the  neck  and  trunk,  terminates  at 
the  posterior  extremity  of  the  trunk 
at  the  anus.  It  may  be  divided 
into  mouth,  pharynx,  esophagus, 
stomach,  small  and  large  intestines. 

In  all  mammals  the  ventral 
cavity -is  divided  by  a  musculo- 
membranous  partition  into  two 
smaller  cavities,  the  thorax  and 
abdomen.  The  former  contains  the 
lungs,  heart  and  its  great  blood- 
vessels, and  the  anterior  part  of 
the  ahmentary  canal,  the  gullet  or 
esophagus;  the  latter  contains  the 
continuation  of  the  alimentary 
canal — that  is,  the  stomach  and 
intestines — and  the  glands  in  con- 
nection with  it,  the  liver  and  pan- 
creas. In  the  posterior  portion  of 
the  abdominal  cavity  are  found  the 
kidneys,  ureters,  and  bladder,  and 
in  the  female  the  organs  of  repro- 
duction. The  thoracic  and  ab- 
dominal cavities  are  each  hned  by 
a  thin    serous   membrane,   known. 


INTRODUCTION.  21 

respectively,  as  the  pleural  and  peritoneal  membranes,  which,  in 
addition,  are  reflected  over  the  surfaces  of  the  organs  contained 
within  them.  The  alimentary  canal  and  the  various  cavities  con- 
nected with  it  are  lined  throughout  by  a  mucous  membrane. 

The  surface  of  the  body  is  covered  by  the  skin.  This  is  com- 
posed of  an  inner  portion,  the  derma,  and  an  outer  portion,  the  epi- 
dermis. The  former  consists  of  fibers,  blood-vessels,  nerves,  etc. ;  the 
latter  of  layers  of  scales  or  cells.  Embedded  within  the  skin  are 
numbers  of  glands,  which  exude,  in  the  different  classes  of  animals, 
sweat,  oily  matter,  etc.  Projecting  from  the  surface  of  the  skin  are 
hairs,  bristles,  feathers,  claws.  Beneath  the  skin  are  found  muscles, 
bones,  blood-vessels,  nerves,  etc. 

The  appendicular  portion  of  the  body  consists  of  two  pairs 
of  symmetric  limbs,  which  project  from  the  sides  of  the  trunk,  and 
which  bear  a  determinate  relation  to  the  vertebral  column.  They  con- 
sist fundamentally  of  bones,  surrounded  by  muscles,  blood-vessels, 
nerves  and  lymphatics.  The  limbs,  though  having  a  common  plan 
of  organization,  are  modified  in  form  and  adapted  for  prehension 
and  locomotion  in  accordance  with  the  needs  of  the  animal. 

Anatomic  Systems. — All  the  organs  of  the  body  which  have 
certain  peculiarities  of  structure  in  common  are  classified  by  anato- 
mists into  systems — e.  g.,  the  bones,  collectively,  constitute  the  bony 
or  osseous  system;  the  muscles,  the  nerves,  the  skin,  constitute, 
respectively,  the  muscle,  the  nerve,  and  the  tegumentary  systems. 

Physiologic  Apparatus. — ]\Iore  important  from  a  physiologic 
point  of  view  than  a  classification  of  organs  based  on  similarities  of 
structure  is  the  natural  association  of  two  or  more  organs  acting 
together  for  the  accomphshment  of  some  definite  object,  and  to 
which  the  term  physiologic  apparatus  has  been  applied.  While  in 
the  community  of  organs  which  together  constitute  the  animal  body 
each  one  performs  some  definite  function,  and  the  harmonious  co- 
operation of  all  is  necessary  to  the  life  of  the  individual,  everywhere 
it  is  found  that  two  or  more  organs,  though  performing  totally  dis- 
tinct functions,  are  cooperating  for  the  accomphshment  of  some 
larger  or  compound  function  in  which  their  individual  functions  are 
blended — e.  g.,  the  mouth,  stomach,  and  intestines,  with  the  glands 
connected  with  them,  constitute  the  digestive  apparatus,  the  object 
or  function  of  which  is  the  complete  digestion  of  the  food.  The 
capillary  blood-vessels  and  lymphatic  vessels  of  the  body,  and  espe- 
cially those  in  relation  to  the  vilh  of  the  small  intestine,  constitute 
the  absorptive  apparatus,  the  function  of  which  is  the  introduction 
of  new  material  into  the  blood.  The  heart  and  blood-vessels  con- 
stitute the  circulatory  apparatus,  the  function  of  which  is  the  dis- 
tribution of  blood  to  all  portions  of  the  body.     The  lungs  and  trachea. 


22  TEXT-BOOK  OF  PHYSIOLOGY. 

together  with  the  diaphragm  and  the  walls  of  the  chest,  constitute 
the  respiratory  apparatus,  the  function  of  which  is  the  introduction 
of  oxygen  into  the  blood  and  the  elimination  from  it  of  carbon  dioxid 
and  other  injurious  products.  The  kidneys,  the  ureters,  and  the 
bladder  constitute  the  urinary  apparatus.  The  skin,  with  its  sweat- 
glands,  constitutes  the  perspiratory  apparatus,  the  functions  of  both 
being  the  excretion  of  waste  products  from  the  body.  The  liver, 
the  pancreas,  the  mammary  glands,  as  well  as  other  glands,  each 
form  a  secretory  apparatus  which  elaborates  some  specific  material 
necessary  to  the  nutrition  of  the  individual.  The  functions  of  these 
different  physiologic  apparatus — e.  g.,  digestion,  absorption  of  food, 
elaboration  of  blood,  circulation  of  blood,  respiration,  production 
of  heat,  secretion,  and  excretion — -are  classified  as  nutritive  functions, 
and  have  for  their  final  object  the  preservation  of  the  individual. 

The  nerves  and  muscles  constitute  the  nervo-muscular  apparatus, 
the  function  of  which  is  the  production  of  motion.  The  eye,  the 
ear,  the  nose,  the  tongue,  and  the  skin,  with  their  related  structures, 
constitute,  respectively,  the  visual,  auditory,  olfactory,  gustatory,  and 
tactile  apparatus,  the  function  of  which,  as  a  whole,  is  the  reception 
of  impressions  and  the  transmission  of  nerve  impulses  to  the  brain, 
where  they  give  rise  to  visual,  auditory,  olfactory,  gustatory,  and 
tactile  sensations  and  volitional  impulses. 

The  brain,  in  association  with  the  sense  organs,  forms  an  appa- 
ratus related  to  mental  processes.  The  larynx  and  its  accessory 
organs — the  lungs,  trachea,  respiratory  muscles,  the  mouth  and 
resonant  cavities  of  the  face — form  the  vocal  and  articulating  appa- 
ratus, by  means  of  which  voice  and  articulate  speech  are  produced. 
The  functions  exhibited  by  the  apparatus  just  mentioned — viz., 
motion,  sensation,  language,  mental  and  moral  manifestations — are 
classified  as  functions  of  relation,  as  they  serve  to  bring  the  individual 
into  conscious  relationship  with  the  external  world. 

The  ovaries  and  the  testes  are  the  essential  reproductive  organs, 
the  former  producing  the  germ-cell,  the  latter  the  sperm  element. 
Together  with  their  related  structures, — the  fallopian  tubes,  uterus, 
and  vagina  in  the  female,  and  the  urogenital  canal  in  the  male, — 
they  constitute  the  reproductive  apparatus  characteristic  of  the  two 
sexes.  Their  cooperation  results  in  the  union  of  the  germ-cell  and 
sperm  element  and  the  consequent  development  of  a  new  being. 
The  function  of  reproduction  serves  to  perpetuate  the  species  to 
which  the  individual  belongs. 

The  animal  body  is  therefore  not  a  homogeneous  organism,  but 
one  composed  of  a  large  number  of  widely  dissimilar  but  related 
organs.  As  all  vertebrate  animals  have  the  same  general  plan  of 
organization,  there  is  a  marked  similarity  both  in  form  and  struc- 
ture among  corresponding  parts  of  different  animals.     Hence  it  is 


INTRODUCTION.   •  23 

that  in  the  study  of  human  anatomy  a  knowledge  of  the  form,  con- 
struction, and  arrangement  of  the  organs  in  different  types  of  animal 
life  is  essential  to  its  correct  interpretation;  it  follows  also  that  in  the 
investigation  and  comprehension  of  the  complex  problems  of  human 
physiology  a  knowledge  of  the  functions  of  the  organs  as  they  manifest 
themselves  in  the  different  types  of  animal  hfe  is  indispensable. 
As  many  of  the  functions  of  the  human  body  are  not  only  complex, 
but  the  organs  exhibiting  them  are  practically  inaccessible  to  in- 
vestigation, we  must  supplement  our  knowledge  and  judge  of  their 
functions  by  analogy,  by  attributing  to  them,  within  certain  limits, 
the  functions  revealed  by  experimentation  upon  the  corresponding 
organs  of  lower  animals.  This  experimental  knowledge,  corrected 
by  a  study  of  the  clinical  phenomena  of  disease  and  the  results  of 
post-mortem  investigations,  forms  the  basis  of  modern  human  physi- 
ologv. 


CHAPTER  11. 

CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY. 

Since  it  has  been  demonstrated  that  every  exhibition  of  functional 
activity  is  associated  with  changes  of  structure,  it  has  been  apparent 
that  a  knowledge  of  the  chemic  composition  of  the  body,  not  only 
when  in  a  state  of  rest,  but  to  a  far  greater  degree  when  in  a  state 
of  activity,  is  necessary  to  a  correct  understanding  of  the  intimate 
nature  of  physiologic  processes.  Though  the  analysis  of  the  dead 
body  is  comparatively  easy,  the  determination  of  the  successive 
changes  in  composition  of  the  living  body  is  attended  with  many 
difficulties.  The  hving  material,  the  bioplasm,  is  not  only  complex 
and  unstable  in  composition,  but  extremely  sensitive  to  all  physical 
and  chemic  influences.  The  methods,  therefore,  which  are  employed 
for  analysis  destroy  its  composition  and  vitahty,  and  the  products 
which  are  obtained  are  peculiar  to  dead  rather  than  to  living  material. 

Chemic  analysis,  therefore,  may  be  directed — 

1.  To  the  determination  of  the  composition  of  the  dead  body. 

2.  To  the  determination  of  the  successive  changes  in  composition 

which  the  living  bioplasm  undergoes  during  functional  activity. 

A  chemic  analysis  of  the  dead  body,  with  a  view  to  disclosing 
the  substances  of  which  it  is  composed,  their  properties,  their  intimate 
structure,  their  relationship  to  one  another,  constitutes  what  might 
be  termed  chemic  anatomy.  An  investigation  of  the  living  ma- 
terial and  of  the  successive  changes  it  undergoes  in  the  performance 
of  its  functions  constitutes  what  has  been  termed  chemic  physi- 
ology or  physiologic  chemistry. 

By  chemic  analysis  the  animal  body  can  be  reduced  to  a  number 
of  liquid  and  solid  compounds  which  belong  to  both  the  inorganic 
and  organic  worlds.  These  compounds,  resulting  from  a  proximate 
analysis,  have  been  termed  proximate  principles.  That  they  may 
merit  this  term,  however,  they  must  be  obtained  in  the  form  under 
which  they  exist  in  the  living  condition.  The  organic  compounds 
consist  of  representatives  of  the  carbohydrate,  fatty,  and  proteid 
groups  of  organic  bodies;  the  inorganic  compounds  consist  of  water, 
various  acids,  and  inorganic  salts. 

The  compounds  or  proximate  principles  thus  obtained  can  be 
further  resolved  by  an  ultimate  analysis  into  a  small  number  of 
chemic  elements  which  are  identical  with  elements  found  in  many 
other  organic  as  well  as  inorganic  compounds.     The  different  chemic 

24 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.         25 

elements  which  are  thus  obtained,  and  the  percentages  in  which  they 
exist  in  the  body,  are  as  follows — viz.,  oxygen,  72  per  cent.;  hydrogen, 
9.10;  nitrogen,  2.5;  carbon,  13.50;  phosphorus,  1.15;  calcium,  1.30; 
sulphur,  0.147;  sodium,  o.io;  potassium,  0.026;  chlorin,  0.085; 
fluorin,  iron,  silicon,  magnesium,  in  small  and  variable  amounts. 


THE  CARBOHYDRATES. 

The  carbohydrate  compounds,  which  enter  into  the  composition 
of  the  animal  body,  are  mainly  starches  and  sugar.  In  many  re- 
spects they  are  closely  related,  and  by  appropriate  means  are  readily 
converted  into  one  another.  In  composition  they  consist  of  the 
elements  carbon,  hydrogen,  and  oxygen.  As  their  name  imphes, 
the  hydrogen  and  oxygen  are  present  in  these  compounds  in  the 
proportion  in  which  they  exist  in  water,  or  as  2  :  i.  The  molecule 
of  the  carbohydrates  above  mentioned  consists  of  either  six  atoms 
of  carbon  or  a  muUiple  of  six;  in  the  latter  case  the  quantity  of 
hydrogen  and  oxygen  taken  up  by  the  carbon  is  increased,  though 
the  ratio  remains  unchanged. 

The  carbohydrates  may  be  divided  into  three  groups — viz.:  (i) 
amyloses,  including  starch,  dextrin,  glycogen,  and  cellulose;  (2) 
dextroses,  including  dextrose,  levulose,  galactose;  (3)  saccharoses, 
including  saccharose,  lactose,  and  maltose.  According  to  the  number 
of  carbon  atoms  entering  into  the  second  group  (six),  they  are  fre- 
quently termed  monosaccharids;  those  of  the  third  group,  disaccharids 
— twice  six ;  those  of  the  first  group,  polysaccharids — multiples  of  six. 

Though  but  few  of  the  members  of  the  carbohydrate  group  are 
constituents  of  the  human  body,  many  are  constituents  of  the  foods; 
on  account  of  their  importance  in  this  respect,  and  their  relation  to 
one  another,  the  chemic  features  of  the  more  generally  consumed 
carbohydrates  will  be  stated  in  this  connection. 

I.  AMYLOSES,  (CeHioOj),,. 

Starch  is  widely  distributed  in  the  vegetable  world,  being  abundant 
in  the  seeds  of  the  cereals,  leguminous  plants,  and  in  the  tubers 
and  roots  of  many  vegetables.  It  occurs  in  the  form  of  microscopic 
granules,  which  vary  in  size,  shape,  and  appearance,  according  to 
the  plant  from  which  they  are  obtained.  Each  granule  presents 
a  nucleus,  or  hilum,  around  which  is  arranged  a  series  of  eccentric 
rings,  alternately  light  and  dark.  The  granule  consists  of  an  envelope 
and  stroma  of  cellulose,  containing  in  its  meshes  the  true  starch 
material — gramdose.  Starch  is  insoluble  in  cold  water  and  alcohol. 
When  heated  with  water  up  to  70°  C,  the  granules  swell,  rupture, 
and  liberate  the  granulose,  which  forms  an  apparent  solution;  if 
present  in  sufficient  quantity,  it  forms  a  gelatinous  mass  termed 


26  TEXT-BOOK  OF  PHYSIOLOGY. 

starch  paste.  On  the  addition  of  iodin,  starch  strikes  a  characteristic 
deep  blue  color;  the  compound  formed — iodid  of  starch — is  weak, 
the  color  disappearing  on  heating,  but  reappearing  on  cooling. 

BoiUng  starch  with  dilute  sulphuric  acid  (twenty-five  per  cent.) 
converts  it  into  dextrose.  In  the  presence  of  vegetable  diastase  or 
animal  ferments,  starch  is  converted  into  maltose  and  dextrose,  two 
forms  of  sugar. 

Dextrin  is  a  substance  formed  as  an  intermediate  product  in 
the  transformation  of  starch  into  sugar.  There  are  at  least  two 
principal  varieties — erythrodextrin,  which  strikes  a  red  color  with 
iodin,  and  achroodextrm,  which  is  without  color  when  treated  with 
this  reagent.  In  the  pure  state  dextrin  is  a  yellow-white  powder, 
soluble  in  water.  In  the  presence  of  vegetable  ferments  erythro- 
dextrin is  converted  into  maltose. 

Glycogen  is  a  constituent  of  the  animal  liver,  and,  to  a  sHght 
extent,  of  muscles  and  of  tissues  generally.  In  the  tissues  of  the 
embryo  it  is  especially  abundant.  When  obtained  in  a  pure  state 
it  is  an  amorphous,  white  powder.  It  is  soluble  in  water,  forming 
an  opalescent  solution.  With  iodin  it  strikes  a  port- wine  color.  In 
some  respects  it  resembles  starch,  in  others  dextrin.  Like  vegetable 
starch,  glycogen  or  animal  starch  can  be  converted  by  dilute  acids 
and  ferments  into  sugar  (dextrose). 

Cellulose  is  the  basic  material  of  the  more  or  less  sohd  framework 
of  plants.  It  is  soluble  in  ammoniacal  solution  of  cupric  oxid,  from 
which  it  can  be  precipitated  by  acids.  It  is  an  amorphous  powder; 
dilute  acids  can  convert  it  into  dextrose. 

2.  DEXTROSES,  CeHuOe. 

Dextrose,  glucose,  or  grape-sugar  is  found  in  grapes,  most 
sweet  fruits,  and  honey,  and  as  a  normal  constituent  of  liver,  blood, 
muscles,  and  other  animal  tissues.  In  the  disease  diabetes  mellitus 
it  is  found  also  in  the  urine. 

When  obtained  from  any  source,  it  is  soluble  in  water  and  in  hot 
alcohol,  from  which  it  crystalhzes  in  six-sided  tables  or  prisms.  As 
usually  met  with,  it  is  in  the  form  of  irregular,  warty  masses.  It  is 
sweet  to  the  taste.  When  examined  with  the  polariscope,  dextrose 
turns  the  plane  of  polarized  light  to  the  right.  It  is  therefore  termed 
dextro-rotatory.  It  has  for  a  long  time  been  known  that  when  sugar, 
cupric  hydroxid,  and  an  alkali — e.g.,  sodium  or  potassium — are 
present  in  solution,  the  sugar  will  abstract  from  the  cupric  hydroxid 
a  portion  of  its  oxygen,  thus  reducing  it  to  a  lower  stage  of  oxidation 
giving  rise  to  cuprous  oxid.  Sugar  has  a  similar  action  on  both 
silver  and  bismuth.  On  this  property  of  sugar  a  standard  solution 
of  cupric  hydroxid  was  suggested  by  Fehhng  which  may  be  employed 
for  both  quahtative  and  quantitative  tests  for  the  presence  of  sugar 
in  solution. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.         27 

Fehling's  Test  Solution. — This  is  a  solution  of  cupric  hydroxid 
made  alkaline  by  an  excess  of  sodium  or  potassium  hydroxid  with 
the  addition  of  sodium  and  potassium  tartrate.  It  is  made  by 
dissolving  cupric  sulphate  34.64  grams,  potassium  hydroxid  125 
grams,  sodium  and  potassium  tartrate  173  grams,  in  distilled  water 
sufficient  to  make  one  liter. 

The  reaction  is  expressed  by  the  following  equation: 

CUSO4  +  2KOH  =  Cu(OH)2  +  KjSO^. 

The  object  of  the  sodium  and  potassium  tartrate  is  to  dissolve 
the  cupric  hydroxid  and  hold  it  in  solution. 

For  qualitative  analysis  it  is  only  necessary  to  boil  a  few  cubic 
centimeters  of  this  solution  in  a  test-tube;  then  add  the  suspected 
solution  and  again  heat  to  the  boihng-point.  If  sugar  be  present, 
the  cupric  hydroxid  is  reduced  to  the  condition  of  a  cuprous  oxid, 
which  shows  itself  as  a  red  or  orange-yellow  precipitate.  The  color 
of  the  precipitate  depends  on  the  relative  excess  of  either  copper  or 
sugar,  being  red  with  the  former,  orange  or  yellow  with  the  latter. 
The  delicacy  of  this  test  is  shown  by  the  fact  that  a  few  minims  of 
this  solution  will  detect  in  i  c.c.  of  water  the  -j^j  of  a  milhgram  of 
sugar. 

For  quantitative  analysis,  10  c.c.  of  Fehhng's  solution,  diluted 
with  40  c.c.  of  water,  are  heated  in  a  porcelain  capsule,  to  which 
the  suspected  solution  is  cautiously  added  from  a  buret  until  the  blue 
color  entirely  disappears.  The  strength  of  this  solution  is  such  that 
I  c.c.  is  decolorized  by  5  milHgrams  of  sugar  (dextrose),  from  which 
the  percentage  of  sugar  in  any  solution  can  be  determined. 

All  the  sugars,  with  the  exception  of  chemically  pure  saccharose, 
may  be  tested  for  with  this  solution. 

The  Fermentation  Test. — All  the  sugars  with  the  exception  of 
lactose  undergo  reduction  to  simpler  compounds,  mainly  alcohol  and 
carbon  dioxid,  under  the  action  of  the  yeast  plant,  Saccharomyces 
cerevisicB.  The  change  with  dextrose  is  expressed  in  the  following 
equation: 

CeHijOe     =     aCjHeO     +      2CO2. 
Dextrose        =        Alcohol         —    Carbon  Dioxid. 

About  95  per  cent,  of  the  dextrose  is  so  changed,  the  remaining 
5  per  cent,  yielding  secondary  products — succinic  acid,  glycerin,  etc. 
As  a  means  of  testing  any  solution  for  the  presence  of  sugar  this 
method  may  be  adopted.  It  is  generally  very  satisfactory.  From 
the  quantity  of  carbon  dioxid  and  alcohol  thus  produced  the  quantity 
of  sugar  in  the  solution  may  be  determined. 

Levulose,  or  fruit-sugar,  is  found  in  association  with  dextrose 
as  a  constituent  of  many  fruits.  It  is  sweeter  than  dextrose  and 
more  soluble  in  both  water  and   dilute  alcohol.     From  alcohohc 


28  TEXT-BOOK  OF  PHYSIOLOGY. 

solutions  it  crystallizes  in  fine,  silky  needles,  though  it  usually  occurs 
in  the  form  of  a  syrup. 

Levulose  is  distinguished  from  dextrose  by  its  property  of  turning 
the  plane  of  polarized  light  to  the  left ;  the  extent  to  which  it  does  so, 
however,  varies  with  the  temperature  and  concentration  of  the 
solution. 

Under  the  influence  of  the  yeast  plant  it  slowly  undergoes  fer- 
mentation, yielding  the  same  products  as  dextrose.  It  also  has  a 
reducing  action  on  cupric  hydroxid. 

Galactose  is  obtained  by  boihng  milk-sugar  (lactose)  with  dilute 
sulphuric  acid.  In  many  chemic  relations  it  resembles  dextrose. 
It  is  less  soluble  in  water,  however,  crystallizes  more  easily,  and  has 
a  greater  dextro-rotatory  power.  It  also  undergoes  fermentation 
with  the  yeast  plant. 

3.  SACCHAROSES,  CisHjjO,,. 

Saccharose,  or  cane-sugar,  is  widely  distributed  throughout 
the  vegetable  world,  but  is  especially  abundant  in  sugar-cane,  sor- 
ghum cane,  sugar-beet,  Indian  corn,  etc.  It  crystallizes  in  large 
monoclinic  prisms.  It  is  soluble  in  water  and  in  dilute  alcohol. 
Saccharose  has  no  reducing  power  on  cupric  hydroxid,  and  hence  its 
presence  can  not  be  detected  by  Fehling's  solution.  It  is  dextro- 
rotatory. Boiled  with  dilute  mineral,  as  well  as  with  organic  acids, 
saccharose  combines  with  water  and  undergoes  a  change  in  virtue  of 
which  it  rotates  the  plane  of  polarized  light  to  the  left,  and  hence  the 
product  was  termed  invert  sugar.  This  latter  has  been  shown  to  be  a 
mixture  of  equal  quantities  of  levulose  and  dextrose.  This  inversion 
of  saccharose  through  hydration  and  decomposition  is  expressed  in 
the  following  equation : 

C12H22O11  +    H2O  =  CeHijOg  -t-  CgHijOg 

Saccharose      -I-     Water    =      Le\Tilose      -f-      Dextrose 

Invert  Sugar. 

Saccharose  is  not  directly  fermentable  by  yeast,  but  through  the 
specific  action  of  a  ferment,  invertin  or  invertase,  secreted  by  the 
yeast  plant,  or  the  inverting  ferment  of  the  small  intestine,  it  under- 
goes inversion,  as  previously  stated,  after  Avhich  it  is  readily  fermented, 
yielding  alcohol  and  carbon  dioxid. 

Lactose  is  the  form  of  sugar  found  exclusively  in  the  milk  of  the 
mammalia,  from  which  it  can  be  obtained  in  the  form  of  hard, 
white,  rhombic  prisms  united  with  one  molecule  of  water.  It  is 
soluble  in  water,  insoluble  in  alcohol  and  ether.  It  is  dextro-rotatory. 
It  reduces  cupric  hydroxid,  but  to  a  less  extent  than  dextrose.  Dilute 
acids  decompose  it  into  equal  quantities  of  dextrose  and  galactose. 
Lactose  is  not  fermentable  with  yeast,  but  in  the  presence  of  the 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.         29 

lactic  acid  bacillus  it  is  decomposed  into  lactic  acid,  and  finally  into 
butyric  acid,  as  expressed  in  the  following  equation: 

CijHsjOn      +      H2O      =     4C3H6O3 

Lactose  ~     Water       =;     Lactic  Acid. 

2C3He03     =     C.HsOz     +      2CO2     +      2H2 

Lactic  Acid        =  Butyric  Acid     —      Carbon     —      Free 

Dioxid  Hydrogen. 

Maltose  is  a  transformation  product  of  starch,  and  arises  when- 
ever the  latter  is  acted  on  by  malt  extract  or  the  diastatic  ferments 
in  sahva  and  pancreatic  juice.  The  change  is  expressed  by  the  fol- 
lowing equation: 

2CeHio05   +    H2O    =    CnHj^On 

Starch.  Water.  Maltose. 

Maltose  crystalhzes  in  the  form  of  white  needles,  which  are 
soluble  in  water  and  in  dilute  alcohol.  It  is  dextro-rotatory.  In 
the  presence  of  ferments  and  dilute  acids  maltose  undergoes  hydra- 
tion and  decomposition,  giving  rise  to  two  molecules  of  dextrose. 
It  has  a  reducing  action  on  cupric  hydroxid.  Fermentation  is  readily 
caused  by  yeast,  but  w^hether  directly  or  indirectly  by  inversion  is 
somewhat  uncertain. 

Osazones. — All  the  sugars  w^hich  possess  the  power  of  reducing 
cupric  hydroxid  are  capable  of  combining  with  phenyl-hydrazin, 
with  the  formation  of  compounds  termed  osazones.  The  osazones 
so  formed  are  crystalline  in  structure,  but  have  different  melting- 
points,  varying  degrees  of  solubility  and  optic  properties,  all  of 
which  serve  to  detect  the  various  sugars  and  to  distinguish  one  from 
the  other.  Of  the  different  osazones,  phenyl-glucosazone  is  the 
most  characteristic,  and  occurs  in  the  form  of  long,  yellow  needles. 
It  may  be  obtained  from  dextrose  by  the  following  method:  To  50 
c.c.  of  a  dextrose  solution  add  2  gm.  of  phenyl-hydrazin  and  2  gm. 
of  sodium  acetate,  and  boil  for  an  hour.  On  cooling,  the  osazone 
cr}^stalhzes  in  the  form  of  long,  yellow  needles. 


THE  FATS. 

The  fats  constitute  a  group  of  organic  bodies  found  in  the  tissues 
of  both  vegetables  and  animals.  In  the  vegetable  world  they  are 
largely  found  in  fruits,  seeds,  and  nuts,  where  they  probably  originate 
from  a  transformation  of  the  carbohydrates.  In  the  animal  body 
the  fats  are  found  largely  in  the  subcutaneous  tissue,  in  the  marrow 
of  bones,  in  and  around  various  internal  organs  and  in  milk.  In 
these  situations  fat  is  contained  in  small,  round  or  polygon-shaped 
vesicles,  which  are  united  by  areolar  tissue  and  surrounded  by  blood- 
vessels. At  the  temperature  of  the  body  the  fat  is  hquid,  but  aftei 
death  it  soon  sohdifies  from  the  loss  of  heat. 


30  TEXT-BOOK  OF  PHYSIOLOGY. 

The  fats  are  compounds  consisting  of  carbon,  hydrogen,  and 
oxygen,  of  which  the  first  is  the  chief  ingredient,  forming  by  weight 
about  75  per  cent.,  while  the  last  is  present  only  in  small  quantity. 
The  fat  found  in  animals  is  a  mixture,  in  varying  proportions 
in  different  animals,  of  three  neutral  fats — stearin,  palmitin,  and 
olein.  Each  fat  is  a  derivative  of  glycerin  and  the  particular  acid 
indicated  by  its  name — e.  g.,  stearic  acid,  in  the  case  of  stearin,  etc. 
The  reaction  which  takes  place  in  the  combination  of  glycerin  and 
the  acid  is  expressed  in  the  following  equation: 

C3H5(HO)3  +   (HCi8H,502)3   =   C,U,{C,siis,02)3  +   3H2O. 

Glycerin.  Stearic  Acid.  Stearin.  Water. 

Hence,  strictly  speaking,  the  fats  are  compound  ethers,  in  which 
the  hydrogen  of  the  organic  acid  is  replaced  by  the  trivalent  radicle, 
tritenyl,  C3H5. 

Stearin,  €3115(0^8113502)3,  is  the  chief  constituent  of  the  more 
solid  fats.  It  is  solid  at  ordinary  temperatures,  melting  at  55°  0., 
then  solidifying  again  as  the  temperature  rises,  until  at  71°  0.  it  melts 
permanently.     It  crystaUizes  in  square  tables. 

Palmitin,  03115(016113^02)3,  is  a  semifluid  fat,  solid  at  45°  0. 
and  melting  at  62°  0.  It  crystalhzes  in  fine  needles,  and  is  soluble 
in  ether. 

Olein,  03115(0^8113302)3,  is  a  colorless,  transparent  fluid,  liquid  at 
ordinary  temperatures,  only  soHdifying  at  0°  0.  It  possesses  marked 
solvent  powers,  and  holds  stearin  and  palmitin  in  solution  at  the 
temperature  of  the  body. 

Saponification. — When  subjected  to  the  action  of  superheated 
steam,  a  neutral  fat  is  saponified — i.  e.,  decomposed  into  glycerin  and 
the  particular  acid  indicated  by  the  name  of  the  fat  used:  e.  g.,  stearic, 
palmitic,  or  oleic.     The  reaction  is  expressed  as  follows: 

C3H5(C,8H3303)3    +     3H3O     =     C3H5(HO)3    +     3(Cl8H3402) 

Olein.  Water.  Glycerin.  Oleic  Acid. 

The  fatty  acids  thus  obtained  are  characterized  by  certain  chemic 
features,  as  follows : 

Stearic  acid  is  a  firm,  white  soHd,  fusible  at  69°  0.  It  is  soluble 
in  ether  and  alcohol,  but  not  in  water. 

Palmitic  acid  occurs  in  the  form  of  white,  ghstening  scales  or 
needles,  melting  at  62°  0. 

Oleic  acid  is  a  clear,  colorless  Hquid,  tasteless  and  odorless  when 
pure.     It  crystalhzes  in  white  needles  at  0°  0. 

If  this  saponification  takes  place  in  the  presence  of  an  alkali, — 
e.  g.,  potassium  hydroxid  or  sodium  hydroxid, — the  acid  produced 
combines  at  once  with  the  alkaH  to  form  a  salt  known  as  a  soap,  while 
the  glycerin  remains  in  solution.     The  reaction  is  as  follows: 
3KHO  +  (C,8H3,02)3  =  sCKC.sHgjO^)  +  3H2O 

Potassium.  Oleic  Acid  Potassium  Oleate.  Water. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.         31 

All  soaps  are,  therefore,  salts  formed  by  the  union  of  alkalies  and 
fatty  acids.  The  sodium  soaps  are  generally  hard,  while  the  potas- 
sium soaps  are  soft.  Those  made  with  stearin  and  palmitin  are  harder 
than  those  made  with  olein.  If  the  soap  is  composed  of  lead,  zinc, 
copper,  etc.,  it  is  insoluble  in  water. 

Emulsification. — ^When  a  neutral  oil  is  vigorously  shaken  with 
water  or  other  fluid,  it  is  broken  up  into  minute  globules  that  are 
more  or  less  permanently  suspended;  the  permanency  depending 
on  the  nature  of  the  hquid.  The  most  permanent  emulsions  are 
those  made  with  soap  solutions.  The  process  of  emulsification  and 
the  part  played  by  soap  can  be  readily  observed  by  placing  on  a 
few  cubic  centimeters  of  a  solution  of  sodium  carbonate  (0.25  per 
cent.)  a  small  quantity  of  a  perfectly  neutral  oil  to  which  has  been 
added  2  or  3  per  cent,  of  a  fatty  acid.  The  combination  of  the  acid 
and  the  alkali  at  once  forms  a  soap.  The  energy  set  free  by  this 
combination  rapidly  divides  up  the  oil  into  extremely  minute  globules. 
A  spontaneous  emulsion  is  thus  formed. 

In  addition  to  the  ordinary  fats,  there  are  present  in  different 
tissues  several  compounds  which,  though  usually  regarded  as  fats, 
nevertheless  differ  materially  from  them  in  composition,  containing, 
as  they  do,  both  nitrogen  and  phosphorus.  These  nitrogenized  or 
phosphorized  fats  are  as  follows: 

Lecithin,  C^^HggN.POg,  is  found  in  blood,  lymph,  red  and 
white  corpuscles,  nerve  tissue,  yolk  of  eggs,  etc.  When  pure,  it 
presents  itself  generally  under  the  form  of  a  white,  crystalHne  powder, 
though  sometimes  as  a  white,  waxy  mass.  Lecithin  is  easily  decom- 
posed, yielding,  with  various  reagents,  glycero-phosphoric  acid,  cholin 
and  stearic  acid. 

Protagon,  Cj^oHa^gNgPOgs,  is  found  most  abundantly  in  the 
brain  tissue,  especially  in  the  white  portion.  It  crystalhzes  from 
warm  alcohohc  solutions,  on  coohng,  in  the  form  of  white  needles, 
generally  arranged  m  groups.  It  melts  at  200°  C,  and  forms  a 
syrupy  Hquid. 

Cerebrin,  Cj^HggN.Og,  is  found  largely  in  the  brain,  in  nerves, 
and  in  pus-corpuscles.  It  is  a  soft,  white,  amorphous  powder,  in- 
soluble in  water,  but  swelling  up  like  starch  in  boiling  water.  When 
boiled  with  dilute  acids,  it  is  decomposed,  yielding  a  fermentable 
dextro-rotatory  sugar,  identical  with  galactose.  Cerebrin  may, 
therefore,  be  regarded  as  a  glucosid. 

THE  PROTEIDS. 

The  proteids  constitute  a  group  of  organic  bodies  which  are 
found  in  both  vegetable  and  animal  tissues.  Though  present  in 
all  animal  tissues,   they  are   especially  abundant   in  muscles   and 


32  TEXT-BOOK  OF  PHYSIOLOGY. 

bones,  where  they  constitute  20  per  cent,  and  30  per  cent,  respectively. 
Though  genetically  related,  and  possessing  many  features  in  common, 
the  different  members  of  the  proteid  group  are  distinguished  by 
characteristic  physical  and  chemic  properties  which  serve  not  only 
for  their  identification,  but  for  their  classification  into  more  or  less 
well-defined  groups  as  follows: 


SIMPLE    PROTEIDS. 
ALBUMINS. 

The  members  of  this  group  are  soluble  in  water,  in  dilute  saUne 
solutions,  and  in  saturated  solutions  of  sodium  chlorid  and  mag- 
nesium sulphate.  They  are  coagulated  by  heat,  and  when  dried 
form  an  amber-colored  mass. 

(a)  Serum-albumin.  This  most  important  proteid  is  found  in 
blood,  lymph,  chyle,  and  some  tissue  fluids.  It  is  obtained 
readily  by  precipitation  from  blood-serum,  after  the  other 
proteids  have  been  removed,  on  the  addition  of  ammonium 
sulphate.  When  freed  from  sahne  constituents,  it  presents 
itself  as  a  pale,  amorphous  substance,  soluble  in  water  and  in 
strong  nitric  acid.  It  is  coagulated  at  a  temperature  of  73° 
C,  as  well  as  by  various  acids — e.  g.,  citric,  picric,  nitric,  etc. 
It  has  a  rotatory  power  of  — 62.6°. 

(b)  Egg-albumin. — Though  not  a  constituent  of  the  human 
body,  egg-albumin  resembles  the  foregoing  in  many  respects. 
When  obtained  in  the  solid  form  from  the  white  of  the  egg, 
it  is  a  yellow  mass  without  taste  or  odor.  Though  similar 
to  serum-albumin,  it  differs  from  it  in  being  precipitated  by 
ether,  in  coagulating  at  54°  C,  and  in  having  a  lower  rotatory 
power,  —35.5°. 

(c)  Lact-albumin. — As  its  name  impHes,  this  proteid  is  found 

in  milk.  It  can  be  precipitated  from  milk-plasma  by  sodium 
sulphate  after  the  precipitation  of  the  other  proteids  by  half 
saturation  with  ammonium  sulphate.     It  slowlv  coagulates  at 

77°  C. 

(d)  Myo-albumin. — This  proteid  is  found  in  muscle-plasma 
from  which  it  subjects  the  plasma  to  fractional  heat  coagu- 
lation.    At  73°  C.  myo-albumin  coagulates. 

GLOBULINS. 

The  members  of  this  group  are  insoluble  in  water  and  in  saturated 
solutions  of  sodium  chlorid  and  magnesium  sulphate  and  ammonium 
sulphate.  They  are  soluble,  however,  in  dilute  sahne  solutions — 
e.  g.,  sodium  chlorid  (i  per  cent.),  potassium  chlorid,  ammonium 
chlorid,  etc.     They  are  coagulated  by  heat. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.        S3 

(a)  Serum-globiilin  or  Paraglobulin. — This  proteid,  as  its  name 

implies,  is  found  in  blood-serum,  though  it  is  present  in  other 
animal  fluids.  When  precipitated  by  magnesium  sulphate 
or  carbon  dioxid,  it  presents  itself  as  a  flocculent  substance, 
insoluble  in  water,  soluble  in  dilute  acids  and  alkaHes,  and 
coagulating  at  75°  C. 

(b)  Fibrinogen. — This  proteid  is  found  in  blood-plasma  in  asso- 

ciation with  serum-globuHn  and  serum-albumin.  It  is  also 
present  in  lymph-tissue  fluids  and  in  pathologic  transudates. 
It  can  be  obtained  from  blood-plasma  which  has  been  pre- 
viously treated  with  magnesium  sulphate  on  the  addition  of 
a  saturated  solution  of  sodium  chlorid.  It  is  soluble  in 
dilute  acids  and  alkahes,  and  coagulates  at  56°  C. 

(c)  Para-myosinogen. — This    proteid   is   a  constituent   of    the 

muscle-plasma  from  which  it  can  be  precipitated  by  a  tem- 
perature of  47°  C. 

(d)  Myosinogen. — This   proteid   is  the  chief  constituent   of  the 

muscle-plasma  and  is  of  great  nutritive  value.  During  the 
living  condition  it  is  hquid,  but  after  death  it  readily  under- 
goes a  chemic  change  and  contributes  to  the  formation  of  an 
insoluble  proteid  known  as  myosin.  It  is  soluble  in  dilute 
hydrochloric  acid  and  dilute  alkalies.     It  coagulates  at  56°  C. 

(e)  Globin. — This  is  a  product  of  the  spontaneous  decomposition 

of  the  coloring-matter  of  the  blood, — hemoglobin, — and  arises 
when  the  latter  is  exposed  to  the  air. 
(/)  Crystallin  or  Globulin. — This  is  obtained  by  passing  a  stream 
of  CO,  through  a  watery  extract  of  the  crystalhne  lens. 

DERIVED  ALBUMINS  OR  ALBUMINATES. 

The  proteids  of  this  group  are  derived  from  both  albumins  and 
globuhns  by  the  gradual  action  of  dilute  acids  and  alkahes,  and  may 
be  regarded  as  compounds  of  a  proteid  with  an  acid  or  an  alkali. 

(a)  Acid-albumin. — This  is  formed  when  a  native  albumin  is 
digested  with  dilute  hydrochloric  acid  (0.2  per  cent.)  or  dilute 
sulphuric  acid  for  some  minutes.  It  is  precipitated  by  neu- 
tralization with  sodium  hydroxid  (o.i  per  cent,  solution). 
After  the  precipitate  is  washed,  it  is  found  to  be  insoluble 
in  distilled  water  and  in  neutral  saline  solutions.  In  acid 
solutions  it  is  not  coagulated  by  heat. 

(b)  Alkali-albumin. — This  is  formed  when  a  native  albumin  is 

treated  with  a  dilute  alkali — e.  g.,  o.i  per  cent,  of  sodium 
hydroxid — for  five  or  ten  minutes.  On  careful  neutrahzation 
with  dilute  hydrochloric  acid,  it  is  precipitated.  It  is  also 
insoluble  in  distilled  water  and  in  saline  solutions;  it  is  not 
coagulable  by  heat. 
3 


34  TEXT-BOOK  OF  PHYSIOLOGY. 

COAGULATED  PROTEIDS. 

Although  these  proteids  are  not  found  as  constituents  of  the 
animal  organism,  they  possess  much  interest  on  account  of  their 
relation  to  prepared  foods  and  to  the  digestive  process.  They  are 
produced  when  solutions  of  egg-albumin,  serum-albumin,  or  globuhns 
are  subjected  to  a  temperature  of  ioo°  C.  or  to  the  prolonged  action 
of  alcohol.  They  are  insoluble  in  water,  in  dilute  acids,  and  in 
neutral  sahne  solutions. 

In  this  same  group  may  be  included  also  those  coagulated  pro- 
teids which  are  produced  by  the  action  of  animal  ferments  on  soluble 
proteids — e.  g.,  fibrin,  myosin,  casein. 

(a)  Fibrin. — Fibrin  is  derived  from  a  soluble  proteid — fibrinogen 

— by  the  action  of  a  special  ferment.  It  is  not  present  under 
normal  circumstances  in  the  circulating  blood,  but  makes  its 
appearance  after  the  blood  is  withdrawn  from  the  vessels 
and  at  the  time  of  coagulation.  It  can  also  be  obtained  by 
whipping  the  blood  with  a  bundle  of  twigs,  on  which  it  accu- 
mulates. When  freed  from  blood  by  washing  under  water, 
it  is  seen  to  consist  of  bundles  of  white  elastic  fibers  or  threads. 
It  is  insoluble  in  water,  in  alcohol,  and  ether.  In  dilute  acids 
it  swells,  becomes  transparent,  and  finally  is  converted  into 
acid-albumin.  In  dilute  alkalies  a  similar  change  takes  place, 
but  the  resulting  product  is  an  alkali-albumin.  Fibrin  pos- 
sesses the  property  of  decomposing  hydrogen  dioxid,  HjOj 
— i.  e.,  liberating  oxygen,  which  accumulates  in  the  form  of 
bubbles  on  the  fibrin.  On  incineration  fibrin  yields  an  ash 
which  contains  calcium  phosphate  and  magnesium  phosphate. 

(b)  Myosin. — Myosin  develops  in  muscles  after  death  and  is  the 
cause  of  the  stiffening  of  the  muscles.  It  has  been  regarded 
as  a  derivative  of  the  soluble  proteid  myosinogen  alone, 
but  there  is  evidence  that  in  its  form  ation  both  paramyosinogen 
and  myosinogen  take  part.  It  is  not  definitely  known  whether 
this  is  the  result  of  the  action  of  a  special  ferment  or  not. 

(c)  Casein. — Casein  is  derived  from  the  chief  proteid  of  milk — 

caseinogen — by  the  action  of  a  special  ferment  known  as 
rennin  or  chymosin.  This  ferment  is  a  constituent  of  gas- 
tric juice. 

PROTEOSES  AND  PEPTONES. 

During  the  progress  of  the  digestive  process,  as  it  takes  place  in 
the  stomach  and  intestines,  there  is  produced  by  the  action  of  the 
gastric  and  pancreatic  juices,  out  of  the  proteids  of  the  food,  a  series 
of  new  proteids,  knowm  as  proteoses  and  peptones.  The  chemic 
properties  of  these  substances  will  be  considered  in  connection  with 
the  process  of  digestion. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.         35 


CONJUGATED  OR  COMBINED  PROTEIDS. 

The  different  members  of  this  group  are  capable  of  being  de- 
composed by  chemic  methods  into  a  proteid  and  a  non-proteid  sub- 
stance; e.  g.,  a  coloring  matter,  a  carbohydrate,  or  a  nuclein.  The 
chemic  character  of  the  non-proteid  substance  furnishes  the  basis 
for  the  following  classification: 

CHROMO-PROTEIDS. 

(a)  Hemaglobin. — Hemoglobin  is  the  coloring  matter  of  the  red 

corpuscles,  of  which  it  constitutes  about  94  per  cent.  It 
possesses  the  power  of  absorbing  oxygen  as  it  passes  through 
the  lung  capillaries  and  of  yielding  it  up  to  the  tissues  as  it 
_  passes  through  the  tissue  capillaries.  In  the  arterial  blood 
it  is  known  as  oxyhemoglobin,  and  in  the  venous  blood  as 
deoxy-  or  reduced-hemoglobin.  When  hydrolysed  by  acids 
or  alkahes,  hemoglobin  undergoes  a  cleavage  into  a  proteid, 
globin,  and  a  pigment  hematin. 

(b)  Myohematin. — Myohematin  is  a  proteid  supposed  to  be 
present  in  muscle.  It  has  never  been  isolated,  hence  its 
chemic  features  are  unknown.  Spectroscopic  examination  in- 
dicates that  it  is  capable  of  absorbing  and  again  yielding 
up  oxygen.  For  this  reason  it  is  believed  to  be  a  derivative 
of  hemoglobin. 

GLUCO-PROTEIDS. 

(a)  Mucin. — Mucin  is  the  proteid  which  gives  the  mucus,  secreted 
by  the  epithelial  cells  of  the  mucous  membranes  and  related 
glands,  its  viscid,  tenacious  character.  It  is  also  a  constituent 
of  the  intercellular  substance  of  the  connective  tissues.  It 
is  readily  precipitated  by  acetic  acid.  When  heated  with 
dilute  acids,  mucin  undergoes  a  cleavage  into  a  simpler  pro- 
teid and  a  carbohydrate  termed  mucose,  which  is  capable 
of  reducing  Fehhng's  solution. 

(b)  Mucoids. — The  mucoids  resemble  the  mucins  though  differ- 
ing from  them  in  solubility  and  in  not  being  precipitable 
from  alkaline  solutions  by  acetic  acid.  They  are  found  in 
the  vitreous  humor,  white  of  egg,  cartilage,  and  in  other 
situations.  They  differ  slightly  one  from  the  other  in  proper- 
ties and  chemic  composition.  They  yield  on  decomposition  a 
carbohydrate. 

NUCLEO-PROTEIDS. 

The  nucleo-proteids  are  obtained  from  the  nuclei  and  cell-sub- 
stance of  tissue-cells.  Chemically  they  are  characterized  by 
the  presence  of  phosphorus  in  relatively  large  amounts.  When 
hydrolysed,  they  separate  into  a  proteid  and  a  nuclein. 


36  TEXT-BOOK  OF  PHYSIOLOGY. 

The  nucleins  derived  from  cell  nuclei  can  be  still  further  sepa- 
rated into  a  simpler  proteid  and  nucleic  acid,  which  latter 
in  turn  yields  phosphoric  acid  and  the  so-called  purin  bases, 
xanthin,  hypoxanthin,  adenin,  and  guanin.  All  nucleins  which 
yield  the  purin  bases  are  termed  true  nucleins. 

The  nucleins  derived  from  caseinogen,  vitellin,  and  probably  cell 
protoplasm  can  be  separated  by  chemic  methods  into  a  pro- 
teid and  phosphoric  acid  only.  For  the  reason  that  they  do 
not  give  origin  to  purin  bases  they  are  termed  pseudo-  or 
paranucleins. 

{a)  Caseinogen. — This  is  the  principal  proteid  of  milk,  in  which 
it  exists  in  association  with  an  alkah,  and  hence  was  formerly 
regarded  as  an  alkah-albumin.  It  is  precipitated  by  acetic 
acid  and  by  magnesium  sulphate.  It  is  coagulated  by 
rennet — that  is,  separated  into  an  insoluble  proteid,  casein  or 
tyrein,  and  a  soluble  albumin.  Calcium  phosphate  seems  to 
be  the  natural  alkali  necessary  to  this  process,  for  if  it  be 
removed  by  dialysis,  or  precipitated  by  the  addition  of 
potassium  oxalate,  coagulation  does  not  take  place. 

(&)  Vitellin. — Vitellin  is  a  constituent  of  the  vitelhs  or  yolk  of 
eggs.  It  diiTers  from  other  proteids  in  the  fact  that  it  is 
semicrystalline  in  character.  Though  usually  regarded  as  a 
nucleo-proteid  it  is  not  definitely  known  whether  or  not  it 
contains  phosphorus  in  its  composition. 


ALBUMINOIDS. 

The  albuminoids  constitute  a  group  of  substances  similar  to  the 
proteids  in  many  respects,  though  differing  from  them  in  others. 
When  obtained  from  the  tissues,  in  which  they  form  an  organic 
basis,  they  are  found  to  be  amorphous,  colloid,  and  when  decom- 
posed yield  products  similar  to  those  of  the  true  proteids.  The 
principal  members  of  this  group  are  as  follows: 

{a)  Collagen,  Ossein. — These  are  two  closely  alhed,  if  not  identical, 
substances,  found  respectively  in  the  white  fibrous  connec- 
tive tissue  and  in  bone.  When  the  tendons  of  muscles,  the 
ligaments,  or  decalcified  bone  are  boiled  for  several  hours, 
the  collagen  and  ossein  are  converted  into  soluble  gelatin, 
which,  when  the  solution  cools,  becomes  solid. 
(h)  Chondrigen. — This  is  supposed  to  be  the  organic  basis  of 
the  more  permanent  cartilages.  When  the  latter  are  boiled, 
they  yield  a  substance  which  gelatinizes  on  cooling,  and  to 
which  the  name  chondrin  has  been  given.  Chondrin,  how- 
ever, is  not  a  pure  gelatin,  but  has  associated  with  it  a  com- 
pound proteid  known  as  chrondro-mucoid. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.         37 

(c)  Elastin  is  the  name  given  to  the  substance  composing  the 
fibers  of  tlie  yellow,  elastic  connective  tissue. 

{d)  Keratin  is  the  substance  found  in  all  horny  and  epidermic 
tissues,  such  as  hairs,  nails,  scales,  etc.  It  differs  from  most 
proteids  in  containing  a  high  percentage  of  sulphur. 

The  average  percentage  composition  of  several  proteids  is  shown 
in  the  following  analyses: 

c.  H.         M.         o.         s. 

Egg-albumin, 52.9  7.2  15.6  23.9  0.4    (Wiirtz). 

Serum-albumin, 53.0  6.8  16.0  22.29  i-77  (Hammersten). 

Casein, 53.3  7.07  15.91  22.03  0.82  (Chittenden  and  Painter). 

Myosin, 52-82  7. 11  16.77  21.90  1.27  (Chittenden  and  Cummins). 

The  molecular  composition  of  the  proteids  is  not  definitely  known, 
and  the  formulae  which  have  been  suggested  are  therefore  only 
approximative.  Leow  assigns  to  albumin  the  formula  C-jHjjjNjg- 
O22S,  while  Schiitzenberger  raises  the  numbers  to  C24oH392Ng5075S3, 
either  of  which  shows  that  the  proteid  molecule  is  extremely  complex. 
As  a  class,  the  proteids  are  characterized  by  the  following  prop- 
erties : 

1.  Indiffusibility. — None   of  the   proteids   normally  assumes   the 

crystalline  form,  and  hence  they  are  not  capable  of  diffusing 
through  parchment  or  an  animal  membrane.  Peptone,  a  product 
of  the  digestion  of  proteids,  is  an  exception  as  regards  its  diffu- 
sibility.  As  met  with  in  the  body,  all  proteids  are  amorphous, 
but  vary  in  consistence  from  the  liquid  to  the  soHd  state.  The 
colloid  character  of  the  proteids  permits  of  their  separation  and 
purification  from  crystalloid  difi'usible  compounds  by  the  process 
of  dialysis. 

2.  Solubility. — Some  of  the  proteids  are  soluble  in  water,  others  in 

solutions  of  the  neutral  salts  of  varying  degrees  of  concentration, 
in  strong  acids  and  alkalies.  All  are  insoluble  in  alcohol  and 
ether. 

3.  Coagulability. — Under  the  influence  of  heat  and  various  acids 

and  animal  ferments,  the  proteids  readily  pass  from  the  soluble 
liquid  state  to  the  insoluble  soHd  state,  attended  by  a  permanent 
alteration  in  their  chemic  composition.  To  this  change  the 
term  coagulation  has  been  given.  The  various  proteids  not 
only  coagulate  at  different  temperatures,  but  with  different 
chemic  reagents— distinctive  features  which  permit  not  only  of 
their  detection,  but  separation.  Proteids  are  capable  of  pre- 
cipitation without  losing  their  solubility  by  ammonium  sulphate, 
sodium  chlorid,  and  magnesium  sulphate. 

4.  Fermentability. — In  the  presence  of  specific  microorganisms — 

■  bacteria — the  proteids,  owing  to  their  complexity  and  instability, 
are  prone  to  undergo  disintegration  and  reduction  to  simpler 


38  .  TEXT-BOOK  OF  PHYSIOLOGY. 

compounds.  This  decomposition  or  putrefaction  occurs  most 
readily  when  the  conditions  most  favorable  to  the  growth  of 
bacteria  are  present — viz.,  a  temperature  varying  from  25°  C. 
to  40°  C,  moisture,  and  oxygen.  The  intermediate  as  well  as 
the  terminal  products  of  the  decomposition  of  the  proteids  are 
numerous,  and  vary  with  the  composition  of  the  proteid  and 
the  specific  physiologic  action  of  the  bacteria.  Among  the 
intermediate  products  is  a  series  of  alkaloid  bodies,  some  of 
which  possess  marked  toxic  properties,  known  as  ptomains. 
The  toxic  symptoms  which  frequently  follow  the  ingestion  of 
foods  in  various  stages  of  putrefaction  are  to  be  attributed  to 
these  compounds.  The  terminal  products  are  represented 
by  hydrogen  sulphid,  ammonia,  carbon  dioxid,  fats,  phosphates, 
nitrates,  etc. 

Color  Tests  for  Proteids. — When  proteids  are  present  in  solu- 
tion, they  may  be  detected  by  the  following  color  reactions — viz.: 

1.  Xanthoproteic-     The  solution  is  boiled  with  nitric  acid  for  several 

minutes,  when  the  proteid  assumes  a  light  yellow  color.  After 
the  solution  has  cooled,  the  addition  of  ammonia  changes  the 
color  to  an  orange  or  amber-red. 

2.  The  rose-red  reaction.     The  solution  is  boiled  with  acid  nitrate 

of  mercury  (Millon's  reagent)  for  a  few  minutes,  when  the 
coagulated  proteid  turns  a  purple-red  color. 

3.  The  blue- violet  reaction.     A  few  drops  of  copper  sulphate  solution 

are  first  added  to  the  proteid  solution,  and  then  an  excess  of 
sodium  hydroxid.  A  blue- violet  color  is  produced,  which  deepens 
somewhat  on  heating,  but  no  further  change  ensues. 


INORGANIC  CONSTITUENTS. 

The  inorganic  compounds  and  mineral  constituents  obtained 
from  the  solids  and  fluids  of  the  body  are  very  numerous,  and,  in 
some  instances,  quite  abundant.  Though  many  of  the  compounds 
thus  obtained  are  undoubtedly  derivatives  of  the  tissues  and  necessary 
to  their  physical  and  physiologic  activity,  others,  in  all  probability, 
are  decomposition  products,  or  transitory  constituents  introduced 
with  the  food.  Of  the  inorganic  compounds,  the  following  are  the 
most  important: 

WATER. 

Water  is  the  most  important  of  the  inorganic  constituents,  as  it 
is  indispensable  to  hfe.  It  is  present  in  all  the  tissues  and  fluids 
without  exception,  varying  from  99  per  cent,  in  the  saliva  to  80 
per  cent,  in  the  blood,  75  per  cent,  in  the  muscles  to  2  per  cent,  in 
the  enamel  of  the  teeth.     The  total  quantity  contained  in  a  body 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.        39 

weighing  75  kilograms  (165  pounds)  is  52.5  kilograms  (115  pounds). 
Much  of  the  water  exists  in  a  free  condition,  and  forms  the  chief  part 
of  the  fluids,  giving  to  them  their  characteristic  degree  of  fluidity. 
Possessing  the  capability  of  holding  in  solution  a  large  number  of 
inorganic  as  well  as  some  organic  compounds,  and  being  at  the  same 
time  dift'usible,  it  renders  an  interchange  of  materials  between  all 
portions  of  the  body  possible.  It  aids  in  the  absorption  of  new 
material  into  the  blood  and  tissues,  and  at  the  same  time  it  transfers 
waste  products  from  the  tissues  to  the  blood,  from  which  they  are 
finally  ehminated,  along  with  the  water  in  which  they  are  dissolved. 
A  portion  of  the  water  is  chemically  combined  with  other  tissue  con- 
stituents and  gives  to  the  tissues  their  characteristic  physical  properties. 
The  consistency,  elasticity,  and  pliability  are,  to  a  large  extent,  con- 
ditioned by  the  amount  of  water  they  contain.  The  total  quantity 
of  water  eliminated  by  the  kidneys,  lungs,  and  skin  amounts  to  about 
3  kilograms  (6|  pounds). 

CALCroM  COMPOUNDS. 

Calcium  phosphate,  Ca3(P04)2,  has  a  very  extensive  distribu- 
tion throughout  the  body.  It  exists  largely  in  the  bones,  teeth,  and 
to  a  sught  extent  in  cartilage,  blood,  and  other  tissues.  Milk  con- 
tains 0.27  per  cent.  The  sohdity  of  the  bones  and  teeth  is  almost 
entirely  due  to  the  presence  of  this  salt,  and  is,  therefore,  to  be 
regarded  as  necessary  to  their  structure.  It  enters  into  chemic  union 
with  the  organic  matter,  as  shown  by  the  fact  that  it  can  not  be 
separated  from  it  except  by  chemic  means,  such  as  hydrochloric 
acid.  Though  insoluble  in  water,  it  is  held  in  solution  in  the  blood 
and  milk  by  the  proteid  constituents,  and  in  the  urine  by  the  acid 
phosphate  of  soda.  The  total  quantity  of  calcium  phosphate  which 
enters  into  the  formation  of  the  body  has  been  estimated  at  2.5 
kilograms.  The  amount  ehminated  daily  from  the  body  has  been 
estimated  at  0.4  gm.,  a  fact  which  indicates  that  nutritive  changes 
do  not  take  place  with  much  rapidity  in  those  tissues  in  which  it  is 
contained. 

Calcium  carbonate,  CaCOg,  is  present  in  practically  the  same 
situations  in  the  body  as  the  phosphate,  and  plays  essentially  the 
same  role.  It  is,  however,  found  in  the  crystalline  form,  aggregated 
in  small  masses  in  the  internal  ear,  forming  the  otoliths,  or  ear  stones. 
Though  insoluble,  it  is  held  in  solution  by  the  carbonic  acid  diffused 
through  the  fluids. 

Calcium  fluorid,  CaF,,  is  found  in  bones  and  teeth. 

SODIUM  COMPOUNDS. 

Sodium  chlorid,  NaCl,  is  present  in  all  the  tissues  and  fluids 
of  the  body,  but  especially  in  the  blood,  0.6  per  cent.,  lymph,  0.5, 


40  TEXT-BOOK  OF  PHYSIOLOGY. 

and  pancreatic  juice,  0.25  per  cent.  The  entire  quantity  in  the  body 
has  been  estimated  at  about  200  gm.  Sodium  chlorid  is  of  much 
importance  in  the  body,  as  it  determines  and  regulates  to  a  large 
extent  the  phenomena  of  diffusion  which  are  there  constantly  taking 
place.  This  is  illustrated  by  the  fact  that  a  solution  of  albumin 
placed  in  the  rectum  without  the  addition  of  this  salt  will  not  be 
absorbed.  When  the  salt  is  added,  absorption  takes  place.  The 
ingested  water  is  absorbed  into  the  blood  largely  in  consequence  of 
the  percentage  of  this  salt  which  it  contains.  The  normal  percentage 
of  sodium  chlorid  in  the  blood-plasma  assists  in  maintaining  the  shape 
and  structure  of  the  red  blood-corpuscles  by  determining  the  amount 
of  water  entering  into  their  composition.  The  same  is  true  of  other 
tissue  elements. 

Sodium  chlorid  also  influences  the  general  nutritive  process,  in- 
creasing the  disintegration  of  the  proteids,  as  shown  by  the  increased 
amount  of  urea  excreted.  During  its  existence  in  the  body  it  under- 
goes chemic  transformations  or  decompositions,  yielding  its  chlorid 
to  form  the  potassium  chlorid  of  the  blood-corpuscles  and  muscles 
and  to  form  the  hydrochloric  acid  of  the  gastric  juice. 

Sodium  phosphate,  Na2HPO^,  is  found  in  all  sohds  and  fluids 
of  the  body,  to  which,  with  but  few  exceptions,  it  imparts  an  alkahne 
reaction.  This  is  especially  true  of  blood,  lymph,  and  tissue  fluids 
generally.  It  is  essential  to  physiologic  action  that  all  tissue  elements 
should  be  bathed  by  an  alkahne  medium. 

Sodium  carbonate,  NajCOg,  is  generally  found  in  association 
with  the  preceding  salt.  As  it  is  an  alkahne  compound,  it  also 
assists  in  giving  to  the  blood  and  lymph  their  characteristic  alkalinity. 
In  carnivorous  animals  the  sodium  phosphate  is  the  more  abundant, 
while  in  the  herbivorous  animals  the  sodium  carbonate  is  the  more 
abundant. 

Sodium  sulphate,  Na^SO^,  is  present  in  many  of  the  tissues  and 
fluids,  especially  in  the  urine.  Though  introduced  in  the  food,  it  is 
also,  in  all  probabihty,  formed  in  the  body  from  the  decomposition 
and  oxidation  of  the  proteids. 

POTASSroM  COMPOUNDS. 

Potassium  chlorid,  KCl,  is  met  with  in  association  with  sodium 
chlorid  in  almost  all  situations  in  the  body.  It  preponderates,  how- 
ever, in  the  tissue  elements,  especially  in  the  muscle  tissue,  nerve 
tissue,  and  red  corpuscles.  The  plasma  with  which  these  structures 
are  bathed  contains  but  a  very  small  amount  of  this  salt,  but,  as 
previously  stated,  a  relatively  large  quantity  of  sodium  chlorid. 
Though  introduced  to  some  extent  in  the  food,  it  is  very  likely  that 
it  is  also  formed  through  the  decomposition  of  the  sodium  chlorid. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY        41 

Potassium  phosphate,  KjHPO^,  is  found  in  association  with 
sodium  phosphate  in  all  the  fluids  and  sohds.  As  it  has  similar 
chemic  properties,  its  functions  are  practically  the  same. 

Potassium  carbonate,  KgCOg,  is  generally  found  with  the  pre- 
ceding salt. 

MAGNESIUM  COMPOUNDS. 

Magnesium  phosphate,  Mg3(POj2>  is  found  in  all  tissues,  in 
association  with  calcium  phosphate,  though  in  much  smaller  quantity. 

Magnesium  carbonate,  MgCOg,  occurs  only  in  traces  in  the 
blood. 

Both  of  these  compounds  have  functions  similar  to  the  calcium 
compounds,  and  exist,  in  all  probabihty,  under  similar  conditions. 


IRON  COMPOUNDS. 

Iron  is  a  constituent  of  the  coloring-matter  of  the  blood.  Traces, 
however,  are  also  found  in  lymph,  bile,  gastric  juice,  and  in  the 
pigment  of  the  eyes,  skin,  and  hair.  The  amount  of  iron  contained 
in  a  body  weighing  75  kilograms  is  about  3  gm.  It  exists  under 
various  forms — e.  g.,  ferric  oxid,  and  in  combination  with  organic 
compounds. 

Chemic  analysis  thus  shows  that  the  chemic  elements  into  which 
the  compounds  may  be  resolved  by  an  ultimate  analysis  do  not  exist 
in  the  body  in  a  free  state,  but  only  in  combination,  and  in  char- 
acteristic proportions,  to  form  compounds  whose  properties  are  the 
resultant  of  those  of  the  elements.  Of  the  four  principal  elements 
which  make  up  97  per  cent,  of  the  body,  O,  H,  N  are  extremely 
mobile,  elastic,  and  possessed  of  great  atomic  heat.  C,  H,  N  are 
distinguished  for  the  narrow  range  of  their  affinities,  and  for  their 
chemic  inertia.  C  possesses  the  great  atomic  cohesion.  O  is  noted 
for  the  number  and  intensity  of  its  combinations. 

As  the  properties  of  the  compounds  formed  by  the  union  of 
elements  must  be  the  resultants  of  the  properties  of  the  elements 
themselves,  it  follows  that  the  ternary  compounds,  starches,  sugars, 
and  fats  must  possess  more  or  less  inertia,  and  at  the  same  time 
instability;  while  in  the  more  complex  proteids,  in  which  sulphur 
and  phosphorus  are  frequently  combined  with  the  four  principal 
elements,  molecular  instabihty  attains  its  maximum.  As  all  the 
foregoing  compounds  possess  in  varying  degrees  the  properties  of 
inertia  and  instability,  it  follows  that  hving  matter  must  possess 
corresponding  properties,  and  the  capability  of  undergoing  unceas- 
ingly a  series  of  chemic  changes,  both  of  composition  and  decom- 
position, in  response  to  the  chemic  and  physical  influences  by  which 
it  is  surrounded,  and  which  underhe  all  the  phenomena  of  Hfe. 


42  TEXT-BOOK  OF  PHYSIOLOGY. 


PRINCIPLES  OF  DISSIMILATION. 

In  addition  to  the  previously  mentioned  compounds, — viz., 
carbohydrates,  fats,  proteids,  and  inorganic  salts, — there  is  obtained 
by  chemic  analysis  from  the  tissues  and  fluids  of  the  body: 

1.  A  number  of  organic  acids,  such  as  acetic,  lactic,  oxahc,  butyric, 

propionic,  etc.,  in  combination  with  alkaline  and  earthy  bases. 

2.  Organic  compounds,  such  as  alcohol,  glycerin,  cholesterin. 

3.  Pigments,  such  as  those  found  in  bile  and  urine. 

4.  Crystalhzable  nitrogenized  bodies,  such  as  urea,  uric  acid,  xanthin, 

hippuric  acid,  creatin,  creatinin,  etc. 
While  some  few  of  these  compounds  may  possibly  be  regarded  as 
necessary  to  the  physiologic  integrity  of  the  tissues  and  fluids,  the 
majority  of  them  are  to  be  regarded  as  products  of  dissimilation  of 
the  tissues  and  foods  in  consequence  of  functional  activity,  and 
represent  stages  in  their  reduction  to  simpler  forms  previous  to  being 
eliminated  from  the  body. 


CHAPTER  III. 
PHYSIOLOGY  OF  THE  CELL. 

A  microscopic  analysis  of  the  tissues  shows  that  they  can  be 
resolved  into  simpler  elements,  termed  cells,  which  may,  therefore, 
be  regarded  as  the  primary  units  of  structure.  Though  cells  vary 
considerably  in  shape,  size,  and  chemic  composition  in  the  different 
tissues  of  the  adult  body,  they  are,  nevertheless,  descendants  from 
typical  cells,  known  as  embryonic  or  undifferentiated  cells,  examples 
of  which  are  the  leukocytes  of  the  blood  and  lymph  and  the  first 
offspring  of  the  fertihzed  ovum.  Ascending  the  Une  of  embryonic 
development,  it  will  be  found  that  every  organized  body  originates 
in  a  single  cell — the  ovum.  As  the  cell  is  the  elementary  unit  of  all 
tissues,  the  function  of  each  tissue  must  be  referred  to  the  function 
of  the  cell.  Hence  the  cell  may  be  defined  as  the  primary  anatomic 
and  physiologic  unit  of  the  organic  world,  to  which  every  exhibition 
of  hfe,  whether  normal  or  abnormal,  is  to  be  referred. 

Structure  of  Cells. — Though  cells  vary  in  shape  and  size  and 
internal  structure  in  dift'erent  portions  of  the  body,  a  typical  cell  may 
be  said  to  consist  mainly  of  a  gelatinous  substance  forming  the  body 
of  the  cell,  termed  protoplasm  or  bioplasm,  in  which  is  embedded  a 
smaller  spheric  body,  the  nucleus.  The  shape  of  the  adult  cell  varies 
according  to  the  tissue  in  which  it  is  found;  when  young  and  free 
to  move  in  a  fluid  medium,  the  cell  assumes  a  spheric  form,  but  when 
subjected  to  pressure,  may  become  cyHndric,  fusiform,  polygonal, 
or  stellate.  Cells  vary  in  size  within  wide  hmits,  ranging  from -3x00 
of  an  inch,  the  diameter  of  a  red  blood- corpuscle,  to  -j^  of  an  inch, 
the  diameter  of  the  large  cells  in  the  gray  matter  of  the  spinal  cord. 
(See  Fig.  2.) 

The  cell  protoplasm  consists  of  a  soft,  semifluid,  gelatinous 
material,  varying  somewhat  in  appearance  in  different  tissues. 
Though  frequently  homogeneous,  it  often  exhibits  a  finely  granular 
appearance  under  medium  powers  of  the  microscope.  Young  cells 
consist  almost  entirely  of  clear  protoplasm.  Mature  cells  contain, 
according  to  the  tissue  in  which  they  are  found,  material  of  an  en- 
tirely different  character — e.  g.,  small  globules  of  fat,  granules  of 
glycogen,  mucigen,  pigments,  digestive  ferments,  etc.  Under  high 
powers  of  the  microscope  the  cell  protoplasm  is  found  to  be  pervaded 
by  a  network  of  fibers,  termed  spongio plasm,  in  the  meshes  of  which 
is  contained  a  clearer  and  more  fluent  substance,  the  hyaloplasm. 

43 


44 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  relative  amount  of  these  two  constituents  varies  in  different 
cells,  the  proportion  of  hyaloplasm  being  usually  greater  in  young 
cells.  The  arrangement  of  the  fibers  forming  the  spongioplasm  also 
varies,  the  fibers  having  sometimes  a  radial  direction,  in  others  a 
concentric  disposition,  but  most  frequently  being  distributed  evenly 
in  all  directions.  In  many  cells  the  outer  portion  of  the  cell  proto- 
plasm undergoes  chemic  changes  and  is  transformed  into  a  thin, 
transparent,  homogeneous  membrane, — the  cell  membrane, — v^hich 
completely  incloses  the  cell  substance.  The  cell  membrane  is 
permeable  to  water  and  watery  solutions  of  various  inorganic  and 
organic  substances.  It  is,  however,  not  an  essential  part  of  the  cell. 
The  nucleus  is  a  small  vesicular  body  embedded  in  the  proto- 
plasm near  the  center  of  the  cell.     In  the  resting  condition  of  the  cell 


Nuclear  membrane.    ^ 


Linin. 


Nuclear  fluid  (matrix). 


Nucleolus. 


Chromatin-cords 
(nuclear  network). 


Nodal  enlargements 
of  the  chromatin. 


Cell  membrane. 
Exoplasm. 

Microsomes. 
Centrosoma. 

Spongioplasm. 
Hyaloplasm. 
Foreign  inclosures. 


Fig.  2. — Diagram  of  a  Cell.     Microsomes  and  spongioplasm  are  only  partly  drawn. 

— (Stohr.) 


it  consists  of  a  distinct  membrane,  composed  of  amphipyrenin,  in- 
closing the  nuclear  contents.  The  latter  consists  of  a  homogeneous 
amorphous  substance, — the  nuclear  matrix, — in  which  is  embedded 
the  nuclear  network.  It  can  often  be  seen  that  a  portion  of  one 
side  of  the  nucleus,  called  the  pole,  is  free  from  this  network.  The 
main  cords  of  the  network  are  arranged  as  V-shaped  loops  about  it. 
These  main  cords  send  out  secondary  branches  or  twigs,  which, 
uniting  with  one  another,  complete  the  network.  The  nuclear  cords 
are  composed  of  granules  of  chromatin, — so  called  because  of  its 
affinity  for  certain  staining  materials, — held  together  by  an  achromatin 
substance  known  as  linin.  Besides  the  nuclear  network,  there  are 
embedded  in  the  nuclear  matrix  one  or  more  small  bodies  composed 


PHYSIOLOGY  OF  THE  CELL.  45 

of  py renin,  known  as  nucleoli.  At  the  pole  of  the  nucleus,  either 
within  or  just  without  in  the  protoplasm,  is  a  small  body,  the  centro- 
some,  or  pole  corpuscle. 

Chemic  Composition  of  the  Cell. — The  composition  of  hving 
protoplasm  is  ditiicult  of  determination,  for  the  reason  that  all  chemic 
and  physical  methods  employed  for  its  analysis  destroy  its  vitality, 
and  the  products  obtained  are  pecuhar  to  dead  rather  than  to  Uving 
matter.  Moreover,  as  protoplasm  is  the  seat  of  constructive  and 
destructive  processes,  it  is  not  easy  to  determine  whether  the  products 
of  analysis  are  crude  food  constituents  or  cleavage  or  disintegration 
products.  Nevertheless,  chemic  investigations  have  shown  that  even 
in  the  living  condition  protoplasm  is  a  highly  complex  compound — 
the  resultant  of  the  intimate  union  of  many  different  substances. 
About  75  per  cent,  of  protoplasm  consists  of  water  and  25  per  cent, 
of  sohds,  of  which  the  more  important  compounds  are  various 
nucleo-proteids  (characterized  by  their  large  percentage  of  phos- 
phorus), globuhns,  traces  of  lecithin,  cholesterin,  and  frequently  fat 
and  carbohydrates.  Inorganic  salts,  especially  the  potassium, 
sodium,  and  calcium  chlorids  and  phosphates,  are  almost  invariable 
and  essential  constituents. 


MANIFESTATIONS  OF  CELL  LIFE. 

Growth,  Nutrition,  and  Reproduction. — All  cells  exhibit  the 
three  fundamental  properties  of  hfe — viz.,  growth,  nutrition,  repro- 
duction. All  cells  when  newly  reproduced  are  extremely  small,  but 
b}^  the  absorption  of  nutritive  material  from  their  surrounding  me- 
dium, the  h-mph,  they  gradually  grow  until  they  attain  their  mature 
size.  This  is  accomplished  by  the  power  which  living  material  pos- 
sesses of  not  only  absorbing  nutritive  material,  but  of  subsequently 
assimilating  it,  organizing  it,  transforming  it  into  material  like  itself 
and  endowing  it  with  its  own  ph}'siologic  properties. 

In  the  physiologic  condition  the  hving  material  of  the  cell,  the 
bioplasm,  is  the  seat  of  a  series  of  chemic  changes  which  vary  in 
activity  from  moment  to  moment,  and  on  the  continuance  of  which 
its  vitality  depends.  Some  of  these  changes  are  destructive  or  dis- 
integrative in  character,  whereby  the  hving  material  is  reduced 
through  a  series  of  descending  chemic  stages  to  simpler  compounds 
such  as  urea,  uric  acid,  carbon-dioxid,  etc.,  and  which  are  finally 
eliminated  from  the  body.  To  these  disintegrative  changes  the 
terms  dissimilation  and  kataholism  are  applied;  other  of  the  changes 
are  constructive  or  integrative  in  character,  whereby  the  hving  ma- 
terial is  repaired  and  restored  to  its  former  condition,  and  out  of 
new"  nutritive  material  through  a  series  of  ascending  chemic  stages. 


46  TEXT-BOOK  OF  PHYSIOLOGY. 

To  these  integrati\'e  changes  the  terms  assimilation  and  anabolism 
are  given. 

The  sum  total  of  all  changes  which  go  on  in  the  cell,  both 
assimilative  and  dissimilative,  are  embraced  under  the  general 
term  nulrltion,  or  metabolism.  During  the  course  of  its  physiologic 
activities  the  bioplasm  of  the  cell  produces  material  of  an  entirely 
different  character  which  varies  with  the  cell,  such  as  fat,  glycogen, 
mucigen,  ferments,  etc.,  which  are  frequently  spoken  of  as  meta- 
bolic products.  Every  cell  presents  in  its  nutritive  activities  an 
epitome  of  the  nutritive  activities  of  the  body  as  a  whole. 

Physiologic  Properties  of  Protoplasm. — All  Hving  protoplasm 
possesses  properties  which  serve  to  distinguish  and  characterize  it — 
viz.,  irritability,  conductivity,  and  motility. 

Irritability,  or  the  power  of  reacting  in  a  definite  manner  to  some 
form  of  external  excitation,  whether  mechanic,  chemic,  or  electric, 
is  a  fundamental  property  of  all  living  protoplasm.  The  character 
and  extent  of  the  reaction  will  vary,  and  will  depend  both  on  the 
nature  of  the  protoplasm  and  the  character  and  strength  of  the 
stimulus.  If  the  protoplasm  be  muscle,  the.  response  will  be  a  con- 
traction; if  it  be  gland,  the  response  will  be  secretion;  if  it  be  nerve, 
the  response  will  be  a  sensation  or  some  other  form  of  nerve  activity. 

Conductivity,  or  the  power  of  transmitting  molecular  disturbances 
arising  at  one  point  to  all  portions  of  the  irritable  material,  is  also 
a  characteristic  feature  of  all  protoplasm.  This  power,  however,  is 
best  developed  in  that  form  of  protoplasm  found  in  nerves,  which 
serves  to  transmit,  with  extreme  rapidity,  molecular  disturbances 
arising  at  the  periphery  to  the  brain,  as  well  as  in  the  reverse  direction. 
Muscle  protoplasm  also  possesses  the  same  power  in  a  high  degree. 

Motility,  or  the  power  of  executing  apparently  spontaneous 
movements,  is  exhibited  by  many  forms  of  cell  protoplasm.  In 
addition  to  the  molecular  movements  which  take  place  in  certain 
cells,  other  forms  of  movement  are  exhibited,  more  or  less  constantly, 
by  many  cells  in  the  animal  body — e.  g.,  the  waving  of  cilia,  the 
ameboid  movements  and  migrations  of  white  blood-corpuscles,  the 
activities  of  spermatozooids,  the  projection  of  pseudopodia,  etc. 
These  movements,  arising  without  any  recognizable  cause,  are  fre- 
quently spoken  of  as  spontaneous.  Strictly  speaking,  however,  all 
protoplasmic  movement  is  the  resultant  of  natural  causes,  the  true 
nature  of  which  is  beyond  the  reach  of  present  methods  of  investi- 
gation. 

Reproduction. — Cells  reproduce  themselves  in  the  higher  ani- 
mals in  two  ways — by  direct  division  and  by  indirect  division,  or 
karyokinesis.  In  the  former  the  nucleus  becomes  constricted,  and 
divides  without  any  special  grouping  of  the  nuclear  elements.  It  is 
probable  that  this  occurs  only  in  disintegrating  cells,  and  never  in 


PHYSIOLOGY  OF  THE  CELL. 


47 


a  physiologic  multiplication.     In  division  by  karyokinesis  (Fig.  3) 
there  is  a  progressive  rearranging  and  definite  grouping  of  the  nucleus, 
the  result  of  which  changes  is  the  division  of  the  centrosome,  the 
chromatin,  and  the  rest  of  the  nucleus  into  two  equal  portions,  which 
form  the  nuclei.     Following  the  division  of  the  nuclei,  the  proto- 
plasm divides.     The  process  may  be  divided  into  three  phases: 
I.  Prophase. — The  centrosome,  at  first  small  and  lying  within  the 
nucleus,  increases  in  size  and  moves  into  the  protoplasm,  where 
it  lies  near  the  nucleus,  surrounded  by  a  clear  zone,  from  which 
delicate  threads  radiate  throudi  an  area  known  as  the  attraction 


Close  Skein 
(^■iewed  from 
the  side). 
Polar  field. 


Loose  Skein  (viewed 
from  above — i.  e.,  from 

the  pole).  Mother  Stars  (\iewed  from  the  side). 


Spindle. 


Mother  Star  (\-iewed       Daughter  Star 
from  above). 


%i.*^ 


Beginning  Completed 

Di\ision  of  the  Protoplasm. 


Fig.  3.— Karyokinetic  Figures  Observed  in  the  Epithelium  of  the  Or.a.l 
Cavity  of  a  Sal,a.maxder.  The  picture  in  the  upper  right-hand  corner  is  from 
a  section  through  a  dividing  egg  of  Siredon  pisciformis.  Neither  the  centrosomes 
nor  the  first  stages  of  the  development  of  the  spindle  can  be  seen  by  this  mag- 
nification.    X  560. — {Stohr.) 


Sphere.  The  nucleus  enlarges  and  becomes  richer  in  chromatin. 
The  lateral  twigs  of  the  chromatin  cords  are  drawn  in,  while 
the  main  cords  become  much  contorted,  These  cords  have  a 
general  direction  transverse  to  the  long  axis  of  the  cell,  and 
parallel  to  the  plane  of  future  cleavage.  They  are  seen  as 
V-shaped  segments  or  loops,  chromosomes,  having  their  closed 
ends  directed  toward  a  common  center,  the  polar  field,  while 
the  other  ends  interdigitate  on  the  opposite  side  of  the  nucleus 
— the  anti-pole.     The  polar  field  corresponds  to  the  area  occu-. 


48  TEXT-BOOK  OF  PHYSIOLOGY. 

pied  by  the  centrosome.  This  arrangement  is  known  as  the 
close  skein;  but  as  the  process  goes  on,  the  chromosomes  become 
thicker,  shorter  and  less  contorted,  producing  a  much  looser 
arrangement,  known  as  the  loose  skein.  During  the  formation 
of  the  loose  skein,  the  centrosome  divides  into  two  portions, 
which  move  apart  to  positions  at  the  opposite  ends  of  the  long 
axis  of  the  nucleus.  At  the  same  time  dehcate  achromatin 
fibers  make  their  appearance,  arranged  in  the  form  of  a  double 
cone,  the  apices  of  which  correspond  in  position  to  the  centro- 
somes.  This  is  known  as  the  nuclear  spindle.  During  the 
prophase  the  nuclear  membrane  and  the  nucleoli  disappear. 

2  The  Metaphase. — The  two  centrosomes  are  at  opposite  ends  of 
the  long  axis  of  the  nucleus,  each  surrounded  by  an  attraction 
sphere,  now  called  the  polar  radiation.  The  chromosomes 
become  yet  shorter  and  thicker,  and  move  toward  the  equator 
of  the  nucleus,  where  they  lie  with  their  closed  ends  toward  the 
axis,  presenting  the  appearance,  when  seen  from  the  poles,  of 
a  star, — the  so-called  mother  star,  or  monaster.  While  moving 
toward  the  equator  of  the  nucleus,  and  often  earher,  each 
chromosome  undergoes  longitudinal  cleavage,  the  sister  loops 
remaining  together  for  a  time.  Upon  the  completion  of  the 
monaster,  one  loop  of  each  pair  passes  to  each  pole  of  the  nucleus, 
guided,  and  perhaps  drawn  by  the  threads  of  the  nuclear  spindle. 
The  separation  of  the  sister  segments  begins  at  their  apices, 
and  as  the  open  ends  are  drawn  apart  they  remain  connected 
by  delicate  achromatin  filaments  drawn  out  from  the  chromo- 
somes. This  separation  of  the  daughter  chromosomes,  and 
their  movement  toward  the  daughter  centrosomes,  is  called 
metakinesis.  As  they  approach  their  destination,  we  have  the 
appearance  of  two  stars  in  the  nucleus — the  daughter  stars,  or 
diasters. 

3.  Anaphase. — The  daughter  stars  undergo,  in  reverse  order,  much 
the  same  changes  that  the  mother  star  passed  through.  The 
chromosomes  become  much  convoluted,  and  perhaps  united  to 
one  another,  the  lateral  twigs  appear,  and  the  chromatin  resumes 
the  appearance  of  the  resting  nucleus.  The  nuclear  spindle, 
with  most  of  the  polar  radiation,  disappears,  and  the  nucleoh 
and  the  nuclear  membrane  reappear,  thus  forming  two  complete 
daughter  nuclei.  Meanwhile  the  protoplasm  becomes  con- 
stricted midway  between  the  young  nuclei.  This  constriction 
gradually  deepens  until  the  original  cell  is  divided,  with  the 
formation  of  two  complete  cells. 


CHAPTER  IV. 

HISTOLOGY  OF  THE  EPITHELIAL  AND  CONNECTIVE 

TISSUES. 

I.  EPITHELIAL  TISSUE. 

The  epithelial  tissue  consists  of  one  or  more  layers  of  cells 
resting  on  a  homogeneous  membrane,  the  other  side  of  which  is 
abundantly  supphed  with  blood-vessels  and  nerves.  The  form 
of  the  epithelial  cell  varies  in  different  situations,  and  may  be 
flattened,  cuboid,  spheroid,  or  columnar.  (See  Figs.  4,  5,  and  6.) 
The  form  of  the  cell  in  all  instances  is  related  to  some  specific 
function.  When  arranged  in  layers  or  strata,  the  cells  are  cemented 
toorether  bv  an  intercellular  substance. 


Fig.  4. — Epithelial  Cells  of  Rabbit,  Isolated.  X  560.  i.  Squamous  cells 
(mucous  membrane  of  mouth).  2.  Columnar  cells  (corneal  epithelium).  3 
Columnar  cells,  with  cuticular  border,  s  (intestinal  epithelium).  4.  Ciliated  cells; 
h,  cilia  (bronchial  epithelium)  .^(^/o/tr.) 


The  epithehal  tissue  forms  a  continuous  covering  for  the  surfaces 
of  the  body.  The  external  investment  (the  skin)  and  the  internal 
investment  (the  mucous  membrane,  which  lines  the  entire  ahmentary 
canal  as  well  as  associated  body  cavities)  are  both  formed,  in  all 
situations,  by  the  homogeneous  basement  membrane,  covered  with 
one  or  more  layers  of  cells.  The  glands  of  the  skin,  the  lungs  and 
the  glands  in  connection  with  the  alimentary  canal  and  the  uro-geni- 
tal  apparatus  are  formed  of  the  same  elemental  structures.  All 
materials,  therefore,  whether  nutritive,  secretory,  or  excretory,  must 
pass  through  epithehal  cells  before  they  can  enter  into  the  formation 
of  the  blood  or  be  eliminated  from  it.  The  nutrition  of  the  epithelial 
tissue  is  maintained  by  the  nutritive  material  derived  from  the  blood 
4  49 


50 


TEXT-BOOK  OF  PHYSIOLOGY. 


diffusing  itself  into  and  through  the  basement  membrane.  Chemi- 
cally, the  epithehal  cells  of  the  epidermis — hair,  nails,  etc. — are 
composed  of  an  albuminoid  material  (keratin),  a  small  quantity  of 
water,  and  inorganic  salts.  In  other  situations,  especially  on  the 
mucous  membranes,  the  cells  consist  largely  of  mucin,  in  associa- 
tion with  other  proteids.  The  consistency  of  epithehum  varies  in 
accordance  with  external  influences,  such  as  the  presence  or  absence 
of  moisture,  pressure,  friction,  etc.  This  is  well  seen  in  the  skin  of 
the  palms  of  the  hands  and  the  soles  of  the  feet — situations  where  it 
acquires  its  greatest  density.  In  the  aHmentary  canal,  in  the  lungs, 
and  in  other  cavities,  where  the  reverse  conditions  prevail,  the  epi- 
thelium is  extremely  soft.  Epithelial  tissues  also  possess  varying 
degrees  of  cohesion  and  elasticity — physical  properties  which  en- 
able them  to  resist  considerable 
pressure  and  distention  without 
^  having    their     physiologic    in- 

-  tegrity    destroyed.       Inasmuch 


I 


Fig.  5. — Stratified  Squamous  Epi- 
thelium (Larynx  of  Man). 
X  240.  I.  Columnar  cells.  2. 
Prickle-cells.     3.  Squamous   cells. 

—{Stohr.) 


Fig.  6. — Stratified  Ciliated  Epi- 
thelium. X  560.  From  the  res- 
piratory nasal  mucous  membrane 
of  man.  i.  Oval  cells.  2.  Spindle- 
shaped  cells.  3.  Columnar  cells. 
—{Stohr.) 


as  these  tissues  are  poor  conductors  of  heat,  they  assist  in  preventing 
too  rapid  radiation  of  heat  from  the  body,  and  cooperate  with 
other  mechanisms  in  maintaining  the  normal  temperature.  The 
physiologic  activity  of  all  epithehal  tissue  depends  on  a  due  supply  of 
nutritive  material  derived  from  the  blood,  which  not  only  maintains 
its  nutrition,  but  affords  those  materials  out  of  which  are  formed  the 
secretions  of  the  glands,  whether  of  the  skin  or  mucous  membrane. 
The  cells  hning  the  blood-vessels,  the  lymph-vessels,  the  peri- 
toneal, pleural,  pericardial,  and  other  closed  cavities  are  usually 
termed  endothelial  cells.  These  cells  are  flat,  irregular  in  shape, 
with  borders  more  or  less  wavv  or  sinuous  in  outline. 


THE  CONNECTIVE  TISSUES.  51 

Functions  of  Epithelial  Tissue. — In  succeeding  chapters  the 
form,  chemic  composition,  and  functions  of  epithelial  cells  will  be 
considered  in  connection  with  the  functions  of  the  organs  of  which 
they  constitute  a  part.  In  this  connection  it  may  be  stated  in  a 
general  way  that  the  functions  of  the  epithehal  tissues  are: 

1 .  To  serve  on  the  surface  of  the  body  as  a  protective  covering  to  the 

underlying  structures  which  collectively  form  the  true  skin,  thus 
protecting  them  from  the  injurious  influences  of  moisture,  air, 
dust,  microorganisms,  etc.,  which  would  otherwise  impair  their 
vitality.  Wherever  continuous  pressure  is  applied  to  the  skin, 
as  on  the  palms  of  the  hands  and  soles  of  the  feet,  the  epithelium 
increases  in  thickness  and  density,  and  thus  prevents  undue 
pressure  on  the  nerves  of  the  true  skin.  The  density  of  the 
epidermis  enables  it  to  resist,  within  limits,  the  injurious  influence 
of  acids,  alkalies,  and  poisons. 

2.  To  promote  absorption.     Inasmuch  as  the  skin  and  mucous  mem- 

branes cover  the  surfaces  of  the  body,  it  is  obvious  that  all 
nutritive  material  entering  the  body  must  first  traverse  the  epi- 
thelial tissue.  Owing  to  their  density,  however,  the  epithelial 
cells  covering  the  skin  play  but  a  feeble  role  as  absorbing  agents 
in  man  and  the  higher  animals.  The  epithelium  of  the  mucous 
membrane  of  the  ahmentary  canal,  particularly  that  of  the  small 
intestine,  is  especially  adapted,  from  its  situation,  consistency, 
and  properties,  to  play  the  chief  role  in  the  absorption  of  new 
materials  into  the  blood.  The  epithelium  Hning  the  air-vesicles 
of  the  lungs  is  engaged  in  promoting  the  absorption  of  oxygen 
and  the  exhalation  of  carbon  dioxid. 

3.  To  form  secretions  and  excretions.     Each  secretory  gland  con- 

nected with  the  surfaces  of  the  body  is  Hned  by  epithelial  cells, 
which  are  actively  concerned  in  the  formation  of  the  secretion 
peculiar  to  the  gland.  Each  excretory  organ  is  similarly  provided 
with  epithelial  cells,  which  are  engaged  either  in  the  production 
of  the  constituents  of  the  excretion  or  in  their  removal  from  the 
blood. 

2.  THE  CONNECTIVE  TISSUES. 

The  connective  tissues,  in' their  collective  capacity,  constitute 
a  framework  which  pervades  the  body  in  all  directions,  and,  as  the 
name  imphes,  serve  as  a  bond  of  connection  between  the  individual 
parts,  at  the  same  time  affording  a  basis  of  support  for  the  muscle, 
nerve,  and  gland  tissues.  The  connective-tissue  group  includes  a 
number  of  varieties,  among  which  may  be  mentioned  the  areolar, 
adipose,  retiform,  white  fibrous,  yellow  elastic,  cartilaginous  and 
osseous.  Notwithstanding  their  apparent  diversity,  they  possess 
many  points  of  similarity.     They  have  a  common  origin,  developing 


52 


TEXT-BOOK  OF  PHYSIOLOGY. 


from  the  same  embryonic  material;  they  have  much  the  same  struc- 
ture, passing  imperceptibly  into  one  another,  and  perform  practically 
the  same  functions. 

Areolar  Tissue. — This  variety  is  found  widely  distributed 
throughout  the  body.  It  serves  to  unite  the  skin  and  mucous  mem- 
brane to  the  structures  on  which  they  rest;  to  form  sheaths  for  the 
support  of  blood-vessels,  nerves,  and  lymphatics;  to  unite  into  com- 
pact masses  the  muscular  tissue  of  the  body,  etc.  Examined  with 
the  naked  eye,  it  presents  the  appearance  of  being  composed  of 
bundles  of  fine  fibers  interlacing  in  every  direction.  In  the  embryonic 
state  the  elements  of  this  form  of  connective  tissue  are  united  by  a 
ground  substance,  gelatinous  in  character.  In  the  adult  state  this 
substance  shrinks  and  largely  disappears,  leaving  intercommunicating, 
spaces  of  varying  size  and  shape,  from  which  the  tissue  takes  its 
name.     When    subjected   to   the   action   of   various   reagents,    and 

examined  microscopically,  the 
bundles  can  be  shown  to  consist 
of  extremely  dehcate,  colorless, 
transparent,  wavy  libers,  which 
are  cemented  together  by  a 
ground     substance     composed 


In 
super- 
posed 
layers 


Fig.  7. — Adipose  Tissue. — {Stdhr.) 


Fig.  8. — Fat-Cells  from  the  Axilla 
OF  Man.  I.  The  equator  of  the 
cell  in  focus.  2.  The  objective 
somewhat  elevated.  3,  4.  Forms 
changed  by  pressure,  p.  Traces 
of  protoplasm  in  the  vicinity  of 
the  flat  nucleus  k. — {Stolir.) 


largely  of  mucin.  Other  fibers  are  also  observed,  which  are  dis- 
tinguished by  a  straight  course,  a  sharp,  well-defined  outhne,  a  ten- 
dency to  branch  and  unite  with  adjoining  fibers,  and  to  curl  up  at 
their  extremities  when  torn.  From  their  color  and  elasticity  they  are 
known  as  yellow  elastic  fibers.  Distributed  throughout  the  meshes  of 
the  areolar  tissue  are  found  flattened,  irregularly  branched,  or  stellate 
corpuscles,  connective -tissue  corpuscles,  plasma  cells,  and  granule  cells. 
Adipose  Tissue. — This  tissue,  which  exists  very  generally 
throughout  the  body,  though  found  most  abundantly  beneath  the 


THE  CONNECTIVE  TISSUES.  53 

skin,  around  the  kidneys,  and  in  the  bones,  is  practically  but  a 
modification  of  areolar  tissue.  In  these  situations  it  presents  itself 
in  small  masses  or  lobules  of  varying  size  and  shape,  surrounded 
and  penetrated  by  the  fibers  of  connective  tissue.  (See  Fig.  7.) 
Microscopic  examination  shows  that  these  masses  consist  of  small 
vesicles  or  cells,  round,  elliptical,  or  polyhedral  in  shape,  depending 
somewhat  on  pressure.  (See  Fig.  8.)  Each  vesicle  consists  ojf  a  thin, 
colorless,  protoplasmic  membrane,  thickened  at  one  point,  in  which  a 
nucleus  can  usually  be  detected.  This  membrane  incloses  a  globule  of 
fat,  which  during  life  is  in  the  liquid  state.  It  is  composed  of  olein, 
stearin,  and  palmitin.  The  origin  of  the  fat  is>  to  be  referred  to  a 
retrograde  change  in  the  protoplasmic  material  of  the  connective-tissue 
cells.  When  this  protoplasm  becomes  rich  in  carbon  and  hydrogen, 
it  is  speedily  converted  into  fat,  which  makes  its  appearance  in  the 
form  of  minute  drops  in  different  por- 
tions of  the  cell.  As  the  drops  accu- 
mulate, at  the  expense  of  the  cell 
protoplasm,  they  gradually  coalesce, 
until  there  remains  but  a  thin  stratum  / 

of  the  protoplasm,  which  forms  the 
wall  of  the  vesicle.  Adipose  tissue 
may,  therefore,  be  regarded  as  areolar  ,% 

tissue,  in  which,  and  at  the  expense  of 

some  of  its  elements,  fat  is  stored  for  fj 

the  future  needs  of  the  organism.     A  . :  ^ 

diminution  of  food,  especially  of  fat  _ 

and   carbohydrates,    is   promptly   fol-  ""^i^ 

lowed  by  an  absorption  of  fat  by  fig.  9.  — Connective -Tissue 
the  blood-vessels  and  by  its  transfer-  Bundles  of  Various  Thick- 

enre  to  the  tissues    where   it    is  either  nesses  of  the  Intermuscu- 

ence  to  tne  tissues,  wneie  it  is  eitner  ^^  connective  Tissue  of 

utilized  for  tissue  construction  or  for  man.    X  240.— (Stohr.) 

oxidation  purposes.     In  the  situations 

in  which  adipose  tissue  is  found  it  seems,  by  its  chemic  and 
physical  properties,  to  assist  in  the  prevention  of  a  too  rapid  radia- 
tion of  heat  from  the  body,  to  give  form  and  roundness,  and  to 
diminish  angularities,  etc. 

Retiform  and  adenoid  tissue  are  also  modifications  of  areolar 
tissue.  The  meshes  of  the  former  contain  but  Httle  ground  sub- 
stance, its  place  being  taken  by  fluids;  the  meshes  of  the  latter 
contain  large  numbers  of  lymph  corpuscles. 

Fibrous  Tissue. — This  variety  of  connective  tissue  is  widely 
distributed  throughout  the  body.  It  constitutes  almost  entirely  the 
ligaments  around  the  joints,  the  tendons  of  the  muscles,  the  mem- 
branes covering  organs  such  as  the  heart,  liver,  nervous  system, 
bones,  etc.     All  fibrous  tissue,  wherever  found,  can  be  resolved  into 


54 


TEXT-BOOK  OF  PHYSIOLOGY. 


elementary  bundles,  Avhich  on  microscopic  examination  are  seen  to 
consist  of  delicate,  wavy,  transparent,  homogeneous  fibers,  which 
pursue  an  independent  course,  neither  branching  nor  uniting  with 
adjoining  fibers.  (See  Fig.  9.)  A  small  amount  of  ground  substance 
serves  to  hold  them  together.  Fibrous  tissue  is  tough  and  inexten- 
sible,  and  in  consequence  is  admirably  adapted  to  fulfil  various 
mechanical  functions  in  the  body.  It  is,  however,  quite  pliant,  bend- 
ing easily  in  all  directions.  When  boiled,  fibrous  tissue  yields  gelatin, 
a  derivative  of  collagen. 

Elastic  Tissue. — -The  fibers  of  elastic  tissue  are  usually  associated 
in  varying  proportions  with  the  white  fibrous  tissue;  but  in  some 
structures — as  the  ligamentum  nuchae,  the  ligamenta  subflava,  the 


Fig.  10. — Elastic  Fibers.  X  560.  A.  Fine  elastic  fibers,  /,  from  intermuscular 
connective  tissue  of  man;  b,  connective-tissue  bundles  swelled  by  treatment  with 
acetic  acid.  B.  Very  thick  elastic  fibers,  /,  from  ligamentum  nuchas  of  ox;  b, 
connective-tissue  bundles.  C.  From  a  cross-section  of  the  ligamentum  nuchae  of 
ox;  /,  elastic  fibers;  b,  connective-tissue  bundles. — (Stohr.) 


middle  coat  of  the  larger  blood-vessels — the  elastic  fibers  are  almost 
the  only  elements  present,  and  give  to  these  structures  a  distinctly 
yellow  appearance.  The  fibers  throughout  their  course  give  off 
many  branches,  which  unite  with  adjoining  branches  to  form  a  more 
or  less  close  network.  As  the  name  implies,  these  fibers  are  highly 
elastic,  and  are  capable  of  being  extended  as  much  as  60  per  cent, 
before  breaking.     (See  Fig.  10.) 

Cartilaginous  Tissue. — This  form  of  connective  tissue  differs 
from  the  preceding  varieties  chiefly  in  its  density.  As  a  rule,  it  is 
firm  in  consistency,  though  somewhat  elastic.  It  is  opaque,  bluish- 
white  in  color,  though  in  thin  sections  translucent.     All  cartilaginous 


THE  CONNECTIVE  TISSUES.  55 

tissues  consist  of  connective-tissue  cells  embedded  in  a  solid  ground 

substance.     According  to  the  amount  and  texture  of   the  ground 

substance,  three  principal  varieties  may  be  distinguished: 

I.  Hyaline  cartilage,  in  which  the  cells,  relatively  few  in  number,  are 

embedded  in  an  abundant  quantity  of  ground  substance  (Fig.  1 1). 

The  body  of  the  cells  is  in  many  instances  distinctly  marked  off 

from  the  surrounding  substance  by  concentric  hues  or  fibers, 

which  form  a  capsule  for  the  cell.     Repeated  division  of  the  cell 

substance  takes  place,  until  the  whole  capsule  is  completely 

occupied  by  daughter  cells.     The  ground  substance  is  pervaded 

by  minute  channels,  which  communicate  on  one  hand  with  the 


li 

\ 


Fig.  II. — Hyaline  C.-^rtilage.  X  240.  A.  Surface  view  of  the  ensiform  process 
of  frog,  fresh;  p,  protoplasm  of  cartilage-cell,  which  entirely  tills  the  lacuna;  k, 
nucleus;  g,  hyaline  matrix.  B.  Portion  of  cross-section  of  human  rib-cartilage 
several  days  after  death;  e.xamined  in  water:  the  protoplasm,  s,  of  the  cartilage- 
cells  has  withdrawn  from  the  walls  of  the  lacunae,  h;  the  nuclei  are  invisible. 
I.  Two  cells  within  one  capsule,  k;  x,  a  developing  partition.  2.  Five  cartilage- 
cells  within  one  capsule;  the  lowest  cell  has  fallen  out,  and  here  only  the  empty 
space  is  seen.  3.  Capsule  cut  obliquely,  and  apparently  thicker  on  one  side. 
4.  Capsule  not  cut,  but  showing  the  cell  within,  g.  Hyaline  matri.x  transformed 
into  rigid  fibers,  /. — (Slohr.) 

spaces  around  the  cells,  and  on  the  other  with  lymph-spaces  in 
the  connective  tissue  surrounding  the  cartilage.  By  means  of 
these  channels,  nutritive  fluid  can  permeate  the  entire  structure. 
Hyahne  cartilage  is  found  on  the  ends  of  the  long  bones,  where 
it  enters  into  the  formation  of  the  joints;  between  the  ribs  and 
sternum,  forming  the  costal  cartilage,  as  well  as  in  the  nose  and 
larynx. 
2.  White  fibro-cartilage,  the  ground  substance  of  which  is  pervaded 
by  white  fibers,  arranged  in  bundles  or  layers,  between  which 


56 


TEXT-BOOK  OF  PHYSIOLOGY 


Fig, 


are  scattered  the  usual  encapsulated  cells.    (See  Fig.  12.)   White 
fibro-cartilage  is  tough,  resistant,  but  flexible,  and  is  found  in 

joints  where  strength  and  fixcd- 
^  ness  are  required.     Hence  it  is 

\    "     f;  present  between  the  vertebrae, 

fy  forming   the   intervertebral 

V  discs,  between  the  condyle  of 

the  lower  jaw  and  the  glenoid 
fossa,  in  the  knee-joint,  around 
the  margins  of  the  joint  cavities, 
etc.     In  these  situations  it  as- 
sists  in    maintaining    the    ap- 
position of  the  bones,  in  giving 
a  certain  degree  of  mobility  to 
the  joints,  and  in  diminishing 
the  effects  of  shock  and  pres- 
sure imparted  to  the  bones. 
,   Yellow  fibro-cartilage,  the  ground 
substance  of  which  is  pervaded 
by  opaque,  yellow  elastic  fibers, 
which  form,  by  the  interlacing 
of  their  branches,  a  compUcated 
network,  in  the  meshes  of  which  are  to  be  found  the  usual  cor- 
puscles.    (See    Fig.    13.)     As    these   fibers   are    elastic,   they 
impart  to  the  cartilage  a  very  considerable  degree  of  elasticity. 


12. — From  a  Horizontal  Sec- 
tion OF  THE  Intervertebral 
Disc  of  Man.  g.  Fibrillar  con- 
nective tissue.  2.  Cartilage-cell 
(nucleus  invisible),  k.  Capsule 
surrounded  by  calcareous  gran- 
ules.    X  240. — (Stohr.) 


f^S^ 


•■^SsiiSE'^ 


^it-^^mff 


Fig.  13. — Elastic  Cartilage.  X  240.  i.  Portion  of  section  of  vocal  process  (ante- 
rior angle)  of  arytenoid  cartilage  of  a  woman  thirty  years  old;  the  elastic  substance 
in  the  form  of  granules.  2  and  3.  Portions  of  sections  of  epiglottis  of  a  woman 
sixty  years  old;  a  fine  network  of  elastic  fibers  in  2,  a  coarser  network  in  3.  z. 
Cartilage-cell,  nucleus  not  visible;  k,  capsule. — [Stohr.) 

Yellow  fibro-cartilage  is  well  adapted,  therefore,  for  entering 
into  the  formation  of  the  external  ear,  epiglottis.  Eustachian 
tube,  etc. — structures  which  require  for  their  functional  activity 
a  certain  degree  of  flexibility  and  elasticity. 


THE  CONNECTIVE  TISSUES.  57 

Osseous  Tissue. — Osseous  tissue,  as  distinguished  from  bone, 
is  a  member  of  the  connective-tissue  group,  the  ground  substance  of 
which  is  permeated  with  insoluble  lime  salts,  of  which  the  phosphate 
and  carbonate  are  the  most  abundant.  Immersed  in  dilute  solutions 
of  hydrochloric  acid,  they  can  be  converted  into  soluble  salts  and  dis- 
solved out.  The  osseous  matrix  left  behind  is  soft  and  phable. 
When  boiled,  it  yields  gelatin. 

A  thin,  transverse  section  of  a  decalcified  bone,  when  examined 
microscopically,  reveals  a  number  of  small,  round,  or  oval  openings, 
which  represent  transverse  sections  of  canals  which  run  through  the 
bone,  for  the  most  part  in  a  longitudinal  direction,  though  frequently 
anastomosing  with  one  another.  These  so-called  Haversian  canals 
in  the  living  state  contain  blood-vessels  and  lymphatics.  (See 
Fig.  14.) 

Periosteum. 
Outer  ground  lamellae. 
"^5;^.  ^-f"^ Haversian  canals. 


Haversian  lamellae. 


I  ,,    Interstitial  lamellie. 

Y^^^  Inner  ground  laraelUe. 

■  ,  '  Marrow. 


Fig.  14.— From  a  Cross-section  of  a  Metacarp  of  Man.     X  50.     The  Haversian 
canals  contain  a  little  marrow  (fat-cells).     Resorption  line  at  h. — {Slohr.) 

Around  each  Haversian  canal  is  a  series  of  concentric  laminae, 
composed  of  white  fibers.  Between  every  two  laminae  are  found  small 
cavities  (lacunse),  from  which  radiate  in  all  directions  small  canals 
(canaliculi),  which  communicate  freely  with  one  another.  The 
Haversian  canals,  with  their  associated  lacunas  and  canalicuh, 
form  a  system  of  intercommunicating  passages,  which  circulate 
lymph  destined  for  the  nourishment  of  bone.  Each  lacuna 
contains  the  bone  corpuscle,  which  bears  a  close  resemblance  to  the 
usual  branched  connective-tissue  corpuscle,  and  whose  function 
appears  to  be  the  maintenance  of  the  nutrition  of  the  bone. 

The  surface  of  every  bone  in  the  living  state  is  invested  with  a 
fibrous  membrane,  the  periosteum,  except  where  it  is  covered  with 
cartilage.  The  inner  surface  of  this  membrane  is  loose  in  texture, 
and  supports  a  fine  plexus  of  capillary  blood-vessels  and  numerous 
protoplasmic  cells — the  osteoblasts.     As  this  layer  is  directly  con- 


58  TEXT-BOOK  OF  PHYSIOLOGY. 

cerned  in  the  formation  of  bone,  it  is  spoken  of  as  the  osteogenetic 
layer. 

A  section  of  a  bone  shows  that  it  is  composed  of  two  kinds  of 
tissue — compact  and  cancellated.  The  compact  is  dense,  resembling 
ivory,  and  is  found  on  the  outer  portion  of  the  bone;  the  cancellated 
is  spongy,  and  appears  to  be  made  up  of  thin,  bony  plates,  which 
intersect  one  another  in  all  directions,  and  is  found  in  greatest  abun- 
dance in  the  interior  of  the  bones.  The  shaft  of  a  long  bone  is  hollow. 
This  central  cavity,  which  extends  from  one  end  of  the  bone  to  the 
other,  as  well  as  the  interstices  of  the  cancellated  tissue,  is  filled  in  the 
hving  state  with  marrow.  The  marrow  or  medulla  is  composed  of  a 
connective-tissue  framework  supporting  blood-vessels.  In  its  meshes 
are  to  be  found  characteristic  bone  ceils  or  osteoblasts,  the  function 
of  which  is  supposed  to  be  the  formation  of  bone.  In  the  long  bones 
the  marrow  is  yellow,  from  the  presence  in  the  connective-tissue 
corpuscle  of  fat  globules,  which  arise  through  the  transformation  of 
the  cell  protoplasm.  In  the  cancellated  tissue,  near  the  extremities 
of  the  long  bones,  this  fatty  transformation  does  not  take  place  to  the 
same  extent,  and  the  marrow  appears  red.  The  cells  of  the  red 
marrow  are  beheved  to  give  birth  indirectly  to  the  red  blood-cor- 
puscles. 

Physical  and  Physiologic  Properties  of  Connective  Tissues. 
— Among  the  physical  properties  may  be  mentioned  consistency, 
cohesion,  and  elasticity.  Their  consistency  varies  from  the  semi- 
liquid  to  the  solid  state,  and  depends  on  the  quantity  of  water  which 
enters  into  their  composition.  Their  cohesion,  except  in  the  softer 
varieties,  is  very  considerable,  and  offers  great  resistance  to  traction, 
pressure,  torsion,  etc.  In  all  the  movements  of  the  body,  in  the  con- 
traction of  muscles,  in  th&  performance  of  work,  the  consistence  and 
cohesion  of  these  tissues  play  most  important  roles.  Wherever  the 
various  forms  of  connective  tissue  are  found,  their  chemic  com- 
position and  structure  are  in  relation  to  their  functions.  If  traction 
be  the  preponderating  force,  the  structure  becomes  fibrous,  as  in 
ligaments  and  tendons,  and  the  cohesion  greatest  in  the  longitudinal 
direction.  If  pressure  be  exerted  in  all  directions,  as  upon  mem- 
branes, the  fibers  interlace  and  offer  a  uniform  resistance.  When 
pressure  is  exerted  in  a  definite  direction,  as  on  the  extremities  of  the 
long  bones,  the  tissue  becomes  expanded  and  cancellated.  The 
lamellae  of  the  cancellated  tissue  arrange  themselves  in  curves  which 
correspond  to  the  direction  of  the  greatest  pressure  or  traction.  Ex- 
tensibihty  is  not  a  characteristic  feature,  except  in  those  forms  con- 
taining an  abundance  of  yellow  elastic  fibers.  The  elasticity  is  an 
essential  factor  in  many  physiologic  actions.  It  not  only  opposes  and 
Hmits  forces  of  traction,  pressure,  torsion,  etc.,  but  on  their  cessation 
returns  the  tissues  or  organs  to  their  original  condition.     Elasticity 


THE  CONNECTIVE  TISSUES.  59 

thus  assists  in  maintaining  the  natural  form  and  position  of  the  organs 
by  counterbalancing  and  opposing  temporarily  acting  forces. 

The  Skeleton. — The  connective  tissues  in  their  entirety  con- 
stitute a  framework  which  presents  itself  under  two  aspects:  (i) 
As  a  soHd,  bony  skeleton,  situated  in  the  trunk  and  hmbs,  affording 
attachment  for  muscles  and  viscera;  (2)  as  a  line,  fibrous  skeleton, 
found  everywhere  throughout  the  body,  connecting  the  various 
viscera  and  affording  support  for  the  epithehal  muscle,  and  nerve 
tissues. 


THE  ANIMAL  BODY  AS  A  MACHINE  FOR  DOING 

WORK. 

The  animal  body  is  characterized  by  the  power  of  executing  a 
great  variety  of  movements,  all  of  which  have  reference  to  a  change 
of  relation  of  one  part  of  the  body  to  another,  or  to  a  change  of  posi- 
tion relatively  to  the  environment,  as  in  the  various  acts  of  locomotion. 
Since  in  the  execution  of  these  movements  the  different  parts  are 
of  necessity  applied  or  directed  to  the  overcoming  of  opposing 
forces  in  the  environment,  the  animal  is  said  to  be  doing  work.  In 
the  conception  of  the  animal  body  as  a  machine  for  the  accomphsh- 
ment  of  work  the  skeleton,  the  muscle  and  nerve  tissues  constitute  the 
three  primary  mechanisms,  all  of  which  bear  certain  definite  relations 
one  to  another. 


CHAPTER  V. 
THE  PHYSIOLOGY  OF  THE  SKELETON. 

The  Skeleton  is  the  passive  framework  of  the  body,  the  axial 
portion  of  which  (the  vertebral  column,  head,  ribs,  and  sternum) 
imparts  more  or  less  fixity  and  rigidity,  while  the  appendicular  por- 
tions (the  bones  of  the  arms  and  legs)  impart  extreme  mobility.  The 
bones  of  the  arms  and  legs  more  especially  may  be  looked  upon  as 
constituting  a  system  of  levers,  the  fulcra  of  which,  the  points  of 
rest  around  which  they  move,  lie  in  the  joints. 

That  a  lever  may  be  effective  as  an  instrument  for  the  accom- 
pHshment  of  work,  it  must  not  only  be  capable  of  moving  around  its 
fulcrum,  but  it  must  at  the  same  time  be  acted  on  by  two  opposing 
forces,  one  passive,  the  other  active.  In  the  movement  of  the  bony 
levers  of  the  animal  body,  the  passive  forces  are  largely  those  con- 
nected with  the  environment,  e.  g.,  gravity,  cohesion,  friction,  elas- 
ticity, etc.  The  active  forces  by  which  these  latter  are  opposed  and 
overcome  through  the  intermediation  of  the  bony  levers  are  found  in 
the  muscles  attached  to  them.  For  the  execution  of  all  these  move- 
ments, it  is  essential  that  the  relation  of  the  various  portions  of  the 
bony  skeleton  to  one  another  shall  be  such  as  to  permit  of  movement 
while  yet  retaining  close  apposition.  This  is  accomplished  by  the 
mechanical  conditions  which  have  been  evolved  at  the  points  of  union 
of  bones,  and  which  are  technically  known  as  articulations  or  joints. 

A  consideration  of  the  body  movements  involves  an  account  of 
(i)  the  static  conditions,  or  those  states  of  equilibrium  in  which  the 
body  is  at  rest — e.  g.,  standing,  sitting;  (2)  the  dynamic  conditions, 
or  those  states  of  activity  characterized  by  movement — e.  g.,  walking, 
running,  etc.  In  this  connection,  however,  only  those  physical  and 
physiologic  pecuharities  of  the  skeleton,  especially  in  its  relation  to 
joints,  will  be  referred  to,  which  underhe  and  determine  both  the 
static  and  dynamic  states  of  the  body. 

Structure  of  Joints. — The  structures  entering  into  the  formation 
of  joints  are : 

I.  Bones,  the  articulating  surfaces  of  which  are  often  more  or  less 
expanded,  especially  in  the  case  of  long  bones,  and  at  the  same 
time  variously  modified  and  adapted  to  one  another  in  accordance 
with  the  character  and  extent  of  the  movements  which  there 
take  place. 

60 


THE  PHYSIOLOGY  OF  THE  SKELETON.  6i 

2.  Hyaline  cartilage,  which  is  closely  appHed  to  the  articulating  end 

of  each  bone.  The  smoothness  of  this  form  of  cartilage  facih- 
tates  the  movements  of  the  opposing  surfaces,  while  its  elasticity 
diminishes  the  force  of  shocks  and  jars  imparted  to  the  bones 
during  various  muscular  acts.  In  a  number  of  joints,  plates 
or  discs  of  white  fibro-cartilage  are  inserted  between  the  surfaces 
of  the  bones. 

3.  A  synovial  membrane,  which  is  attached  to  the  edge  of  the  hyaline 

cartilage,  entirely  inclosing  the  cavity  of  the  joint.  This  mem- 
brane is  composed  largely  of  connective  tissue,  the  inner  surface 
of  which  is  lined  by  endotheUal  cells,  which  secrete  a  clear, 
colorless,  viscid  fluid — the  synovia.  This  fluid  not  only  fills  up 
the  joint-cavity,  but,  flowing  over  the  articulating  surfaces, 
diminishes  or  prevents  friction. 

4.  Ligaments, — tough,  inelastic  bands,  composed  of  white  fibrous 

tissue, — which  pass  from  bone  to  bone  in  various  directions  on 
the  dift'erent  aspects  of  the  joint.  As  white  fibrous  tissue  is  in- 
extensible  but  phant,  hgaments  assist  in  keeping  the  bones  in 
apposition,  and  prevent  displacement  while  yet  permitting  of 
free  and  easy  movements. 
Classification  of  Joints. — All  joints  may  be  divided,  according 

to  the  extent  and  kind  of  movements  permitted  by  them,  into  (i) 

diarthroses;  (2)  amphiarthroses;  (3)  synarthroses. 

I.  Diarthroses. — In  this  division  of  the  joints  are  included  all  those 
which  permit  of  free  movement.  In  the  majority  of  instances 
the  articulating  surfaces  are  mutually  adapted  to  each  other. 
If  the  articulating  surface  of  one  bone  is  convex,  the  opposing 
but  corresponding  surface  is  concave.  Each  surface,  therefore, 
represents  a  section  of  a  sphere  or  a  cylinder,  which  latter  arises 
by  rotation  of  a  line  around  an  axis  in  space.  According  to  the 
number  of  axes  around  which  the  movements  take  place  all 
diarthrodial  joints  may  be  divided  into: 

I.  Uniaxial  Joints. — In  this  group  the  convex  articulating  surface  is 
a  segment  of  a  cylinder  or  cone,  to  which  the  opposing  surface 
more  or  less  completely  corresponds.  In  such  a  joint  the  single 
axis  of  rotation,  though  nearly,  is  not  exactly  at  right  angles 
to  the  long  axis  of  the  bone,  and  hence  the  movements — flexion 
and  extension — which  take  place  are  not  confined  to  one  plane. 
Joints  of  this  character — e.  g.,  the  elbow,  knee,  ankle,  the  pha- 
langeal joints  of  the  fingers  and  toes — are,  therefore,  termed 
ginglymi,  or  hinge-joints.  Owing  to  the  obliquity  of  their 
articulating  surfaces,  the  elbow  and  ankle  are  cochleoid  or  screw- 
ginglymi.  Inasmuch  as  the  axes  of  these  joints  on  the  opposite 
sides  of  the  body  are  not  coincident,  the  right  elbow  and  left 
ankle  are  right-handed  screws;  the  left  elbow  and  right  ankle, 


62  TEXT-BOOK  OF  PHYSIOLOGY. 

left-handed  screws.  In  the  knee-joint  the  form  and  arrangement 
of  the  articulating  surfaces  are  siich  as  to  produce  that  modifica- 
tion of  a  simple  hinge  known  as  a  spiral  hinge,  or  helicoid.  As 
the  articulating  surfaces  of  the  condyles  of  the  femur  increase  in 
convexity  from  before  backward,  and  as  the  inner  condyle  is 
longer  than  the  outer,  and,  therefore,  represents  a  spiral  surface, 
the  line  of  translation  or  the  movement  of  the  leg  is  also  a  spiral 
movement.  During  flexion  of  the  leg  there  is  a  simultaneous 
inward  rotation  around  a  vertical  axis  passing  through  the  outer 
condyle  of  the  femur;  during  extension  a  reverse  movement  takes 
place.  Moreover,  the  slightly  concave  articulating  surfaces  of 
the  tibia  do  not  revolve  around  a  single  fixed  transverse  axis,  as 
in  the  elbow- joint,  for  during  flexion  they  shde  backward,  during 
extension  forward,  around  a  shifting  axis,  which  varies  in  posi- 
tion with  the  point  of  contact. 

In  some  few  instances  the  axis  of  rotation  of  the  articulating 
surface  is  parallel  with  rather  than  transverse  to  the  long  axis  of 
the  bone,  and  as  the  movement  then  takes  place  around  a  more 
or  less  conic  surface,  the  joint  is  termed  a  trochoid  or  pulley— e.  g., 
the  odonto-atlantal  and  the  radio-ulnar.  In  the  former  the  collar 
formed  by  the  atlas  and  its  transverse  ligament  rotates  around 
the  vertical  odontoid  process  of  the  axis.  In  the  latter  the  head 
of  the  radius  revolves  around  its  own  long  axis  upon  the  ulna, 
giving  rise  to  the  movements  of  pronation  and  supination  of  the 
hand.  The  axis  around  which  these  two  movements  take  place 
is  continued  through  the  head  of  the  radius  to  the  styloid  process 
of  the  ulna. 

2.  Biaxial  Joints. — In  this  group  .the  articulating  surfaces  are  un- 

equally curved,  though  intersecting  each  other.  When  the  sur- 
faces lie  in  the  same  direction,  the  joint  is  termed  an  ovoid  joint 
— e.  g.,  the  radio-carpal  and  the  atlanto-occipital.  As  the  axes 
of  these  surfaces  are  vertical  to  each  other,  the  movements  per- 
mitted by  the  former  joint  are  flexion,  extension,  adduction,  and 
abduction,  combined  with  a  slight  amount  of  circumduction; 
the  latter  joint  permits  of  flexion  and  extension  of  the  head,  with 
inclination  to  either  side.  When  the  surfaces  do  not  take  the 
same  direction,  the  joint,  from  its  resemblance  to  the  surfaces  of 
a  saddle,  is  termed  a  saddle-joint — e.  g.,  the  trapezio-metacarpal. 
The  movements  permitted  by  this  joint  are  also  flexion,  exten- 
sion, adduction,  abduction,  and  circumduction. 

3.  Polyaxial  Joints. — In  this  group  the  convex  articulating  surface 

is  a  segment  of  a  sphere,  which  is  received  by  a  socket  formed 
by  the  opposing  articulating  surface.  In  such  a  joint,  termed  an 
enarthrodial  or  ball-and-socket  joint, — e.  g.,  the  shoulder-joint, 
hip-joint, — the  distal  bone  revolves  around  an  indefinite  number 


THE  PHYSIOLOGY  OF  THE  SKELETON.  63 

of  axes,  all  of  which  intersect  one  another  at  the  center  of  rotation. 
For  simplicity,  however,  the  movement  may  be  described  as 
taking  place  around  axes  in  the  three  ordinal  planes — viz.,  a 
transverse,  a  sagittal,  and  a  vertical  axis.  The  movements  around 
the  transverse  axis  are  termed  flexion  and  extension;  around  the 
sagittal  axis,  adduction  and  abduction;  around  the  vertical 
axis,  rotation.  When  the  bone  revolves  around  the  surface  of  an 
imaginary  cone,  the  apex  of  which  is  the  center  of  rotation  and 
the  base  the  curve  described  by  the  hand,  the  movement  is 
termed  circumduction. 

2.  Amphiarthroses. — In  this  division  are  included  all  those  joints 

which  permit  of  but  slight  movement — e.  g.,  the  intervertebral, 
the  interpubic,  and  the  sacro-iHac  joints.  The  surfaces  of  the 
opposing  bones  are  united  and  held  in  position  largely  by  the 
intervention  of  a  firm,  elastic  disc  of  fibro-cartilage.  Each  joint 
is  also  strengthened  by  hgaments. 

3.  Synarthroses. — In  this  division  are  included  all  those  joints  in 

which  the  opposing  surfaces  of  the  bones  are  immovably  united, 
and  hence  do  not  permit  of  any  movement — e.  g.,  the  joints 
between  the  bones  of  the  skull. 

The  Vertebral  Column. — In  all  static  and  dynamic  states  of  the 
body  the  vertebral  column  plays  a  most  essential  role.  Situated  in 
the  middle  of  the  back  of  the  trunk,  it  forms  the  foundation  of  the 
entire  skeleton.  It  is  composed  of  a  series  of  superimposed  bones, 
termed  vertebrae,  which  increase  in  size  from  above  downward  as 
far  as  the  brim  of  the  pelvic  cavity.  Superiorly,  it  supports  the  skull; 
laterally,  it  affords  attachment  for  the  ribs,  which  in  turn  support  the 
weight  of  the  upper  extremities;  below,  it  rests  upon  the  pelvic  bones, 
which  transmit  the  weight  of  the  body  to  the  inferior  extremities. 
The  bodies  of  the  vertebras  are  united  one  to  another  by  tough  elastic 
discs  of  fibro-cartilage,  which,  collectively,  constitute  about  one- 
quarter  of  the  length  of  the  vertebral  column.  The  vertebras  are  held 
together  by  Hgaments  situated  on  the  anterior  and  posterior  surfaces 
of  their  bodies,  and  by  short,  elastic  ligaments  between  the  neural 
arches  and  processes.  These  structures  combine  to  render  the 
vertebral  column  elastic  and  flexible,  and  enable  it  to  resist  and 
diminish  the  force  of  shocks  communicated  to  it. 

The  amphiarthrodial  character  of  the  intervertebral  joints  endows 
the  entire  column  with  certain  forms  of  movement  which  are  neces- 
sary to  the  performance  of  many  body  activities.  While  the  range 
of  movement  between  any  two  vertebree  is  shght,  the  sum  total  of 
movement  of  the  entire  series  of  vertebras  is  considerable.  In  dift'erent 
regions  of  the  column  the  character,  as  well  as  the  range  of  move- 
ment, varies  in  accordance  with  the  form  of  the  vertebrae  and  the 
inclination  of  their  articular  processes.     In  the  cervical  and  lumbar 


64  TEXT-BOOK  OF  PHYSIOLOGY. 

regions  extension  and  flexion  are  freely  permitted,  though  the  former 
is  greater  in  the  cervical,  the  latter  in  the  lumbar  region,  especially 
between  the  fourth  and  fifth  vertebrae.  Lateral  flexion  takes  place 
in  all  portions  of  the  column,  but  is  particularly  marked  in  the  cer- 
vical region.  A  rotatory  movement  of  the  column  as  a  whole  takes 
place  through  an  angle  of  about  twenty-eight  degrees.  This  is  most 
evident  in  the  lower  cervical  and  dorsal  regions. 

The  skeleton  may,  therefore,  be  regarded  as  a  highly  developed 
framework,  which  determines  not  only  the  form  of  the  body,  and 
affords  support  and  protection  to  the  various  softer  organs  and 
tissues,  but  also,  through  the  mobility  of  its  joints,  permits  of  a  great 
variety  of  compHcated  movements. 


CHAPTER  VI. 
GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 

The  muscle-tissue,  which  closely  invests  the  bones  of  the  body 
and  which  is  famihar  to  all  as  the  flesh  of  animals,  is  the  immediate 
cause  of  the  active  movements  of  the  body.  This  tissue  is  grouped 
in  masses  of  varying  size  and  shape,  which  are  technically  known  as 
muscles.  The  majority  of  the  muscles  of  the  body  are  connected 
with  the  bones  of  the  skeleton  in  such  a  manner  that,  by  an  alteration 
in  their  form,  they  can  change  not  only  the  position  of  the  bones  with 
reference  to  one  another,  but  can  also  change  the  individual's  relation 
to  surrounding  objects.  They  are,  therefore,  the  active  organs  of 
both  motion  and  locomotion,  in  contradistinction  to  the  bones  and 
joints,  which  are  but  passive  agents  in  the  performance  of  the  corre- 
sponding movements.  In  addition  to  the  muscle  masses  which  are 
attached  to  the  skeleton,  there  are  also  other  collections  of  muscle- 
tissue  surrounding  cavities  such  as  the  stomach,  intestine,  blood- 
vessels, etc.,  which  impart  to  their  walls  motihty,  and  so  influence  the 
passage  of  material  through  them. 

Muscles  produce  movement  of  the  structures  to  which  they  are 
attached  by  the  property  with  which  they  are  endowed  of  changing 
their  shape,  shortening  or  contracting  under  the  influence  of  a  stim- 
ulus transmitted  to  them  from  the  nervous  system.  Muscles  are 
divided  into: 

1.  Voluntary  muscles,   comprising  those   the  activity  of  which   is 

called  forth  by  an  act  or  effort  of  volition. 

2.  Involuntary  muscles,  comprising  those  the  activity  of  which  is 

entirely  independent  of  the  voHtion. 
The  voluntary  muscles  are  also  known  from  their  attachment  to 
the  skeleton  as  skeletal,  and  from  their  microscopic  appearance  as 
striped  or  striated  muscles.  Though  for  the  most  part  these  muscles 
are  red,  there  are  certain  muscles  in  man  and  other  animals  which 
are  pale  in  color.  The  involuntary  muscles,  from  their  relation  to  the 
viscera  of  the  body,  are  known  also  as  visceral,  and  from  their  micro- 
scopic appearance  as  plain,  smooth,  or  non-striated  muscles. 

THE  VOLUNTARY  OR  SKELETAL  MUSCLE. 

All  skeletal  muscles  consist  of  a  central  fleshy  portion,  the  body 
or  belly,  provided  at  either  extremity  with  a  tendon  in  the  form  of  a 
5  65 


66 


TEXT-BOOK  OF  PHYSIOLOGY. 


cord  or  membrane.  The  body  is  the  active,  contractile  region,  the 
source  of  the  movement;  the  tendon  is  the  inactive  region,  the  passive 
transmitter  of  the  movement  to  the  bones. 

A  skeletal  muscle  is  a  complex  organ  consisting  of  a  framework  of 
connective  tissue,  supporting  muscle-fibers,  blood-vessels,  nerves,  and 
lymphatics.  The  general  body  of  the  muscle  is  covered  by  a  dense 
layer  of  connective  tissue,  the  epi-mysium,  which  blends  with  and 
partly  forms  the  tendon.  From  the  under  surface  of  this  covering, 
septa  of   connective  tissue  pass  inward,  dividing  and  grouping  the 

fibers  into  larger  and  smaller 
^s\  bundles,  termed  fasciculi.  The 

fasciculi,  invested  by  a  special 
sheath,  the  peri-mysium,  are 
prismatic  in  shape  and  on 
cross-section  present  an  ir- 
regular outline.  The  muscle- 
fibers  composing  the  fasciculi 
are  separated  one  from  an- 
other and  supported  by  a  very 
delicate  connective  tissue,  the 
endo-mysium.  The  connec- 
tive tissue  thus  surrounding 
and  penetrating  the  muscle 
binds  the  fibers  into  a  dis- 
tinct organ  and  affords  sup- 
port to  all  remaining  struc- 
tures (Fig.  15). 

Histology  of  the  Skeletal 
Muscle-fiber. — The  muscle- 
fibers  for  the  most  part  are 
arranged  parallel  one  to  an- 
other and  in  a  direction  cor- 
responding to  the  long  axis  of 
the  muscle.  They  vary  in 
length  from  30  to  40  milli- 
meters and  in  breadth  from 
20  to  30  micromilhmeters.  There  are  exceptional  fibers,  however, 
which  have  a  much  greater  length.  As  the  fibers  have  but  a  hmited 
length  in  the  vast  majority  of  muscles,  each  end,  more  or  less  pointed 
or  beveled,  is  united  to  adjoining  fibers  by  cement.  In  this  way  a 
muscle  is  increased  in  length. 

When  examined  with  the  microscope,  the  muscle-fiber  is  seen  to  be 
cyhndric  or  prismatic  in  shape  and  to  consist  of  a  thin  transparent 
membrane,  the  sarcolemma,  in  which  is  contained  the  true  muscle 
or  sarcous  substance.     The  sarcolemma  is  elastic  and  adapts  itself 


Fig.  15. — From  a  Cross-section  of  the 
Adductor  Muscle  of  a  Rabbit.  P. 
Peri-mysiura,  containing  two  blood-ves- 
sels, at  g;  m,  muscle-fibers;  many  are 
shrunken  and  between  them  the  endo- 
mysium,  p,  can  be  seen;  at  x  the  sec- 
tion of  muscle-fiber  has  fallen  out. 
X  60.— (Stohr.) 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


67 


3 


i',. 


Fig.  16.- 

a.    iJarK     uanu. 


Jh 


OF  A  Rabbit. 
Light    band. 


c.  Intermediate  line.     n.  Nucleus. 
— Landois  and  Stirline. 


to  all  changes  of  form  the  sarcous  substance  undergoes.  Beneath 
the  sarcolemma  there  are  several  nuclei  surrounded  by  granular 
material.  Each  fiber  also  presents  a  series  of  transverse  bands 
alternately  dim  and  bright  which 
give  to  it  a  striated  appearance. 
If  the  bright  bands  are  examined 
with  high  magnifying  powers,  each 
one  is  seen  to  be  crossed  by  a  fine 
dark  fine  which  at  the  time  of  its 
discovery  was  regarded  as  the 
optic  expression  of  a  membrane 
attached  laterally  to  the  sarco- 
lemma. It  has  since  been  re- 
solved into  a  row  of  granules 
(Fig.  16). 

The  muscle-fiber  also  presents 
a  longitudinal  striation  which  in- 
dicates that  it  is  composed  of 
finer  elements  placed  side  by  side, 

termed  fibrillae.     The  fibrillas  extend  throughout  the  entire  length 
of  the  fiber,  though  they  are  not  of  uniform  thickness  (Fig.  17).     That 

portion  of  the  fibril  corresponding 
in  position  to  the  dim  band  is 
thick,  prismatic,  or  rod-hke  in 
shape,  and  termed  a  sarcostyle; 
that  portion  corresponding  in  po- 
sition to  the  bright  band  is  ex- 
tremely thin  and  narrow  and  pre- 
sents at  its  middle  a  sHght  enlarge- 
ment or  granule.  The  fibrillae  are 
embedded  in  a  clear  transparent 
fluid  which,  from  its  supposed 
nutritive  character,  is  termed  sar- 
coplasm.  The  diminution  in  cah- 
ber  of  the  fibrillae  at  different 
levels  permits  of  the  accumulation 
and  storage  of  a  larger  amount 
of  nutritive  material  than  could 
otherwise  be  the  case.  It  is  for 
this  reason  that  the  fiber  at  these 
points  presents  a  brighter  appear- 
ance. 

When  the  muscle-fiber  is  ex- 
amined under  crossed  Nichol  prisms,  the  dim  band  appears  bright 
and  the  bright  band  appears  dim  against  a  dark  background,  indi- 


FiG.  17. — A.  Diagram  of  arrangement 
of  the  contractile  substance  ac- 
cording to  the  \'iew  of  Rollett;  the 
granular  figures  represent  the  con- 
tractile elements,  the  intervening 
light  areas  the  sarcoplasm.  B. 
Small  muscle-fiber  of  man;  the 
corresponding  parts  in  the  two 
figures  are  indicated;  /,  i,  I,  respec- 
tively the  transverse,  the  interme- 
diate, and  lateral  discs,  n.  Muscle 
nuclei. — (Piersol.) 


68 


TEXT-BOOK  OF  PHYSIOLOGY. 


MUSCLE 
FIBER 


..-CAPILLARY   BLOOD 
VESSEL 


eating  that  the  former  is  doubly  refracting  or  anisotropic,  the  latter 
singly  refracting  or  isotropic. 

The  Blood-supply. — Muscles  in  the  physiologic  condition  re- 
quire for  the  maintenance  of  their  activity  a  large  amount  of  nutritive 
material.  This  is  obtained  directly  from  the  lymph  and  indirectly 
from  the  blood  furnished  by  the  blood-vessels.  The  vascular  supply 
to  the  muscles  is  very  great  and  the  disposition  of  the  capillary 
vessels  with  reference  to  the  muscle-fiber  is  very  characteristic. 
The  arterial  vessels,  after  entering  the  muscle,  are  supported  by  the 
peri-mysium;  in  this  situation  they  give  off  short,  transverse 
branches,  which  immediately  break  up  into  a  capillary  network  of 
rectangular  shape  within  which  the  muscle-fibers  are  contained.  The 
relation  of  the  capillary  vessel  to  the  muscle-fiber  is  shown  in  Fig.  i8. 

The  muscle-fiber,  in  inti- 
LYMPH  SPACE  mate  relation  with  the  capil- 

lary, is  bathed  with  lymph 
derived  from  it.  Its  contrac- 
tile substance,  however,  is 
separated  from  the  lymph  by 
its  own  investing  membrane, 
through  which  all  interchange 
of  nutritive  and  waste  mate- 
rials must  take  place. 

The      nutritive      material 
passes    through  the  capillary 
wall    into    the    lymph-space, 
then  through  the  sarcolemma 
into  the  interior  of  the  fiber, 
where   it   comes  into  relation 
with  the  Hving   muscle  mate- 
rial. The  waste  products  aris- 
ing in  the  muscle  as  a  result 
of   nutritive  changes  pass  in 
the  reverse  direction  into  the 
blood,  by  which  they  are  car- 
ried away  to  ehminating  organs.     Lymphatics  are  present  in  mus- 
cle, but  confined  to  the  connective  tissue,  in  the  spaces  of  which  they 
take  their  origin. 

The  Nerve-supply. — The  nerves  which  carry  the  stimuh  to  a 
muscle  enter  near  its  geometric  center.  Many  of  the  fibers  pass 
directly  to  the  muscle-fibers  with  which  they  are  connected;  others 
are  distributed  to  blood-vessels.  Every  muscle-fiber  is  supphed 
with  a  special  nerve-fiber  except  in  those  instances  where  the  nerve- 
trunks  entering  a  muscle  do  not  contain  as  many  fibers  as  the  muscle. 
In  such  cases  the  nerve-fibers  divide  near  their  termination  until  the 


Fig.  i8. 


-Relation  of  the  Blood-vessel 
TO  THE  Muscle-fiber. 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  69 

number  of  branches  equals  the  number  of  muscle-fibers.  The 
individual  muscle-fiber  is  penetrated  near  its  center  by  the  nerve 
where  it  terminates;  the  ends  being  practically  free  from  nerve  in- 
fluence. The  stimulus  that  comes  to  the  muscle-fiber  acts  primarily 
upon  its  center,  the  effect  of  which  then  travels  in  both  directions 
to  the  ends.  The  manner  in  which  the  nerve-fibers  terminate  in 
muscle  will  be  more  fully  described  in  connection  with  the  histology 
of  the  nerve  tissue. 


CHEMIC  COMPOSITION  OF  MUSCLE. 

The  chemic  composition  of  living  muscle  is  but  imperfectly  under- 
stood owing  to  the  fact  that  shortly  after  death  some  of  its  constituents 
undergo  a  spontaneous  coagulation  and  for  the  reason  that  the 
methods  employed  for  analysis  also  tend  to  alter  its  composition. 
To  human  muscle,  the  following  average  percentage  composition  has 
been  given : 

Water, __-  73.5 

Proteids,  including  those  of  sarcolemma,  connective- 
tissue,  pigments, 18.02 

Gelatin, 1.99 

Fat, 2.27 

Extractives, 0.22 

Inorganic  salts, 3-i2.     (Halliburton.) 

(The  composition  of  muscles  of  different  animals,  consumed  as 
foods,  will  be  found  in  the  chapter  on  Foods.) 

When  fresh  muscle  is  freed  from  fat  and  connective  tissue,  frozen, 
rubbed  up  in  a  mortar,  and  expressed  through  hnen,  a  shghtly  yellow 
syrupy  alkahne  or  neutral  liquid  is  obtained  which  has  been  termed 
muscle-plasma.  This  fluid  at  normal  temperatures  coagulates 
spontaneously,  the  phenomena  resembhng  in  many  respects  those 
observed  in  the  coagulation  of  blood-plasma.  The  coagulum  subse- 
quently contracts  and  squeezes  out  an  acid  muscle-serum.  The 
coagulated  proteid  is  known  as  myosin  and  belongs  to  the  class  of 
globuhns.  Inasmuch  as  it  is  not  present  in  hving  muscle  and  only 
makes  its  appearance  under  conditions  not  strictly  physiologic,  it  is 
regarded  as  a  derivative  of  a  pre-existing  proteid  which  has  been 
termed  myosinogen.  According  to  Halhburton,  the  proteids  of  living 
muscle  are  four  in  number,  distinguished  by  their  varying  solubilities 
in  different  salts  and  by  the  varying  temperatures  at  which  they 
coagulate.  From  muscle-plasma  may  then  be  obtained:  (i)  Para- 
myosinogen and  (2)  myosinogen,  the  former  coagulating  at  47°  C,  the 
latter  at  56°  C.  It  is  myosinogen  which  is  converted  into  myosin 
under  the  influence  of  some  special  ferment,  though  both  enter  into 
the  formation  of  the  muscle-clot.  From  the  muscle-serum  may  also 
be  obtained  at  68°  C.  a  globulin  body  termed  myoglobulin  and  a 


70  TEXT-BOOK  OF  PHYSIOLOGY. 

small  quantity  of  myoalbumin.  Among  the  proteids  may  be  men- 
tioned hemoglobin,  which  gives  the  color  to  the  muscles.  Spectro- 
scopic investigation  reveals  the  presence  of  a  special  pigment,  myo- 
hematin,  which  is  supposed  to  have  a  respiratory  function,  inasmuch 
as  its  absorption  bands  change  by  oxidation  and  reduction. 

Among  the  extractives  containing  nitrogen  may  be  mentioned 
creatin,  creatinin,  xanthin,  carnin,  urea,  uric  acid,  carnic  acid,  etc. 
Among  the  extractives  free  of  nitrogen,  glycogen,  dextrose,  inosite, 
lactic  acid,  fat,  are  the  most  important.  Inorganic  salts  are  relatively 
abundant,  of  which  potassium  is  the  most  abundant  among  the  bases, 
and  phosphoric  acid  among  the  acids. 


THE  PHYSICAL  AND  PHYSIOLOGIC  PROPERTIES  OF  MUSCLE- 
TISSUE. 

Consistency. — The  consistency  of  muscle-tissue  during  hfe 
varies  considerably  in  accordance  with  different  states  of  the  muscle. 
In  a  state  of  tension  it  is  hard  and  resistant ;  in  the  absence  of  tension 
it  is  soft  and  fluctuating  to  the  sense  of  touch.  Tension  alone  gives 
rise  to  hardness. 

Cohesion. — The  cohesion  of  a  muscle  is  largely  dependent  on  the 
quantity  of  connective  tissue  it  contains.  A  band  of  fresh  human 
muscle  one  square  centimeter  in  cross-section  was  able  to  resist  a 
weight  of  14  kilograms  without  rupture  (McAhster).  Cohesion 
resists  the  forces  of  traction  and  pressure. 

Elasticity. — Muscle,  in  common  with  many  other  organic  as 
well  as  inorganic  substances,  is  capable  of  being  extended  beyond  the 
normal  length  through  the  action  of  external  forces  and  of  resuming 
the  normal  length  w^hen  these  forces  cease  to  act.  All  such  bodies 
are  said  to  be  elastic;  and  the  greater  the  variations  between  the 
natural  and  acquired  lengths,  the  greater  is  their  elasticity  said  to  be. 
Muscle  therefore  possesses  extensibihty  and  elasticity.*  If  the 
muscle  of  a  frog,  preferably  the  sartorius,  the  fibers  of  which  are 
arranged  in  a  practically  parallel  manner,  be  fastened  at  one  ex- 
tremity by  a  clamp,  and  then  extended  by  a  series  of  successive 
weights  which  differ  by  a  common  increment,  it  will  be  found  that 
the  extensibihty  of  muscle  does  not  follow  the  law  of  elasticity  as 
determined  for  inorganic  bodies;  i.  e.,  directly  proportional  to  the 
weight  and  to  the  length  of  the  body  extended;  but  that  while  in- 
creasing in  length  with  each  successive  weight,  the  increase  is  always 
in  a  diminishing  ratio.  Thus,  for  example,  as  shown  in  Fig.  19: 
The  extension  produced  by  5  grams  is  5  milhmeters,  that  produced 
by  10  grams  is  only  4  milhmeters  more,  and  so  on  with  additional 

*By  this  latter  term  is  here  meant  the  power  by  virtue  of  which  the  muscle 
returns  to  its  original  length   and  is  used  synonymously  with  perfect  retractibility. 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


71 


.    10. — Extension    Curve 
OF  Muscle. — (Gad.) 


weights  until  the  increase  in  passing  from  25  to  30  grams  is  only  i 
millimeter.  The  extensibihty  is  thus  shown  to  be  proportionately 
greater  with  small  than  with  larger  weights.  It  is,  however,  actually 
greater  with  the  larger  weights.  The  ex- 
tention  curve  A  B  formed  by  joining  the  —ptrm 
ends  of  the  muscle  approximates  that  of  a 
parabola.  The  muscle  in  returning  to  its 
original  length  also  show^s  a  variation  from 
the  behavior  of  inorganic  bodies.  With 
the  successive  removal  of  the  weights,  the 
elasticity  of  the  muscle  asserts  itself  with 
gradually  increasing  energy  until  its  nor- 
mal length  is  nearly,  if  not  entirely,  re- 
gained (Fig.  20).  Though  it  is  usually 
stated  that  the  elasticity  of  muscle  is  in- 
complete, it  must  be  borne  in  mind  that 
the  experiments  have  usually  been  made 
on  muscles  removed  from  the  body,  de- 
prived of  blood  and  nerve  influences, 
and  hence  under  abnormal  conditions. 
It  is  highly  probable  that  in  the  living 
body  muscles  possess  perfect  elasticity 
which  enables  them  to  completely  return 
to  their  normal  length  after  extension.  The  extension  and  elastic 
recoil  of  muscle  depends  on  the  maintenance  of  physiologic  condi- 
tions. If  the  nutrition  is  impaired  by  fatigue,  deficient  blood-sup- 
ply, or  any  pathologic  condition,  the  elasticity  is  at  once  impaired. 

Tonicity. — This  is  a  property  pos- 
sessed by  all  muscles  in  the  body  in  con- 
sequence of  being  stretched  to  a  slight 
extent  beyond  their  normal  length.  This 
may  be  due  to  the  action  of  antagonistic 
muscles  or  to  their  mode  of  growth,  the 
muscles  growing  somewhat  more  slowly 
than  the  bones  to  which  they  are  at- 
tached. That  muscles  are  so  stretched 
is  shown  by  the  shortening  which  at  once 
takes  place  when  their  tendons  are  di- 
vided. This  muscle  tonus  or  tension  is 
closely  connected  with  the  elasticity  and 
plays  an  important  role  in  muscle  con- 
traction; being  always  on  the  stretch,  the  muscle  loses  no  time  in 
acquiring  that  degree  of  tension  necessary  to  immediate  action  on 
the  bone  to  which  it  is  attached.  The  working  power  of  a  muscle  is 
also  increased  by  the  presence,  within  limits,  of  some  resistance  to 


Fig.  20. — Curve  of  Elas- 
ticity Produced  by 
Continuous  Extension 
AND  Recoil  of  a  Frog's 
Muscle,  o  x.  Abscissa 
before;  x',  after  extension. 
— {Landois  and  Stirling.) 


72  TEXT-BOOK  OF  PHYSIOLOGY. 

the  act  of  contraction.  According  to  Marey,  the  amount  of  work  is 
considerably  increased  when  the  muscle  energy  is  transmitted  by  an 
elastic  body  to  the  mass  to  be  moved,  while  at  the  same  time  the  shock 
of  the  contraction  is  lessened.  The  position  of  a  passive  limb  is  the 
resultant  also  of  the  elastic  tension  of  antagonistic  groups  of  muscles. 

Another  explanation  for  the  tonicity  of  muscle  is  found  in  the  fact 
that  the  skeletal  muscles  of  the  body  receive  continuously  nerve 
impulses  from  the  thermogenic  centers.  The  chief  function  of  the 
tonicity  would  thus  be  the  production  of  heat,  other  functions  which 
the  tone  subserves  being  merely  secondary. 

Irritability,  Contractility. — These  are  terms  employed  to  de- 
note that  property  of  muscle-tissue  in  virtue  of  which  it  responds  by  a 
change  of  form,  becoming  shorter  and  thicker  on  the  application  of 
any  external  agent  which  acts  as  a  stimulus.  On  the  withdrawal 
of  the  stimulus  the  muscle  again  undergoes  a  change  of  form, 
becoming  longer  and  narrower,  and  returns  to  its  original  con- 
dition. All  muscles  which  possess  this  capability  are  irritable  and 
contractile;  and  all  agents  which  excite  the  muscle  to  action  are 
stimuli.  The  rapid  change  of  form  which  a  highly  irritable  muscle 
undergoes  in  response  to  the  action  of  a  stimulus  of  short  duration  is 
usually  termed  a  twitch  or  pulsation.  With  appropriate  apparatus 
it  can  be  shown  that  the  muscle  at  the  time  of  the  twitch  becomes 
warmer  and  exhibits  electric  phenomena.  The  muscle  is  therefore 
an  apparatus  for  the  conversion  of  potential  into  kinetic  energy: 
viz.,  heat,  electricity,  and  mechanic  motion. 

Though  usually  associated  with  the  activity  of  the  nervous  system, 
and  to  some  extent  dependent  on  it,  irritability  is  nevertheless  an 
independent  endowment  of  the  muscle  and  persists  for  a  longer 
or  shorter  period,  as  shown  by  many  experiments,  after  all  nerve 
connections  have  been  destroyed.  Among  the  proofs  which  may  be 
presented  in  support  of  this  view  is  the  following:  The  introduction 
of  the  drug  curara  into  the  body  of  an  animal  produces  in  a  short 
time  complete  paralysis.  Experiment  has  shown  that  curara  sus- 
pends the  conductivity  of  the  intramuscular  terminations  of  the 
nerve-fiber  and  thus  separates  the  muscle  entirely  from  the  nerve. 
Though  the  animal  is  incapable  of  executing  a  single  movement,  its 
muscles  respond  promptly  on  the  apphcation  of  a  stimulus.  More- 
over, portions  of  muscles  exhibit  irritability  in  which  there  is  no  trace 
of  nerve  structure.  This  is  the  case  with  the  ends  of  the  sartorius 
muscle  of  the  frog  and  the  anterior  end  of  the  retractor  muscle  of 
the  eyeball  of  the  cat.  These  and  other  facts  demonstrate  the  in- 
dependence of  muscle  irritabihty. 

In  the  hving  body  irritabihty  and  nutritive  activity,  with  which 
it  is  closely  associated,  are  maintained  by  a  due  supply  of  oxygen, 
of  nutritive  material,  the  removal   of  waste  products,   and  a  nor- 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  73 

mal  temperature.  The  muscles  of  the  cold-blooded  animals,  and 
especially  the  frog,  retain  their  irritabihty  for  a  much  longer  period 
than  the  muscles  of  the  warm-blooded  animals.  This  is  the  case  also 
with  the  individual  muscles  after  removal  from  the  body  of  the  animal. 
The  reason  for  this  is  found  in  all  probability  in  the  difference  in  the 
rate  of  their  nutritive  activities  and  in  the  quantity  of  nutritive  mate- 
rial stored  up  in  their  cells.  The  duration  of  the  irritability  of  isolated 
muscles  can  be  considerably  prolonged  by  keeping  them  moist. 

Muscle  Stimuli. — Though  consisting  of  a  highly  irritable  tissue, 
muscles  do  not  possess  spontaneity  of  action.  They  require  for  the 
manifestation  of  their  characteristic  activity  the  application  of  a 
stimulus.  In  the  living  body  all  contractions,  at  least  of  the  skeletal 
muscles,  occurring  under  normal  or  physiologic  conditions  are  caused 
by  the  action  of  "nerve  impulses"  transmitted  by  the  nerves  from  the 
central  nervous  system  to  the  muscles.  The  nerve  impulse  is  the 
normal  or  physiologic  stimulus.  After  removal  from  the  body  and 
freed  from  nerve  connections  muscles  can  be  excited  to  action  by 
various  agents — e.  g.,  mechanic,  chemic,  thermic,  electric.  These 
are  artificial  or  non-physiologic  stimuli. 

1 .  Mechanic  Stimuli. — Cutting,  pinching,  sharply  tapping  the  muscle 

\vill  cause  it  to  contract,  providing  the  stimulus  has  sufficient 
intensity.  With  each  stimulation  a  short,  fleeting  contraction 
ensues.  If  repeated  with  sufficient  rapidity,  a  series  of  con- 
tinuous but  irregular  pulsations  are  produced. 

2.  Chemic   Stimuli. — Various    chemic    substances   in   solution    will 

excite  single  or  continuous  pulsations  if  the  strength  of  the  solu- 
tion is  not  such  as  to  destroy  at  once  the  irritabihty.  They  owe 
their  efficiency  as  stimuh  to  the  rapidity  with  which  they  alter 
the  composition  of  the  muscle-substance.  Among  these  may  be 
mentioned  solutions  of  potassium  and  sodium,  weak  solutions  of 
the  mineral  and  organic  acids,  ammonium  vapor,  distilled  water, 
glycerin,  and  sugar. 

3.  Thermic  Stimuli. — The  application  of  a  heated  object,  such  as  a 

hot  wire,  causes  the  muscle  to  rapidly  contract. 

4.  Electric  Stimuli. — The  most  efficient  stimulus  and  the  one  least 

injurious  to  the  tissue  is  the  electric  current.     Either  the  con- 
stant or  the  induced  current  may  be  used.* 
The  Constant  Current. — If  the  ends  of  the  wires  in  connection  with 
an  electric  cell  be  provided  Avith  non-polarizable  electrodes  and  the 


*  Since  the  study  of  the  physiologic  properties  of  both  muscle-tissue  and  nerve- 
tissue  involves  the  employment  of  electricity  as  a  stimulus,  it  becomes  necessary  for 
the  student  to  familiarize  himself  with  certain  forms  of  apparatus  by  which  it  is  gen- 
erated, controlled,  and  applied.  For  the  purpose  of  not  interrupting  the  continuity 
of  the  text  this  information  is  embodied  in  an  appendix.  The  facts  therein  contained 
should  be  mastered  bv  the  student. 


74  TEXT-BOOK  OF  PHYSIOLOGY. 

latter  placed  on  opposite  ends  of  a  freshly  prepared  sartorius  muscle 
of  a  frog  which  has  been  previously  curarized,  it  will  be  found  on 
closing  or  making  the  circuit  that  the  muscle  will  exhibit  a  short  quick 
pulsation.  During  the  actual  passage  of  the  current,  especially  if 
it  is  weak,  there  may  be  no  apparent  change  in  the  muscle.  If  the 
current  is  strong,  the  muscle  may,  on  the  contrary,  remain  in  a  state 
of  continuous  contraction.  With  the  opening  or  breaking  of  the 
current  the  muscle  at  once  relaxes,  or  perhaps  again  contracts  and 
then  relaxes.  The  extent  of  the  contraction  depends  mainly  on  the 
strength  of  the  current,  being  greater  with  strong,  less  with  weak 
currents.  When  the  current  is  sufificiently  strong  to  elicit  both 
making  and  breaking  contractions,  it  is  found  that  the  contraction 
occurring  on  the  make  or  closure  of  the  circuit  is  always  greater 
than  that  occurring  on  the  break  or  opening  of  the  circuit.  More- 
over, it  has  been  shown  in  many  ways  that  the  contraction  occur- 
ring on  the  closure  of  the  circuit  has  its  origin  at  the  point  where 
the  current  is  leaving  the  muscle — i.  e.,  in  the  immediate  neighbor- 
hood of  the  negative  pole  or  cathode — and  propagates  itself  to  the 
opposite  extremity;  while  the  contraction  occurring  on  the  opening 
of  the  circuit  has  its  origin  at  the  point  where  the  current  is  entering 
the  muscle,  i.  e.,  in  the  neighborhood  of  the  positive  pole  or  anode. 

The  Induced  Current. — If  the  primary  spiral  of  the  inductorium 
be  connected  with  an  electric  cell  and  the  secondary  spiral  be  con- 
nected with  a  muscle,  it  will  be  found  that  the  current  induced  in  the 
secondary  circuit,  both  on  the  make  and  break  of  the  primary,  will 
also  cause  the  muscle  to  sharply  and  rapidly  pulsate  if  the  two  spirals 
are  sufhciently  near  each  other.  Observation,  however,  makes  it 
evident  that  the  pulsation  occurring  with  the  break  of  the  primary 
circuit  is  more  energetic  than  that  occurring  with  the  make,  a  result 
the  opposite  of  that  obtained  with  the  constant  current.  This  is 
not  due  to  any  difference  in  the  electricity,  however,  but  to  pecuharities 
in  the  construction  of  the  inductorium.  When  the  primary  circuit 
is  interrupted  with  sufficient  frequency,  as  it  can  be  by  throwing  into 
the  circuit  Neef's  hammer  or  some  other  form  of  interrupter,  the  con- 
tractions excited  by  the  induced  currents  may  be  made  to  succeed  one 
another  so  rapidly  that  they  become  fused  together,  producing  a 
spasm  or  tetanus  of  the  muscle.  The  rapidity  with  which  the  induced 
current  appears  and  disappears,  its  brief  duration,  the  ease  with  which 
its  strength  can  be  regulated,  combine  to  render  it  a  most  efficient 
stimulus  for  either  muscle  or  nerve. 

Conductivity. — All  muscle  protoplasm  possesses  conductivity. 
The  change  excited  in  a  muscle-fiber  by  the  arrival  of  a  nerve  impulse 
is  at  once  conducted  with  great  rapidity  in  opposite  directions  to 
the  end  of  the  fiber;  the  advance  of  the  excitation  process  is  im- 
mediately succeeded  by  the  contraction  process,  the  change  of  form 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  75 

which  constitutes  the  contraction.  With  the  disappearance  of  the 
former,  the  latter  also  disappears  and  the  muscle  resumes  its  pre- 
vious passive  condition.  There  is  no  evidence,  however,  that  the 
excitation  process  travels  transversely, — that  is,  into  adjoining  fibers, 
— being  prevented  from  doing  so  by  the  presence  of  the  hmiting 
membranes,  the  sarcolemmata.  The  fact  that  each  muscle-fiber 
receives  its  own,  or  at  least  a  branch  of  a  nerve-fiber,  and  hence  its 
own  nerve  impulse  or  stimulus,  would  also  indicate  that  the  excitation 
process  can  not  be  conducted  longitudinally  into  adjoining  fibers, 
or  at  least  with  sufficient  rapidity  for  the  purposes  of  ordinary  muscle 
actions.  Nevertheless  if  a  long  muscle,  such  as  the  sartorius,  from  a 
curarized  frog  be  stimulated  at  one  end  with  an  induced  electric  cur- 
rent, the  excitation  and  the  contraction  processes  will  be  conducted 
with  extreme  rapidity  to  the  opposite  end  of  the  muscle.  The  rapidity 
of  conduction  in  human  muscles  has  been  estimated  at  from  10  to  13 
meters  per  second,  and  in  frog's  muscle  at  from  3  to  4.5  meters  per 
second.  The  contraction  process,  the  thickening  of  the  muscle,  is 
termed  the  contraction  wave.  As  it  is  the  result  of  the  excitation 
process  and  immediately  succeeds  it,  its  rate  of  conduction  must  be 
the  same  as  that  given  above.  With  appropriate  apparatus  the 
duration  of  the  wave  at  any  given  point  has  been  shown  to  be,  in  the 
frog's  muscle,  one-tenth  of  a  second  and  its  length  three-tenths  of  a 
meter. 

PHENOMENA  ATTENDING  A  MUSCLE  CONTRACTION. 
PHYSICAL  PHENOMENA. 

The  most  obvious  change  in  a  muscle  during  the  contraction  is 
that  relating  to  its  form.  The  muscle  not  only  becomes  shorter,  but 
at  the  same  time  thicker.  The  extent  to  which  it  may  shorten  when 
unopposed  may  amount  to  30  per  cent,  or  more  of  its  original  length. 
The  increase  in  thickness  practically  compensates  for  the  diminution 
in  length,  for  there  is  no  observable  diminution  in  volume.  The 
change  in  form  of  the  entire  muscle  results  from  a  corresponding 
change  of  form  of  its  individual  fibers  as  determined  by  microscopic 
examination,  each  of  which  becomes  shorter  and  thicker.  The 
successive  changes  in  both  the  muscle  and  the  individual  fibers  are 
represented  in  Fig.  21. 

When  the  contraction  begins,  the  dim  band  increases  and  the 
bright  band  diminishes  in  width.  This  Engelmann  attributes  to 
the  passage  of  fluid  material  from  the  bright  into  the  dim  band.  At 
the  time  of  relaxation  there  is  a  return  of  this  material  and  the  bands 
assume  their  original  shape  and  volume.  As  the  contraction  wave 
reaches  its  maximum  the  optic  properties  of  the  bright  and  dim  bands 
change.     The   former  now  becomes   darker   and   less   transparent 


76 


TEXT-BOOK  OF  PHYSIOLOGY. 


until  at  the  crest  of  the  wave  it  assumes  the  appearance  of  a  distinct 
dark  band;  the  latter  now  becomes  clear  and  bright  in  comparison. 
This  change  in  the  appearance  of  the  fiber  is  due  to  an  increase  in 
refrangibihty  of  the  bright  and  a  decrease  in  the  refrangibility  of  the 
dim  band  coincident  with  the  passage  of  the  fluid  from  the  former 
into  the  latter.  There  is  at  the  height  of  the  contraction  a  complete 
reversal  in  the  positions  of  the  striations.  At  a  certain  stage  between 
the  beginning  and  the  crest  of  the  wave  the  strias  almost  entirely  dis- 
appear, giving  to  the  fiber  an  appearance  of  homogeneity.  There  is, 
however,  no  change  in  refractive  power  as  shown  by  the  polarizing 
apparatus.  When  the  contraction  wave  has  reached  the  stage  of 
greatest  intensity,  there  is^a  reversal  of  the  above  phenomena  as  the 
fiber  returns  to  its  former  condition,  that  of  relaxation. 


^iri 


IIIIIIMHIjIIIIIIII 


m 


Fig.  21. — Showing  the  Changes  in  a  Muscle  and  Muscle-fiber  during  Con- 
traction. 


Elasticity. — During  the  contraction  of  a  muscle  there  is  a  greater 
or  less  alteration  in  its  elasticity,  as  shown  by  the  fact  that  it  is  ex- 
tended to  a  greater  degree  by  the  same  weight  in  the  active  than  in 
the  passive  condition.  The  degree  to  which  the  extensibihty  is  in- 
creased and  the  elasticity  decreased  is  dependent  on  the  amount  of 
the  resisting  force.  These  facts,  as  determined  experimentally,  are 
represented  in  Fig.  22.  Let  A  B  and  A  b  represent  the  length  of  the 
normal  unweighted  muscle,  passive  and  active  states  respectively;  the 
line  B  B',  the  extension  curve  of  the  passive  muscle  produced  by 
successive  weights,  5,  10,  15,  20,  25,  30  grams,  differing  by  a  com- 
mon increment;  the  fine  b  B',  the  extension  curve  of  the  active  con- 
tracted muscle  when  weighted  with  the  same  weights ;  A'  B'  the  length 
of  the  muscle  when  the  weight  is  sufficiently  great  to  prevent  shorten- 
ing. It  will  be  observed  from  these  facts  that  while  the  muscle  is 
extended  in  both  the  passive  and  active  states  by  corresponding 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


77 


weights,  the  extension  during  the  latter  is  progressively  greater,  until 
with  a  given  weight  the  length  of  the  muscle  is  the  same.  Under 
such  circumstances,  there  being  no  shortening  of  the  muscle,  the 
energy  of  its  contraction  manifests  itself  physically  merely  as  tension. 
In  the  successive  actions  of  the  muscle  represented  in  the  same  figure 
there  is  to  be  observed  also  a  combination  of  a  change  of  lensfth  and 


Fig.  22. — Extension  Curves:  B  B',  of  the  resting;  b  B',  of  the  contracting  muscle. 


a  change  of  tension,  the  ratio  of  the  one  to  the  other  being  deter- 
mined by  the  amount  of  the  supported  weights.  When  the  weight 
is  slight  in  amount,  the  shortening  of  the  muscle  reaches  a  maximum 
and  the  tension  a  minimum;  when  the  weight  is  large  in  amount,  the 
reverse  conditions  obtain. 


THE  CONTRACTION  PROCESS.     METHODS  OF  INVESTIGATION. 

The  contraction  of  a  muscle  as  it  takes  place  in  the  living  body  and 
under  normal  physiologic  conditions  is  a  complex  act,  persisting  for 
a  variable  length  of  time  in  accordance  with  the  number  of  stimuh 
transmitted  to  it  in  a  given  unit  of  time,  and  as  determined  experi- 
mentally is  the  resultant  of  the  fusion  of  a  greater  or  less  number  of 
separate  and  individual  contractions  or  pulsations.  To  this  enduring 
contraction  the  term  tetanus  has  been  given.  With  the  aid  of  ap- 
propriate apparatus  it  has  become  possible  to  obtain  and  record 
single  muscle  contractions,  to  analyze  and  decompose  them  into  their 
constituent  elements,  or  to  combine  them  in  such  a  manner  as  to  pro- 
duce practically  a  normal  physiologic  tetanus.  As  in  the  experi- 
mental study  of  the  phenomena  of  a  muscle  contraction  it  frequently 
becomes  necessary  to  remove  the  muscle  from  the  body  of  the  animal, 
the  muscles  of  warm-blooded  animals  are  not  w^ell  adapted  for  this 
purpose,  owing  to  the  rapid  alteration  in  composition  they  undergo. 


78 


TEXT-BOOK  OF  PHYSIOLOGY. 


with  a  consequent  loss  of  irritability,  when  deprived  of  their  normal 
blood-supply.  The  excised  muscles  of  cold-blooded  animals,  par- 
ticularly of  the  frog, — in  which,  owning  to  the  relatively  slow  rate  of 
the  nutritive  activities,  the  irritabihty  and  contractility  endure  for  a 
long  period  of  time,  even  though  deprived  of  blood, — are  particularly 
valuable  for  experimental  studies.  The  muscles  generally  employed 
are  the  gastrocnemius,  the  sartorius,  and  the  hyoglossus.  If  kept 
at  a  normal  temperature  and  moistened  with  0.6  per  cent,  solution  of 
sodium  chlorid,  such  a  muscle  will  contract  for  a  long  period  of  time 
on  the  application  of  any  form  of  stimulus,  but  especially  the  electric. 
Graphic  Record  of  a  Muscle  Contraction. — Inasmuch  as  the 
changes  in  the  form  of  a  muscle  during  a  single  contraction  take  place 
with  extreme  rapidity,  their  succession,  peculiarities,  and  time  re- 


FiG.   23. — Myograph.     K.  Recording  cylinder.      M.  Moist  chamber, 
ing  lever.     W.  Weight.     I.  Induction  coil. 


L.  Record 


lations  cannot  be  determined  with  any  degree  of  accuracy  by  the 
unaided  eye.  This  difficulty  can  largely  be  overcome  by  the  employ- 
ment of  the  graphic  method,  the  principle  of  which  consists  in  record- 
ing the  movements  by  means  of  a  pen  on  some  appropriate  moving 
and  receiving  surface.  To  accomphsh  this  object  the  muscle  is  at- 
tached at  one  extremity  by  a  clamp  to  a  firm  support,  and  at  the  other 
extremity  to  a  weighted  lever,  which  is,  however,  sufficiently  light  to 
enable  it  to  take  up,  reproduce,  and  magnify  its  movements.  The 
end  of  the  lever  provided  with  a  pen  is  apphed  to  a  smooth  surface, 
such  as  glazed  paper  on  a  cyhnder  or  plate,  and  covered  with  lamp- 
black. If  the  surface  is  stationary,  the  contraction  is  recorded  as  a 
vertical  line;  if  it  is  placed  in  movement  at  a  uniform  rate  by  clock- 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


79 


work,  the  contraction  is  recorded  in  the  form  of  a  curve,  the  width  of 
the  arms  of  which  will  depend  on  the  rate  of  movement.  The  time 
relations  of  the  phases  of  the  contraction  can  be  obtained  by  placing 
beneath  the  lever  a  pen  attached  to  an  electro-magnet  thrown  into 
action. by  a  tuning-fork  vibrating  in  hundredths  of  a  second.  In 
order  to  determine  the  rapidity  with  which  the  contraction  follows 
the  stimulation,  it  is  essential  that  the  moment  of  the  latter  be  also 
recorded.  This  is  accomphshed  by  an  automatic  key,  the  opening 
or  closing  of  which  develops  the  stimulus  which  excites  the  muscle. 
A  combination  of  these  different  appliances  constitutes  a  myograph 
and  the  curve  of  contraction  a  myogram.     (See  Fig.  23.) 

The  Isotonic  Myogram. — With  the  object  of  obtaining  a  curve 
of  successive  changes  in  the  length  of  a  muscle  during  a  single  con- 
traction and  at  the  same  time  avoiding  changes  in  tension,  the  weight 


Fig.  24. — The  Isotonic  Myogram. 

attached  to  the  lever  should  be  appHed  close  to  its  axis,  a  mechanic 
condition  which  practically  maintains  a  uniform  tension  throughout 
the  contraction.  To  this  method  the  term  "isotonic"  has  been  given 
and  the  curve  so  obtained  an  isotonic  myogram.* 

The  Character  of  an  Isotonic  Myogram. — With  the  muscle 
arranged  as  previously  described  and  stimulated  directly  with  a  single 
induction  shock,  the  contraction  will  be  recorded  in  the  form  of  a 
curve  similar  to  that  represented  in  Fig.  24,  in  which  the  line  t  t  repre- 
sents the  abscissa  of  time;  a,  the  moment  of  stimulation;  and  bed, 
the  degree  of  shortening  at  each  successive  moment.  The  undulating 
hne  shows  the  time  relations,  the  distance  from  crest  to  crest  represent- 


*  In  the  ordinary  method  of  recording  a  muscular  movement,  i.  e.,  with  the  weight 
attached  to  the  lever  immediately  beneath  the  muscle  and  known  as  the  "loaded 
method,"  a  certain  momentum  is  imparted  to  the  weight,  which  continues  after  the 
muscle  has  ceased  to  act,  both  when  shortening  and  relaxing,  and  so  imparts  to  the  re- 
cording lever  additional  movements  which  \atiate  the  true  character  of  the  curve. 


8o  TEXT-BOOK  OF  PHYSIOLOGY. 

ing  hundreths  of  a  second.     The  curve  may  be  divided  into  three 
portions: 

1.  A  short  but  measurable  portion  between  the  point  of  stimulation 

and  the  first  evidence  of  the  shortening,  a  b,  known  as  the 
"latent  period."  The  duration  of  this  period  for  the  skeletal 
muscle  of  the  frog  was  originally  determined  to  be  o.oi  second, 
but  with  the  employment  of  more  accurate  apparatus  it  has  been 
reduced  to  0.0025  to  0.004  second.  During  this  period  it  is 
supposed  that  certain  chemic  changes  are  taking  place  prepara- 
tory to  the  exhibition  of  the  movement.  The  duration  of  the 
latent  period  is  influenced  by  a  variety  of  conditions,  e.  g.,  tem- 
perature, fatigue,  strength  of  stimulus,  etc. 

2.  An  ascending  portion,  b  c,  the  contraction  or  period  of  increasing 

energy.  The  contraction  as  shown  by  the  character  of  the  curve 
begins  slowly,  then  proceeds  rapidly,  and  again  slowly  as  the 
shortening  reaches  its  maximum.  The  contraction  may  be  said 
to  end  when  the  tangent  to  the  curve  becomes  parallel  with  the 
abscissa. 

3.  A  descending  portion,  c  d,  the  relaxation  or  period  of  decreasing 

energy.     The  relaxation  as  shown  by  the  character  of  the  curve 
begins  slowly,  then  proceeds  rapidly,  and  again  slowly  as  the 
muscle  attains  its  original  length.     The  termination  of  the  re- 
laxation is  at  the  point  where  the  curve  cuts  the  abscissa.     The 
curve  beyond  this  point  may  be  comphcated  by  the  presence  of 
one  or  more  residual  or  after- vibrations,  which  are  probably  due 
to  the  inertia  of  the  lever  as  well  as  to  changes  in  the  muscle 
elasticity. 
The  duration  of  the  period  of  shortening  is  about  0.04  second,  and 
of  the  period  of  relaxation  0.05  second.     A  single  pulsation  of  the 
isolated  muscle  of  the  frog  therefore  occupies,  from  the  moment  of 
stimulation  to  termination,  the  tenth  of  a  second.     Muscles  of  many 
other  animals  have  a  contraction  period  the  duration  of  which  varies 
considerably  from  this.     Thus,  in  man  the  time  of  a  single  contrac- 
tion is  one-twentieth  of  a  second,  in  some  insects  one  three-hundreth 
of  a  second,  and  in  the  turtle  one  second.     Pale  muscles  have  a  shorter 
period  than  the  red. 

Influences  Modifying  the  Contraction  Process. — The  con- 
traction process  in  its  entirety  as  well  as  in  its  individual  parts  is 
considerably  modified  by  external  conditions,  among  which  may  be 
mentioned  the  following: 

I.  Stimulus.  As  the  contraction  is  the  response  of  the  muscle  to 
a  stimulus,  the  vigor  of  the  former  is  proportional,  within  limits, 
to  the  strength  of  the  latter.  Thus,  with  single  induction  shocks 
the  height  of  the  contraction  or  the  degree  of  shortening  increases 
as  the  strength  of  the  stimulus  increases  from  a  minimum  to  a 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  8i 

maximum  value  (Fig.  25).  The  rate  at  which  the  muscle  is 
stimulated  with  a  given  stimulus  will  also  influence  the  character 
of  the  contraction  process.  If  the  intervals  between  the  successive 
stimulations  be  such  as  permit  the  muscle  to  recover  from  the 
effects  of  the  contraction,  it  may  contract  as  many  as  a  thousand 
times  without  showing  any  particular  variation  from  the  normal 
form.  In  the  earlier  period  of  stimulation  there  is  apparently 
a  decrease  in  the  irritability  and  consequently  in  the  energy  of 
the  contraction  for  a  short  time.  This  is  followed  by  an  in- 
crease in  the  irritability,  as  shown  by  a  gradual  increase  in  the 
height  of  the  curve  until  a  certain  maximum  is  reached  and 
maintained  (Fig.  26).  These  so-called  staircase  contractions  have 
been  observed  in  the  muscles  of  both  cold-blooded  and  warm- 
blooded animals.  In  time,  however,  as  the  muscle  becomes 
fatigued  the  effects  of  repeated  contractions  manifest  themselves 
in  a  lengthening  of  the  latent  period,  a  diminution  in  the 
rapidity  and  extent  of   the  contraction,  and  an  increase  in  the 


Fig.  25. — Showing  the  Ef- 
fects OF  Increasing 
Strength  of  Stimulus. 


Fig. 


26. — Showing    Staircase    Con- 
tractions. 


time  of  relaxation.  If  the  intervals  between  successive  stimu- 
lations be  not  sufficient  for  the  muscle  to  recover  itself,  the  same 
phenomena  arise,  though  more  quickly. 
Temperature.  The  temperature  at  which  all  phases  of  the  con- 
traction process,  as  represented  by  the  myogram,  attain  their 
physiologic  maximum  value  is  about  30°  C.  If  the  temperature 
of  the  muscle  falls  to  20°  C.  there  is  a  corresponding  decHne  in 
activity,  as  shown  by  an  increase  of  the  latent  period,  a  decrease 
in  the  height  of  curve, — i.  e.,  in  the  shortening  of  the  muscle, — 
an  increase  both  in  the  contraction  and  relaxation  periods.  As 
the  temperature  approaches  0°  C.  the  height  of  the  curve  again 
suddenly  increases,  indicating,  for  some  unknown  reason,  an 
increase  in  the  irritabihty.  This  is,  however,  scarcely  a  physio- 
logic condition.  At  a  temperature  of  40°  C.  to  50°  C.  the  muscle 
suddenly  contracts  and  passes  into  the  condition  of  heat  rigor. 
The  proteid  constituents  of  the  muscle  are  coagulated  and  the 

irritability  destroyed  (Fig.  27). 
6 


82 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  Load.  The  extent  to  which  a  muscle  is  loaded  or  weighted 
will  not  only  determine  the  height  of  the  contraction,  but  also  the 
time  relations  of  all  its  phases.  This  is  apparent  from  an  ex- 
amination of  Fig.  28,  in  which  it  is  shown  that  with  an  increase 
in  load  there  is  a  decrease  in  the  height  of  the  contraction,  an 
increase  in  the  latent  period,  and  a  general  decrease  in  the  dura- 
tion of  both  the  periods  of  rising  and  falling  energy. 


Fig. 


-Single  Contractions  of  the  Gastrocnemius  at  Different  Tempera- 
tures.   Time  Tracing,  200  per  Second. — (Brodie.) 


Muscle  Fatigue.  Prolonged  or  excessive  activity  of  our  own 
muscles  is  accompanied  by  a  feehng  of  stiffness  or  soreness  and 
lassitude.  There  is  at  the  same  time  a  diminution  in  the  rate  and 
vigor  of  the  contractions  and  the  power  of  doing  work.  To  this 
condition  the  term  fatigue  has  been  given.  The  cause  of  the 
fatigue  is  attributed  to  a  diminution  in  the  amount  of  the  energy- 
holding  compounds  as  well  as  to  the  production  and  accumulation 


Fig.  28. — Contractions  of  a  Gastrocnemius  with  Different  Loads. — (Brodie.) 


of  waste  products  resulting  from  activity.  Among  the  waste 
products  phosphoric  acid,  potassium  phosphate,  lactic  acid,  and 
carbon  dioxid  are  the  most  important.  The  more  rapidly  they 
are  removed,  the  sooner  is  a  fatigued  muscle  restored  to  its  normal 
condition.  It  is  highly  probable  that  the  nerve-centers  are  more 
easily  fatigued  than  the  muscles.     The  condition  of  fatigue  with 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


83 


its  attendant  phenomena  is  shown  by  an  excised  frog  muscle 
when  stimulated  for  a  long  period  of  time  by  induction  shocks 
at  intervals  of  one  second.  In  a  variable  period  of  time  the 
muscle  shows  an  increase  in  the  duration  of  the  latent  period, 
a  diminution  of  the  height  of  the  contraction,  in  the  power  of 
doing  work,  and  an  increase  in  the  time  required  for  relaxation. 
If  the  stimulation  continues,  the  contractions  gradually  dechne 
as  the  muscle  becomes  exhausted.     (See  Fig.  29.) 


Fig.  29. — Fatigue  Curves.     Every  Twentieth  Contraction  Recorded. 


5.  Nutrition. — The  irritability  of  a  muscle  which  conditions  the  con- 
traction process  is  dependent  on  the  maintenance  of  its  nutrition ; 
hence  a  continuous  and  sufiticient  supply  of  nutritive  material 
and  a  rapid  removal  of  waste  products  are  essential  conditions 
for  the  exhibition  of  normal  contractions.  A  diminution  of 
blood-supply  or  an  accumulation  of  waste  products  sooner  or 
later  impairs  the  irritabihty  and  diminishes  the  vigor  and  extent 
of  the  contraction.  Various  drugs — e.  g.,  veratrin,  barium,  etc. 
— introduced  into  the  circulation  and  finding  their  way  into  the 
muscle  modify  the  contraction  process,  as  shown  by  a  very  great 
increase  in  the  duration  of  the  relaxation  period. 
The  Isometric   Myogram. — With  the  object    of   obtaining  a 

curve  of   the   increase   and   de- 
crease in  the  tension  of  a  muscle 

during  a  single  contraction,  with 

the  exclusion  as  far  as  possible 

of  a  change  in  length,  the  muscle 

may  be  made  to  contract  against  a 

strong  spring  or  similar  resistance 

sufficient    to   practically   though 

not  absolutely  prevent  shorten- 
ing.    To  this  method  the   term 

isometric  has  been  given,  and  the 

curve  so  obtained  an   isometric 

myogram  or  a  tonogram.      The 

recording  portion  of  the  lever  is  prolonged  some  distance  so  that  the 


Fig.  30. — a.  Diagram  of  Isotonic;  b, 
of  Isometric  Muscle  Curves. — 
{Landois  and  Stirling.) 


84  TEXT-BOOK  OF  PHYSIOLOGY. 

very  slight  upward  movement  at  its  axis,  close  to  which  the  muscle 
is  attached,  will  be  considerably  magnified.  That  the  ordinate  value 
of  an  isometric  curve  may  be  known,  the  apparatus  must  be  graduated 
by  subjecting  the  spring  to  a  series  of  weights  playing  over  a  pulley 
supported  by  the  muscle  clamp. 

The  curve  of  the  variation  in  tension  obtained  by  the  isometric 
method  is  shown  in  Fig.  30,  b,  in  which  the  two  curves  are  con- 
trasted. The  form  of  the  curve  indicates  that  the  muscle  attains  its 
maximum  of  tension  more  rapidly  than  its  maximum  of  shortening; 
that  the  tension  endures  for  a  certain  period  of  time  unchanged ;  that 
the  fall  in  tension  takes  place  more  rapidly  than  the  muscle  relaxes. 

The  Work  Accomplished  by  a  Muscle  during  the  Time  of  a 
Single  Contraction. — By  work  is  meant  the  overcoming  of  opposing 
forces.  In  the  physiologic  activities  of  the  body  the  muscles  at  each 
contraction  not  only  overcome  the  resistances  of  antagonistic  muscles, 
the  weight  of  the  limbs,  the  friction  of  joints,  etc.,  but  in  addition 
overcome  various  external  resistances  connected  with  the  environ- 
ment— e.  g.,  gravity,  cohesion,  friction,  elasticity,  etc.  The  muscles 
may  therefore  be  regarded  as  machines  for  the  accomphshment  of 
work.  Experimentally  the  work  done  by  an  isolated  muscle  may  be 
calculated  by  multiplying  the  weight  by  the  height  through  which  it 
is  lifted.  In  the  following  table  it  will  be  observed  that  the  extent 
to  which  a  muscle  will  shorten  in  response  to  a  maximal  stimulus 
is  greatest  when  it  is  unweighted ;  but  as  weights  differing  by  a  com- 
mon increment  are  added,  the  height  of  the  contraction  diminishes 
until  with  a  given  w'eight  it  is  nil.  The  work  done  is  show^n  in  the 
following  table: 


Weight. 

He 

GHT. 

Work 

DONE. 

0  grams 

14 

mm. 

0 

gram- 

millimeters 

50        " 

9 

450 

100        " 

7 

700 

150        " 

5 

750 

200        " 

2 

400 

250        " 

0 

0 

From  the  preceding  figures  it  is  evident  that  the  mechanical  work  of 
a  muscle  increases  with  increasing  weights  up  to  a  certain  maximum, 
and  then  declines  to  zero.  Equally  when  the  muscle  contracts  to  its 
maximum  without  being  weighted,  and  when  it  does  not  contract  at 
all  from  being  overweighted,  no  work  is  done.  Between  these  two 
extremes  the  muscle  performs  varying  amounts  of  work. 

The  maximum  amount  of  force  which  a  muscle  puts  forth  during  a 
contraction  is  naturally  measured  by  the  amount  of  work  done;  but 
as  this  varies  with  the  degree  to  w^hich  the  muscle  is  weighted,  another 
measure  has  been  adopted,  to  which  the  term  absolute  muscle  force 
or  static  force  has  been  given.     The  absolute  force  is  measured  by 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  85 

the  weight  which  is  sufficient  to  prevent  the  muscle  from  shortening. 
This  is  best  determined  by  the  method  of  after-loading  in  which  the 
muscle  is  not  extended  by  the  weight  previous  to  the  contraction. 
It  has  been  found  that  the  absolute  force  of  a  muscle  is  directly  de- 
pendent on  the  number  and  not  the  length  of  the  fibers  it  contains 
and  proportional  to  the  physiologic  transverse  section  of  the  muscle. 
The  transverse  section  of  a  muscle  is  obtained  by  dividing  the  volume 
1058  (obtained  by  dividing  its  weight  by  the  specific  weight  of  mus- 
cle-tissue) by  the  average  length  of  the  fibers. 

For  purposes  of  comparison  it  is  customary  to  refer  the  absolute 
force  to  units  of  diameter — viz.,  one  square  centimeter.  Rosenthal 
estimates  the  force  for  the  square  centimeter  of  the  muscle  of  the  frog 
at  from  2  to  8  kilograms;  for  the  muscles  of  man  at  6  to  8  kilograms; 
Koster  at  about  ten  kilograms  for  the  muscles  of  the  leg  and  7  or  8 
kilograms  for  the  muscles  of  the  arm. 

Action  of  Successive  Stimuli. — If  a  series  of  successive  stimuh 
be  applied  to  a  muscle,  the  effect  will  vary  according  to  the  rapidity 
with  which  they  follow  one  another.     As  previously  stated,  if  the 


Summation  Curve. 


interval  preceding  each  stimulus  be  sufficiently  long  to  enable  the 
muscle  to  recover  from  the  effects  of  the  previous  contraction,  there 
will  be  no  eft'ect  for  a  long  time  except  a  shght  increase,  in  the  early 
period,  of  the  irritabihty  as  shown  by  the  increased  height  of  the 
curve  or  shortening  of  the  muscle.  If,  however,  a  second  stimulus 
be  applied  to  a  muscle  during  the  period  of  relaxation,  a  second  con- 
traction immediately  follows  which  is  added  to  or  superposed  on  the 
first ;  the  eft'ect  produced  will  be  greater  than  that  produced  by  either 
stimulus  separately. 

A  third  stimulus  applied  during  the  relaxation  of  the  second  con- 
traction produces  a  third  contraction  which  adds  itself  to  the  second, 
and  so  on  (Fig.  31).  The  increment  of  increase  in  the  extent  of  the 
successive  contractions  gradually  diminishes,  however,  until  the  muscle 
reaches  a  maximum  of  contraction.  The  superposition  of  the  second 
contraction  on  the  first,  the  third  on  the  second,  and  so  on,  is  termed 
summation  0}  effects.  Experiment  has  shown  that  the  greatest  effect 
of  a  second  stimulus — that  is,  the  highest  contraction — is  produced 


86  TEXT-BOOK  OF  PHYSIOLOGY. 

when  the  stimulus  is  apphed  during  the  last  third  of  the  period  of 
rising  energy,  when  the  sum  of  the  two  contractions  is  almost  twice  as 
great  as  the  first  contraction  (Fig.  32).  The  effects  following  both 
maximal  and  submaximal  stimuh  indicate  that  the  muscle  cannot 
attain  its  maximum  of  shortening  except  through  a  summation  of 
several  stimuh.  If  a  second  maximal  stimulus  enter  a  muscle  dur- 
ing the  latent  period  following  the  first,  the  effect  produced  will  be 
no  greater  than  that  produced  by  a  single  stimulus.  The  muscle 
during  this  period  is  said  to  be  refractory  or  non-responsive  to  a 
second  stimulus.  If,  however,  the  stimuli  are  submaximal  they  add 
themselves  together,  and  though  the  effect  is  but  a  single  contrac- 
tion, it  is  larger  than  either  would  have  produced  separately.  This 
is  termed  the  summation  of  stimuli. 

Still  further,  if  a  series  of  subminimal  stimuh,  each  of  which  is 
alone  insufficient  to  produce  a  contraction  of  the  muscle,  be  applied 
in  rapid  succession,  a  contraction  frequently  results.  This  is  termed 
the  summation  of  subminimal  stimuli. 

Tetanus. — When  a  muscle  is  stimulated  by  a  series  of  induced 
currents  at  the  rate  of  four  or  six  per  second,  it  undergoes  a  corre- 
sponding number  of  contractions  of  about  equal  extent.  If  the  rate 
of  stimulation  is  increased  up  to  the  point  when  the  interval  between 
each  stimulus  is  less  than  the  duration  of  the  entire  contraction  pro- 
cess, the  muscle  does  not  have  time  to  completely  relax  before  the 
arrival  of  the  succeeding  stimulus,  and  hence  remains  in  a  more  or 
less  contracted  state,  during  which  it  exhibits  a  series  of  alternate 
partial  contractions  and  relaxations.  To  this  condition  of  muscle 
activity  the  term  incomplete  tetanus  or  clonus  is  apphed.  A  graphic 
record  of  an  incomplete  tetanus  is  given  in  Fig.  33. 

In  such  a  tracing  it  is  observed  that  the  second  stimulation,  oc-. 
curring  before  the  muscle  had  time  to  relax,  gave  rise  to  a  second 
contraction,  which  was  superposed  on  the  first;  the  same  result  fol- 
lowed the  third  stimulus,  the  fourth,  the  fifth,  and  so  on.  Owing 
largely  to  this  summation  of  the  contractions  there  is  a  gradual  rise 
in  the  height  of  the  contraction  curve.  This  condition  of  the  muscle, 
viz.,  continued  contraction,  combined  with  diminished  power  of 
relaxation,  is  termed  contracture.  The  tracing  also  shows  that  as 
the  stimulus  continues,  the  base  hne,  that  connecting  the  lowest 
points  of  the  contractions,  gradually  rises  and  takes  the  form  of  a 
curve  which  increases  in  height  with  the  stimulation.  The  apex 
line,  that  connecting  the  highest  points  of  the  contractions,  also  rises 
at  the  same  time,  indicating  a  continuous  increase  in  the  height  of 
the  contractions.  The  duration  of  incomplete  tetanus  depends  on 
a  variety  of  circumstances,  e.  g.,  character  of  muscle,  rate  and  strength 
of  stimulation,  etc.,  but  mainly  on  the  rapidity  with  which  the  muscle 
becomes  fatigued.     With  the  oncoming  of  fatigue  the  muscle  begins 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  87 

to  relax,  and  ultimately  returns  to  its  normal  condition,  notwith- 
standing the  continued  stimulation.  If  the  stimulation  be  with- 
drawn, the  muscle  does  not  at  once  return  to  its  original  length  but 
remains  more  or  less  contracted  for  a  variable  time.  This  contrac- 
tion after  stimulation  is  known  as  the  contraction-remainder. 

If  the  stimulation  be  still  further  increased  in  frequency,  the 
individual  contractions  become  fused  together  and  the  curve  described 
by  the  lever  becomes  a  continuous  hne.  (See  Fig.  7,^.)  Notwith- 
standing the  fact  that  the  individual  contractions  are  no  longer  visible, 
it  can  be  shown  by  other  methods  that  the  muscle  is  undergoing  a 
series  of  shght  alternate  contractions  and  relaxations  or  vibrations 
at  least.  After  a  varying  length  of  time  the  muscle  becomes  fatigued, 
relaxes,  and  returns  to  its  natural  condition  even  though  the  stimu- 
lation continues.  The  number  of  stimuh  per  second  necessary  to 
develop  tetanus  will  depend  under  normal   circumstances   on   the 


Fig.  ^5. — -Curves  Showing  the  Analysis  of  Tetanus  of  a  Frog's  Muscle  (Gas- 
trocnemius). The  numbers  under  the  curves  indicate  the  number  of  shocks  per 
second  appHed  to  the  muscle.  There  is  almost  complete  tetanus  with  twenty- 
five  per  second,  and  it  is  a  little  lower  than  the  previous  one  because  the  muscle 
was  slightly  fatigued. — {Stirling.) 

period  of  duration  of  the  individual  contractions.  The  longer  this 
period,  the  less  the  number  of  stimuli  required,  and  the  reverse. 
Hence  the  number  of  stimuli  will  vary  for  different  classes  of  animals 
and  for  different  muscles  in  the  same  animal,  e.  g.,  2  or  3  for  the 
muscles  of  the  tortoise,  10  for  the  muscles  of  the  rabbit,  15  to  20  for 
the  frog,  70  to  80  for  birds,  330  to  340  for  insects. 

Voluntary  Tetanus. — The  voluntary  contractions  as  they  occur 
in  the  hving  body  are  to  be  regarded  as  states  of  tetanus  more  or  less 
complete;  for  the  simplest  voluntary  contraction,  however  rapidly 
it  takes  place,  has  always  a  longer  duration  than  a  single  con- 
traction caused  by  a  single  induction  shock.  As  tetanus  experi- 
mentally produced  is  the  result  of  a  certain  number  of  successive 
stimulations  per  second,  it  is  assumed  that  a  voluntary  tetanus  is  the 
result  of  the  transmission  to  the  muscles  of  a  certain  number  of  nerve 


88  TEXT-BOOK  OF  PHYSIOLOGY. 

stimuli  per  second.  In  other  words,  the  voluntary  tetanus  is  also  the 
result  of  a  discontinuous  stimulation.  The  number  of  stimuh  trans- 
mitted to  a  muscle  has  been  estimated  by  the  employment  of  the 
graphic  method  to  vary  from  8  to  13  per  second,  10  being  about  the 
average.  Unless  the  contraction  process  of  human  muscle  differs 
from  that  of  frogs,  it  is  difficult,  however,  to  see  how  10  stimulations 
per  second  can  give  rise  to  even  an  incomplete  tetanic  contraction. 

Muscle  Sound. — Providing  a  muscle  be  kept  in  a  state  of  tension 
during  its  contraction,  the  intermittent  variations  in  tension  cause 
the  muscle  to  emit  an  audible  sound.  This  so-called  muscle-sound  or 
tone  is  an  evidence  that  the  stimulation  of  the  muscle  is  not  continu- 
ous, but  discontinuous.  If  the  muscle  is  tetanized  by  induction 
shocks,  the  pitch  of  the  tone  will  correspond  with  the  number  of 
stimuh.  A  voluntary  contraction  is  attended  by  a  tone  having  a 
vibration  frequency  of  about  36  to  40  per  second,  which  is  regarded 
as  the  first  overtone  of  the  muscle  tone,  which  would  have  a  vibration 
frequency  in  consequence  of  from  18  to  20  per  second.  This  was 
formerly  regarded  as  an  indication  of  the  rate  of  stimulation  of  volun- 
tary contraction.     This  view,  however,  is  no  longer  sustained. 


CHEMIC  PHENOMENA. 

The  chemic  changes  which  underlie  the  transformation  of  energy 
in  the  living  muscle  even  when  in  a  state  of  rest  are  active  and  com- 
plex, though  but  little  is  known  as  to  their  exact  character.  As  shown 
by  an  analysis  of  the  blood  flowing  to  and  from  the  resting  muscle, 
it  has,  while  flowing  through  the  capillaries,  lost  oxygen  and  gained 
carbon  dioxid.  The  amount  of  oxygen  absorbed  by  the  muscle 
(9  per  cent.)  is  greater  than  the  amount  of  carbon  dioxid  (6.7  per 
cent.)  given  off.  Notwithstanding  the  relation  of  the  oxygen  ab- 
sorbed to  the  carbon  dioxid  produced,  there  is  no  parallelism  between 
these  two  processes,  as  the  carbon  dioxid  will  be  given  off  in  the 
absence  of  free  oxygen  or  in  an  atmosphere  of  nitrogen. 

In  the  active  or  contracting  muscle  all  the  chemic  changes  are 
increased,  as  shown  both  by  an  increased  absorption  of  oxygen  and 
an  increased  production  of  carbon  dioxid,  though  the  ratio  existing 
between  them  differs  considerably  from  that  of  the  resting  muscle. 
Thus,  according  to  Ludwig,  an  active  muscle  absorbs  12.26  per 
cent,  of  oxygen  and  gives  oft"  10.8  per  cent,  carbon  dioxid.  During 
the  activity  of  a  muscle  its  tissue  changes  from  a  neutral  to  an  acid 
reaction,  from  the  development  of  sarcolactic  acid  and  possibly 
phosphoric  acid.  The  degree  of  the  acidity  depends  to  some  extent 
on  the  duration  of  the  contraction  periods.  Chemic  analysis  of  a 
tetanized  muscle  shows  that  it  contains  less  glycogen  than  a  resting 
muscle,  and  that  it  contains  a  larger  amount  of  water.     Coincident 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  89 

with  muscular  contraction,  the  blood-vessels  become  widely  dilated, 
leading  to  a  large  increase  in  the  blood-supply  and  a  rapid  removal 
of  the  products  of  decomposition. 

Rigor  Mortis. — A  short  time  after  death  the  muscles  pass  into  a 
condition  of  extreme  rigidity  or  contraction  which  lasts  from  one  to 
five  days.  In  this  state  they  offer  great  resistance  to  extension. 
Their  tonicity  disappears,  their  cohesion  diminishes,  and  their  irri- 
tabihty  ceases.  The  time  of  the  appearance  of  this  postmortem 
rigidity  varies  from  a  quarter  of  an  hour  to  seven  hours.  Its  onset 
and  duration  are  influenced  by  the  condition  of  the  muscle  irrita- 
bihty  at  the  time  of  death.  When  the  irritabihty  is  impaired  from 
any  cause,  such  as  chronic  disease  or  defective  blood-supply,  the 
rigidity  appears  promptly  but  is  of  short  duration.  After  death  from 
acute  diseases  it  is  apt  to  be  delayed,  but  will  continue  for  a  longer 
period.  The  rigidity  first  appears  in  the  muscles  of  the  lower  jaw 
and  neck;  next  in  the  muscles  of  the  abdomen  and  upper  extremities; 
finally  in  the  trunk  and  lower  extremities.  It  disappears  in  prac- 
tically the  same  order.  Chemic  changes  of  a  marked  character 
accompany  this  process.  The  muscle  becomes  acid  in  reaction  from 
the  development  of  sarcolactic  acid  and  there  is  a  large  increase  in 
the  amount  of  carbon  dioxid  given  off.  The  immediate  cause  of  the 
rigidity  appears  to  be  coagulation  of  the  myosinogen  within  the  sarco- 
lemma  with  the  formation  of  an  insoluble  proteid,  myosin.  In  the 
early  stages  of  the  coagulation  restitution  is  possible  by  the  circula- 
tion of  arterial  blood  through  the  vessels.  The  final  disappearance 
of  this  postmortem  rigidity  is  due  to  the  action  of  acids  which  render 
the  myosin  soluble,  and  possibly  to  the  action  of  various  micro- 
organisms which  give  rise  to  putrefactive  changes. 

Source  of  the  Muscle  Energy. — Notwithstanding  many  in- 
vestigations, the  nature  of  the  materials  which  are  the  immediate 
source  of  the  muscle  energy  is  not  known.  The  absence  of  any  notice- 
able increase  in  the  quantity  of  urea  or  other  nitrogen-holding  com- 
pounds excreted  renders  it  probable  that  the  energy  does  not  come 
from  the  metaboHsm  of  proteid  materials.  The  marked  production 
of  carbon  dioxid  and  sarcolactic  acid  points  to  the  decomposition  of 
some  unstable  compound,  of  a  carbohydrate  character,  rich  in  carbon 
and  oxygen.  It  has  been  suggested  that  glycogen  furnishes  the 
energy,  inasmuch  as  this  substance,  generally  present  in  muscle,  dis- 
appears during  activity.  A  muscle  which  has  been  tetanized  contains 
less  glvcogen  than  the  corresponding  muscle  at  rest.  A  muscle  which 
has  been  separated  from  the  nervous  system  by  division  of  its  nerves 
and  thus  prevented  from  contracting  accumulates  glycogen.  Bunge 
is  of  the  opinion  that  though  the  carbohydrates  are  the  main,  they  are 
not  the  only  sources  of  muscle  energy.  If  there  is  a  deficiency  or 
absence  of  carbohydrate  food,  the  muscle  will  utihze  fat  and  pro- 


90  TEXT-BOOK  OF  PHYSIOLOGY. 

teid,  for  experiment  has  shown  that  the  available  glycogen  is  entirely 
consumed  the  second  or  third  day.  The  mechanism  by  which  the 
energy  is  liberated,  whether  by  decomposition  or  direct  oxidation,  is 
unknown.  The  fact  that  muscle  will  contract  in  an  atmosphere 
free  of  oxygen,  that  no  free  oxygen  can  be  obtained  from  muscle, 
would  support  the  idea  that  the  mechanism  is  one  of  decomposition. 
Hermann  suggests  that  the  energy  of  a  contraction  is  liberated  by 
the  sphtting  and  subsequent  re-formation  of  a  complex  body  belonging 
neither  to  the  carbohydrates  nor  fats,  but  to  the  proteids — to  this  hypo- 
thetic body  the  term  inogen  is  given.  This  complex  molecule,  the 
product  of  the  nutritive  activity  of  the  muscle-cell  in  undergoing 
decomposition,  would  yield  carbon  dioxid,  sarcolactic  acid,  and  a 
proteid  residue  resembhng  myosin.  On  the  cessation  of  the  con- 
traction the  muscle-cell  recombines  the  proteid  residue  with  oxygen, 
carbohydrates,  and  fats,  and  again  forms  the  energy-holding  com- 
pound, inogen.'  The  phenomena  of  rigor  mortis  support  this  view. 
At  the  moment  of  this  contraction  the  muscle  gives  off  COj  in  large 
amount,  develops  sarcolactic  acid  and  myosin.  There  is  thus  a  close 
analogy  between  the  two  processes ;  in  other  words,  a  contraction  is  a 
partial  death  of  the  muscle.  If  this  view  is  correct,  then  the  oxygen 
is  required  mainly  for  heat  production  through  oxidation  processes. 


THERMIC  PHENOMENA. 

The  potential  energy  Uberated  during  a  contraction  is  transformed 
into  kinetic  energy — viz.,  heat  and  mechanic  motion.  Though  heat 
production  is  taking  place  even  during  the  passive  condition,  prob- 
ably through  oxidation  processes,  it  is  largely  increased  by  muscle 
activity.  The  skeletal  muscle  of  the  frog,  the  gastrocnemius,  shows 
after  tetanization  an  increase  in  temperature  from  0.14°  C.  to  0.18° 
C,  and  after  a  single  contraction  from  0.001°  C.  to  0.005°  C.  The 
amount  of  heat  thus  produced  will  vary  with  a  variety  of  conditions, 
as  strength  of  stimulus,  tension,  work  done,  etc. 

Stimulus. — It  has  been  experimentally  determined  that  an  in- 
crease in  the  strength  of  the  stimulus  from  a  minimal  to  a  maximal 
value  increases  the  amount  of  heat  hberated.  This  is  the  direct  result 
of  increased  chemic  change  naturally  following  increased  stimulation. 

Tension. — The  greater  the  tension  of  a  muscle,  the  greater,  other 
conditions  being  the  same,  is  the  amount  of  heat  liberated.  If  the 
muscle  is  securely  fastened  at  both  extremities  so  that  shortening  is 
practically  impossible  during  the  stimulation,  the  maximum  of  heat 
production  is  reached.  In  the  tetanic  state  the  great  increase  in  tem- 
perature is  due  to  the  tension  of  antagonistic  and  strongly  contracted 
muscles.  In  both  instances,  mechanic  motion  being  prevented,  the 
liberated  energy  is  transformed  into  heat. 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  91 

Mechanic  Work. — If  the  muscle  is  permitted  to  shorten  and 
raise  a  weight,  some  of  the  energy  Hberated  takes  the  form  of  mechanic 
motion.  If  the  weight  is  removed  at  the  height  of  the  contraction, 
external  work  is  accomphshed.  The  greater  the  weight  raised, 
within  limits,  the  greater  is  the  percentage  of  energy  which  takes  the 
direction  of  mechanic  motion.  In  accordance  with  the  law  of  the 
conservation  of  energy,  the  heat  produced,  stated  in  calories,  plus 
the  energy  required  in  the  raising  of  the  weight,  expressed  in  kilogram- 
meters  of  work,  must  equal  the  potential  energy  transformed. 

A  muscle  during  a  tetanic  contraction  of  short  duration  accom- 
plishes more  work  than  during  a  single  contraction;  the  weight  in 
each  case  being  the  same.  In  the  former  condition  the  height  of 
contraction  through  summation,  and  hence  the  work  done,  is  greater 
than  in  the  latter.  The  work  done  by  a  short  tetanic  contraction  may 
be  two  or  three  times  that  of  a  single  contraction,  but  after  the 
muscle  reaches  its  maximum  degree  of  shortening  and  then  con- 
tinues in  a  state  of  tetanus,  no  further  w^ork  is  done.  Internal 
work  is  done,  however,  as  shown  by  an  increase  in  the  temperature. 

When  a  weight  which  is  hfted  by  a  muscle  during  a  single  con- 
traction is  allowed  to  act  on  the  muscle  during  the  relaxation,  no 
external  work  is  accomphshed.  All  the  energy  set  free  manifests 
itself  as  heat.  Internal  work  is  done,  as  shown  by  the  fact  that  the 
muscle  becomes  fatigued. 


ELECTRIC  PHENOMENA. 

Electric  Currents  from  Injured  Muscles. — The  energy  liber- 
ated during  a  muscle  contraction  is  not  only  transformed  into 
heat  and  mechanic  motion,  but  to  some  extent  also  into  electric 
energy.  The  presence  of  points  of  different  potential  on  the 
surface  of  the  muscle,  the  necessary  condition  for  the  development 
of  electric  currents,  is  tested  by  means  of  non-polarizable  elec- 
trodes connected  by  wires  with  a  sensitive  galvanometer  or  capil- 
lary electrometer.  When  such  electrodes  are  brought  in  contact 
with  a  muscle  properly  prepared,  there  is  at  once  developed  and  con- 
ducted to  the  galvanometer  an  electric  current  the  intensity  and  direc- 
tion of  which  are  indicated  by  the  deflection  of  the  galvanometer  needle. 
The  existence  of  this  current  is  most  conveniently  demonstrated  with 
single  muscles  the  fibers  of  which  are  parallel — e.  g.,  the  sartorius,  or 
the  semimembranosus  of  the  frog.  If  the  tendinous  ends  of  either  of 
these  muscles  be  removed  by  a  section  made  at  right  angles  to  the  long 
axis,  a  muscle  prism  is  obtained  which  presents  a  natural  longitudi- 
nal surface  and  two  artificial  transverse  surfaces.  A  hne  drawn  around 
the  surface  of  such  a  muscle  prism  at  a  point  midway  between  the 
two  transverse  sections  constitutes  the  equator. 


92 


TEXT-BOOK  OF  PHYSIOLOGY. 


When  the  natural  longitudinal  and  artificial  transverse  surfaces  are 
connected  with  the  wires  of  a  galvanometer  the  terminals  of  which  are 
provided  with  non-polarizable  electrodes,  an  electric  current  is  at  once 
developed.  In  all  instances  the  current,  as  shown  by  the  deflection 
of  the  needle,  originates  at  the  transverse  surface,  passes  through 
the  muscle  to  the  longitudinal  surface,  thence  through  the  galvan- 
ometer to  the  transverse  surface.  The  longitudinal  surface  is,  there- 
fore, electropositive,  the  transverse  surface  electronegative.  The 
two   points    exhibiting    the    greatest    difference   of    potential,    and 

hence  the  most  powerful  current,  he 
in  the  equator  and  in  the  center  of  the 
transverse  surface.  Currents  of  grad- 
ually diminishing  intensity  are  ob- 
tained when  the  electrode  placed  on 
the  longitudinal  surface  is  removed 
toward  either  end.  Feeble  currents 
are  developed  when  two  points  situ- 
ated at  unequal  distances,  either  on 
corresponding  or  opposite  sides  of  the 
equator,  are  connected;  in  either  case 
the  current  flows  from  the  point  lying 
nearest  the  equator  to  the  point  farth- 
est from  it.  Similar  currents  are  ob- 
tained when  two  points  on  the  cross- 
section  situated  at  unequal  distances 
from  the  central  axis  are  connected,  in 
which  case  the  direction  of  the  current 
will  be  from  the  point  lying  nearest 
the  periphery  toward  the  center.  On 
the  contrary,  no  current  is  developed 
when  two  points  on  the  longitudinal 
surface  equally  distant  from  the  equa- 
tor, or  two  points  on  the  transverse 
surface  equally  distant  from  the  cen- 
tral axis,  are  connected.  Such  points 
are  said  to  be  isoelectric.  These  facts 
are  shown  in  Fig.  34.  The  natural  ends  of  the  muscle,  enclosed  by 
sarcolemma  and  tendon,  do  not  exhibit,  if  carefully  preserved  from 
injury,  the  negativity  characteristic  of  the  artificial  transverse  ends. 
Similar  electric  conditions  are  exhibited  by  the  muscles  of  man 
and  other  mammals,  by  the  muscles  of  birds,  reptiles,  amphibia,  etc. 
The  currents  developed  by  connecting  the  equator  on  the  longitu- 
dinal surface  with  the  axis  of  the  transverse  surface  have  an  electromo- 
tive force  in  the  frog  muscle  of  from  0.037  to  0.075  of  a  Daniell  cell. 
The  electric  currents  in  the  muscle  are  intimately  associated  with 


Fig.  34. — Diagram  to  Illustrate 
THE  Current  in  Muscle. 
The  arrowheads  indicate  the 
direction;  the  thickness  of  the 
lines  indicates  the  strength  of 
the  currents. — {Landois  and 
Stirling.) 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  93 

the  chemic  changes  underlying  its  nutrition,  and  hence  their  intensity 
rises  and  falls  with  all  the  conditions  which  maintain  or  impair  mus- 
cle nutrition  and  irritabihty.  The  currents  observed  in  the  injured 
muscle  during  the  inactive  state  have  been  termed  currents  0}  rest. 
du  Bois-Reymond  regarded  them  as  preexistent,  intimately  connected 
with  the  Hving  condition  of  the  muscle,  and  essential  to  the  performance 
of  its  functions,  and  to  be  explained  by  the  view  that  the  entire  muscle 
is  composed  of  molecules  each  of  which  exhibits  the  same  difference 
of  potential  on  its  longitudinal  and  transverse  surfaces  as  the  muscle 
prism  itself.  Hermann  denies  the  existence  of  currents  in  normal 
resting  muscle  and  attributes  them  to  injuries  of  the  surface,  due  to 
methods  of  preparation,  in  consequence  of  which  the  tissue  dies  and 
becomes  electronegative  to  the  uninjured  area,  which  remains  electro- 
positive. These  currents  Hermann  terms  "demarcation  currents." 
Negative  Variation  of  the  Muscle  Current. — If  a  muscle 
exhibiting  a  current  of  injury  be  excited  to  activity  by  tetanizing  in- 
duced currents  apphed  to 

the  opposite  end  of  the  /"T"^ 

muscle,  it  will  be  ob- 
served that  as  the  con- 
traction wave  passes  over 
the  muscle  there  is  a 
movement  of  the  galvan- 
ometer needle  toward  the 
zero  point,  indicating  a 
diminution  of  the  poten- 
tial on  the  longitudinal  ^"^  ^-l,*-"-- 
surface.       To    this    dimi-      Fig.  35.— The  Negative  Variation  of  the  De- 

v,,,+;^v,   ;v,  +v,^  r.f>.^»,^fV,   r.f  M.A.RCATION    Current.      A.  The    contraction 

nution  m  the  strength  of  ^^^.^^  ^^^^  ^^  .^  ^^^^^^  ^^^^^^^  ^^^  ^1^^^^^^^ 

the      current      the      term  at  B  causes  a  diminution  of  potential. 

negative     variation    was 

given.  On  the  withdrawal  of  the  stimulus  the  needle  again 
returns  in  a  short  time  to  its  former  position.  The  diminution 
of  potential  on  the  longitudinal  surface  of  the  muscle  is  now 
attributed  to  the  passage  of  the  excitation  and  contraction  pro- 
cesses, to  a  temporary  disintegration  of  the  muscle  substance  (Fig. 
35).  With  their  disappearance  and  the  subsequent  restoration  of 
the  nutrition  of  the  muscle,  the  former  electric  condition  returns. 

The  primary  deflection  of  the  galvanometer  needle  is  due  to  the 
demarcation  current  which  arises  as  a  result  of  the  difference  in 
electric  potential  produced  by  the  destructive  chemic  changes  taking 
place  at  the  cut  end  of  the  muscle.  The  negative  variation  is  caused 
by  the  fact  that  the  activity  of  the  muscle,  with  its  attendant  chemic 
changes,  will  always  be  greater  in  the  uninjured  equatorial  region, 
and  hence  will  always  tend  to  counterbalance  the  original  source  of 
difference  in  electric  potential. 


94 


TEXT-BOOK  OF  PHYSIOLOGY. 


Electric  Currents  from  Non-injured  Muscles. — Though  per- 
fectly normal  resting  muscle,  according  to  Hermann,  is  isoelectric, 
nevertheless  electric  currents  are  developed  during  activity  to  which 
he  has  given  the  term  action  currents,  and  which  are  attributed  to 
the  propagation  of  the  contraction  wave. 

^r'j/*Action  Currents.— When  two  isoelectric  points  on  the  longitu- 
dinal surface  of  a  muscle  are  connected  with  a  galvanometer  and  a 
single  stimulus  applied  directly  to  one  extremity,  it  can  be  shown 
that  as  the  contraction  wave  passes  beneath  A,  Fig.  36,  the  muscle- 
tissue  at  that  point  becomes  electronegative  toward  B  and  a  cur- 
rent at  once  passes  through  the  galvanometer  from  B  to  A,  as 
shown  by  the  deflection  of  the  needle  toward  A.  As  the  con- 
traction wave  passes  beneath  B  it  in  turn  becomes  electronegative, 


Fig.  36. — The  Condition  Leading  to  the  Development  of  the  First  Action 

Current. 


and  a  temporary  condition  of  equal  potential  is  established  when  the 
needle  returns  to  the  zero  point.  In  a  very  short  time  the  nutrition  of 
A  is  restored  and  becomes  electropositive  toward  B,  when  a  current 
will  pass  through  the  galvanometer  in  the  opposite  direction  from 
A  to  B,  as  shown  by  the  movement  of  the  needle  toward  B,  Fig.  37. 
As  the  contraction  wave  passes  beyond  B  its  nutrition  is  restored  and 
becomes  of  equal  potential  with  A.  The  term  phasic  is  apphed  to 
these  currents.  The  first  current  flows  in  the  muscle  in  the  direction 
of  progress  of  the  contraction  wave — first  phase ;  the  second  current 
flows  in  the  reverse  direction — second  phase;  the  current  is  therefore 
diphasic.  When  a  muscle  is  tetanized,  there  is  but  a  single  current 
observed,  which,  however,  endures  so  long  as  the  tetanic   contrac- 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


95 


tion  is  maintained.  To  this  current  the  term  decremential  is  given. 
When  a  muscle  is  excited  to  action  by  the  nerve  impulse  which  en- 
ters at  its  center,  two  contraction  waves  are  developed,  one  in  each 
half  of  the  muscle,  and  hence  there  are  two  sets  of  diphasic  action 
currents. 

The  presence  of  action  currents  in  the  muscle  of  the  Hving  body 
during  a  single  contraction  was  demonstrated  by  Hermann  in  the  mus- 
cles of  the  forearm.  The  arrangement  of  the  experiment  was,  briefly, 
as  follows :  The  forearm  was  surrounded  by  two  twine  electrodes  sat- 
urated with  zinc  solution,  one  being  placed  at  the  physiologic  middle 
— the  nervous  equator — the  other  at  the  wrist.  Both  electrodes  were 
then  connected  with  the  galvanometer.  When  the  brachial  plexus  was 
stimulated  in  the  axillary  space,  the  deflections  of  the  galvanometer 


Fig  37. — The  Condition  Leading  to  the  Development  of  the  Second  Action 

Current. 


needle,  when  analyzed  with  the  repeating  rheotome,  indicated  phasic 
currents  with  a  single  contraction.  In  the  first  phase — atterminal — 
the  wrist  became  positive  and  the  current  passed  in  the  muscle  toward 
its  termination;  and  in  the  second — abterminal — it  became  negative 
and  the  current  now  passed  in  the  reverse  direction.  The  action 
currents  which  are  observed  in  the  frog's  muscle  were  thus  shown 
to  be  present  in  the  living  human  muscle,  with  this  difference,  how- 
ever: that  the  second  phase, — abterminal, — instead  of  being  weaker 
in  man,  is  equally  strong  with  the  atterminal.  This  experiment  also 
revealed  the  fact  that  the  rapidity  of  propagation  of  the  excitation 
wave  was  much  greater  in  man,  amounting  to  about  twelve  meters 
per  second.  Hermann  therefore  denies  the  preexistence  of  electric 
currents  and  regards  them  as  due  to  localized  temporary  disintegra- 


96  TEXT-BOOK  OF  PHYSIOLOGY. 

tion  of  the  muscle  in  consequence  of  activity,  as  they  disappear  on 
the  restoration  of  the  muscle  to  its  normal  condition. 

Work  Done  Daily. — The  muscle  system  in  its  entirety  is  to  be 
regarded  as  a  machine  for  the  transformation  of  potential  into  kinetic 
energy,  and  in  so  doing  accomphshes  work.  Through  the  inter- 
mediations of  the  bones  of  the  skeleton  which  play  the  part  of  levers 
the  individual  not  only  changes  his  position  in  space,  but  overcomes 
to  some  extent  the  resistances  offered  by  the  environment.  The 
employment  of  artificial  levers,  tools,  as  distinguished  from  natural 
levers,  bones,  materially  adds  to  the  effectiveness  of  the  muscle 
machine.  The  amount  of  work  which  a  man  of  average  physical 
development  weighing  72  kilos  can  perform  in  eight  hours  has  been 
variously  estimated.  It  will  naturally  vary  according  to  the  character 
of  the  occupation.  If  the  work  done  be  calculated  from  the  number 
of  kilograms  raised  one  meter,  the  average  laboring-man  performs 
about  300,000  kilogrammeters. 


SPECIAL  ACTION  OF  MUSCLE  GROUPS. 

The  individual  muscles  of  the  axial  and  appendicular  portions  of 
the  body  are  named  with  reference  to  their  shape,  action,  structure, 
etc.;  e.  g.,  deltoid,  flexor,  penniform,  etc.  In  different  localities  a 
group  of  muscles  having  a  common  function  is  named  in  accordance 
with  the  kind  of  motion  it  produces  or  to  which  it  gives  rise:  e.  g., 
groups  of  muscles  which  alternately  diminish  or  increase  the  angular 
distance  between  two  bones  are  known  respectively  as  flexors  and 
extensors;  such  muscle  groups  are  usually  found  in  association 
with  ginglymus  joints.  Muscles  which  rotate  the  bone  to  which 
they  are  attached  around  its  own  axis  without  producing  any  great 
change  of  position  are  known  as  rotators,  and  are  found  in  association 
with  enarthrodial  or  ball-and-socket  joints.  Muscles  which  impart 
an  angular  movement  to  the  extremities  to  and  from  the  median  line 
of  the  body  are  termed  adductors  and  abductors  respectively. 

In  addition  to  the  actions  of  individual  groups  of  muscles  in  pro- 
ducing special  movements,  in  some  regions  of  the  body,  several 
groups  of  muscles  are  coordinated  for  the  accomplishment  of  certain 
definite  functions;  e.  g.,  the  functions  of  respiration,  mastication,  etc. 
The  coordination  of  axial  and  appendicular  muscles  enables  the 
individual  to  assume  certain  postures,  such  as  standing,  sitting,  and 
lying;  to  engage  in  various  acts  of  locomotion,  as  walking,  running, 
dancing,  swimming. 

Levers. — The  function  or  special  mode  of  action  of  individual 
muscles  can  be  understood  only  when  the  bones  with  which  they  are 
connected  are  regarded  as  levers  whose  fulcra  or  fixed  points  lie  in 
the  joints  where  the  movement  takes  place,  and  the  muscles  as  sources 


w 

F 

A 

P 

i 

A 

W 

1 

P 

(0 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  97 

of  power  for  imparting  movement  to  the  levers  with  the  object][of 
overcoming  resistance. 

In  mechanics  levers  of  three  kinds  or  orders  are  recognized 
according  to  the  relative  positions  of  the  fulcrum  or  axis  of  motion, 
the  applied  power,  and  the  weight  to  be  moved.     (See  Fig.  38.) 

In  levers  of  the  first  order  the  fulcrum,  F,  lies  between  the  weight 
or  resistance,  W,  and  the  power  or  moving  force,  P.     The  distance 
P  F  is  known  as  the  power  arm  and  the  dis- 
tance W  F  as  the  weight  arm.     As  examples 
of  this  form  of  lever  found  in   the  human 
body  may  be  mentioned : 

1.  The   elevation   of   the   trunk   from   the       .  _ 

flexed   position.      The   axis   of    move-      a  W  p^^^ 

ment,  the  fulcrum,  lies  in  the  hip-joint; 

the  weight,  that  of  the  trunk,  acting  as      ^ 5 — t(3) 

if  concentrated  at  the  center  of  gravity,  ^^^  38.— The  Three  Or- 
which  lies  close  to  the  tenth  dorsal  ver-  '  ders  of  Levers. 

tebra;  the  power,  the  muscles  attached 

to  the  tuberosity  of  the  ischium.  The  opposite  movement  is 
equally  one  of  the  first  order,  but  the  relative  positions  of  P  and 
W  are  reversed. 

2.  The  head  in  its  movement  backward  and  forward  on  the  atlas. 
In  levers  of  the  second  order  the  weight  Hes  between  the  power 

and  the  fulcrum.     As  illustration  of  this  form  of  lever  may  be  men- 
tioned : 

1.  The  depression  of  the  lower  jaw,  in  which  movement  the  fulcrum  is 

the  temporomaxillar\'  articulation;  the  resistance,  the  tension 
of  the  elevator  muscles;  the  power,  the  contraction  of  the  de- 
pressor muscles. 

2.  The  raising  of  the  body  on  the  toes,  in  which  movement  the  ful- 

crum is  the  toes,  the  weight  that  of  the  body  acting  through  the 
ankle,  the  power  the  gastrocnemius  muscle  applied  to  the  heel 
bone. 
In  levers  of  the  third  order  the  power  is  applied  at  a  point  lying 

between  the  fulcrum  and  the  weight.     As  examples  of  this  form  of 

lever  may  be  mentioned : 

1.  The  flexion  of  the  forearm,  in  which  the  fulcrum  is  the  elbow- 

joint,  the  power  the  biceps  and  brachialis  anticus  muscles  ap- 
phed  at  their  points  of  insertion,  the  weight  that  of  the  forearm 
and  hand. 

2.  The  extension  of  the  leg  on  the  thigh. 

When  levers  are  employed  in  mechanic  operations,  the  object 

aimed  at  is  the  overcoming  of  a  great  resistance  by  the  application  of 

a  small  force  acting  through  a  great  distance,  so  as  to  obtain  mechanic 

advantage.     In  the  mechanism  of  the  human  body  the  reverse  gener- 

7 


98  TEXT-BOOK  OF  PHYSIOLOGY. 

ally  obtains,  viz.,  the  overcoming  of  a  small  resistance  by  the  appli- 
cation of  a  large  force  acting  through  a  short  distance.  As  a  result 
there  is  a  gain  in  the  extent  and  rapidity  of  the  movement  of  the 
lever.  The  power,  however,  owing  to  its  point  of  application,  acts 
at  a  great  mechanic  disadvantage  in  many  instances,  especially  in 
levers  of  the  third  order. 

Postures. — Owing  to  its  system  of  joints,  levers,  and  muscles  the 
human  body  can  assume  a  series  of  positions  of  equilibrium,  such  as 
standing  and  sitting,  to  which  the  term  posture  has  been  given.  In 
order  that  the  body  may  remain  in  a  state  of  stable  equilibrium  in 
any  posture,  it  is  essential  that  the  vertical  line  passing  through  its 
center  of  gravity  shall  fall  within  the  base  of  support. 

Standing  is  that  position  of  equilibrium  in  which  a  line  drawn 
through  the  center  of  gravity  of  the  entire  body  falls  within  the  base 
of  support.  This  position  is  maintained  largely  by  the  mechanical 
conditions  of  the  joints,  apparently  for  the  purpose  of  reducing  to  a 
minimum  muscular  action,  so  that  it  can  be  prolonged  for  some  time 
without  giving  rise  to  fatigue.  In  the  military  position,  which  may 
be  assumed  as  the  normal  position,  all  the  joints  must  be  in  such  a 
condition  of  extension  and  fixation  that  the  body  will  represent  a 
rigid  column  resting  on  the  astragalus  and  supported  by  the  arch  of 
the  foot.     This  is  accomplished : 

1.  By  balancing  the  head  on  the  apex  of  the  vertebral  column.     This 

is  done  by  the  action  of  the  muscles  on  the  back  of  the  neck. 
The  muscular  effort  is,  however,  very  sHght,  as  the  center  of 
gravity  of  the  head  lies  but  a  short  distance  in  front  of  the 
articulation. 

2.  By  making  the  vertebral  column  erect  and  rigid.     This  is  brought 

about  by  the  action  of  the  common  extensor  muscles  of  the  trunk. 
In  this  condition  the  center  of  gravity  lies  just  in  front  of  the 
tenth  dorsal  vertebra.     The  head,  trunk,  and  upper  extremities 
are  now  supported  by  the  hip-joints;  and  in  order  that  this  sup- 
port may  give  to  the  body  a  certain  degree  of  stable  equilibrium, 
independent  of  muscular  action,  the  line  of  gravity  falls  behind  the 
line  uniting  the  center  of  rotation  of  the  two  joints.     In  conse- 
quence the  body  would  fall  backward  were  it  not  prevented  by 
the  tension  of  the  iliofemoral  hgament  and  the  fascia  lata. 
The  line  of  gravity,  continued  downward,  passes  through  the  knee- 
joint  posterior  to  the  axis  of  rotation,  and  hence  the  body  would  now 
fall  backward  were  it  not  prevented  by  the  tension  of  the  lateral 
ligaments  and  the  contraction  of   the  quadriceps  femoris   muscle. 
Though  the  body  is  supported  by  the  astragalus,  the  line  of  grav- 
ity does  not  pass  through  the  line  uniting  the  two  joints,  for  in  so 
doing  constant  muscular  effort  would  be  required  to  maintain  stable 
equilibrium;  passing  a  short  distance  in  advance  of  this  Hne,  there 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  99 

would  be  a  tendency  of  the  body  to  fall  forward,  which  is  prevented 
by  the  extensor  muscles  of  the  foot.  When  the  body  is  in  the  erect 
or  mihtary  position,  the  center  of  gravity  lies  between  the  sacrum 
and  last  lumbar  vertebra.  Standing  is  thus  an  act  of  balancing,  and 
requires  not  only  the  static  conditions  of  joints,  but  the  dynamic 
conditions  of  various  groups  of  muscles,  and  hence  is  not  a  position 
of  absolute  ease  and  cannot  be  maintained  for  any  length  of  time 
without  experiencing  discomfort  and  fatigue.  Sitting  erect  is  an 
attitude  of  equilibrium  in  which  the  body  is  balanced  on  the  tubera 
ischii,  when  the  head  and  trunk  together  form  a  rigid  column. 

Locomotion  is  the  act  of  transferring  the  body  as  a  whole  through 
space,  and  is  accomplished  by  the  combined  action  of  its  own  muscles. 
The  acts  involved  consist  of  walking,  running,  jumping,  etc. 

Walking  is  a  complicated  act  involving  almost  all  the  voluntary 
muscles  of  the  body  either  for  purposes  of  progression  or  for  bal- 
ancing the  head  and  trunk,  and  may  be  defined  as  a  progression  in 
a  forward  horizontal  direction  due  to  the  alternate  action  of  both 
legs.  In  walking  one  leg  becomes  for  the  time  being  the  active  or 
supporting  leg,  carrying  the  trunk  and  head;  the  other  the  passive 
but  progressing  leg,  to  become  in  turn  the  active  leg  when  the  foot 
touches  the  ground.  Each  leg  is  therefore  alternately  in  an  active 
and  passive  state. 

Running  is  distinguished  from  walking  by  the  fact  that  at  a  given 
moment  both  feet  are  off  the  ground  and  the  body  is  raised  in  the  air. 


THE  VISCERAL  MUSCLE. 

The  visceral  muscle,  as  the  name  implies,  is  found  in  the  walls 
of  hollow  viscera,  where  it  is  arranged  in  the  form  of  a  membrane 
or  sheet.  It  is  present  in  the  walls  of  the  alimentary  canal,  blood- 
vessels, respiratory  tract,  ureter,  bladder,  vas  deferens,  uterus, 
fallopian  tubes,  iris,  etc.  In  some  situations  it  is  especially  thick 
and  well  developed — e.  g.,  uterus  and  pyloric  end  of  the  stomach;  in 
others  it  is  thin  and  slightly  developed. 

The  Histology  of  the  Visceral  Muscle-fiber. — When  examined 
with  the  microscope,  the  muscle  sheet  is  seen  to  be  composed  of 
fibers,  narrow,  elongated,  and  fusiform  in  shape.  As  a  rule,  they 
are  extremely  small,  measuring  only  from  40  to  250  micromillimeters 
in  length  and  from  4  to  8  micromillimeters  in  breadth.  The  center 
of  each  fiber  presents  a  narrow,  elongated  nucleus.  The  muscle- 
protoplasm  w^hich  makes  up  the  body  of  the  fiber  appears  to  be 
enclosed  by  a  delicate  elastic  membrane  resembling  in  some  respects 
the  sarcolemma  of  the  skeletal  muscle.  In  some  animals  the  visceral 
fiber  presents  a  longitudinal  striation  suggesting  the  existence  of 
fibrillae  surrounded  by  sarcoplasm  (Fig.  39).     The  fibers  are  united 


lOO 


TEXT-BOOK  OF  PHYSIOLOGY. 


longitudinally  and  transversely  by  a  cement  material.  The  muscle 
is  increased  in  thickness  by  the  superposition  of  successive  layers. 
At  varying  intervals  the  fibers  are  grouped  into  bundles  or  fasciculi 
by  septa  of  connective  tissue  (Fig.  40).  Blood-vessels  ramify  in  the 
connective  tissue  and  furnish  the  necessary  nutritive  material. 

The  visceral  muscle  receives  stimuli  from  the  spinal  cord,  not 
directly,  however,  as  in  the  case  of  the  skeletal  muscle,  but  indirectly 


Fig.  39. — Two  Smooth  Muscle-fibers  from  Small  Intestine  of  Frog.  X  240. 
Isolated  with  35  per  cent,  potash-lye.  The  nuclei  have  lost  their  characteristic 
form  through  the  action  of  the  lye. — (Stohr.) 


through  the  intermediation  of  ganghon  cells,  vv^hich  may  be  located 
at  some  distance  from  the  muscle  or  near  the  walls  of  the  viscera. 
Non-medullated  fibers  from  the  ganglion  pass  directly  into  the 
muscle,  where  they  frequently  unite  to  form  a  general  plexus.  From 
this  plexus  fine  branches  take  their  origin  and  ultimately  become 
physiologically  associated  with  the  muscle-fiber. 

Physiologic  Properties. — The  visceral  muscles  which  have 
been  subjected  to  experiment  are  mainly  those  of  the  stomach,  in- 
testine, bladder,  ureter,  and  iris.     From  the  results  of  the  experiments 

which  have  been  published,  it  is 
evident  that  all  visceral  muscles 
possess  elasticity,  tonicity,  irrita- 
bihty,  and  conductivity. 

The  elasticity  of  the  bladder 
muscle  of  the  cat  was  strikingly 
shown  in  the  experiments  pub- 
lished by  Dr.  Colin  C.  Stewart. 
When  this  muscle  was  weighted 
with  weights  differing  by  a  com- 
mon increment,  it  was  extended 
on  the  addition  of  each  weight, 
though  to  a  progressively  less  extent.  On  the  removal  of  the  weights 
the  muscle  eventually  returned  to  its  former  length.  The  records  of 
the  extension  were  similar  to,  if  not  identical  with,  those  of  the  skele- 
tal muscle. 

Tonicity  is  a  property  common  to  all  visceral  muscles.  Each 
muscle  is  continuously  in  a  state  of  contraction  intermediate  between 
that  of  complete  contraction  and  that  of  relaxation.  In  how  far  this 
is  due  to  local  and  inherent  causes  or  to  stimuli  reflected  from  the 


Connective-tissue 
septum. 


Nucleus. 

Smooth  muscle-fiber 
in  transverse  section. 


Fig.  40. — Section  of  the  Circular 
Layer  of  the  Muscular  Coat  of 
THE  Human  Intestine.  — {Stohr.) 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  loi 

nervous  system  as  a  result  of  peripherally  acting  causes  is  not  in 
individual  instances  readily  determinable.  From  time  to  time  the 
tonicity  varies,  increasing  and  decreasing  in  response  to  these  various 
stimuh  and  in  accordance  with  the  functional  activities  of  the  organs 
in  which  the  muscle  is  found. 

The  irritability  manifests  itself  by  a  change  of  form,  and  doubt- 
less by  the  hberation  of  heat  on  the  appHcation  of  any  form  of  stimulus 
— ^mechanic,  chemic,  thermic,  electric. 

The  conductivity  is  less  marked  in  the  visceral  than  in  the  skeletal 
muscle,  and,  contrary  to  what  is  observed  in  the  latter,  the  conduction 
extends  laterally  as  well  as  longitudinally  from  fiber  to  iiber.  This 
is  shown  by  stimulation  of  the  exposed  intestine.  Shortly  after  the 
stimulus  is  applied  the  muscle  contracts  longitudinally — i.  e.,  in  a 
direction  at  right  angles  to  the  long  axis  of  the  intestine,  partially 
obliterating  its  lumen.  From  this  point  the  conduction  process  indi- 
cated by  the  contraction  wave  passes  in  opposite  directions  for  some 
distance  along  the  canal.  As  to  whether  this  is  accomplished  by 
protoplasmic  processes  extending  from  fiber  to  fiber,  or  whether  the 
uniting  membrane  differs  in  conducting  power  from  the  sarcolemma, 
is  as  yet  a  matter  of  doubt.  From  the  fact  that  the  upper  two-thirds 
of  the  ureter,  though  free  of  nerve-cells,  exhibits  lateral  conduction, 
it  is  evident  that  it  may  take  place  independent  of  the  nervous  system. 

The  Contraction  of  the  Visceral  Muscle. — The  general  character 
of  the  contraction  may  be  witnessed  on  opening  the  abdomen  of  a 
recently  killed  animal,  especially  the  rabbit.  Shortly  after  exposure 
to  the  air  the  walls  of  the  intestine  begin  to  contract  in  a  most  vig- 
orous manner.  The  contraction  wave  beginning  at  various  points 
is  propagated  in  both  directions,  running  along  the  intestinal  wall 
for  a  variable  distance.  A  succession  of  similar  waves  may  be  ob- 
served for  some  minutes.  To  the  alternate  contraction  and  relaxa- 
tion of  the  muscle-fibers,  which  are  circularly  arranged,  the  term 
peristalsis  is  usually  given.  The  excised  stomach  of  a  dog  kept 
under  suitable  conditions  will  exhibit  similar  movements.  The 
same  holds  true  of  the  bladder  muscle  of  the  cat,  the  muscle  of  the 
ureter,  etc.  Careful  observation  shows  a  certain  periodicity  in  the 
movements.  Inasmuch  as  the  cause  is  not  apparent,  these  contrac- 
tions are  termed  spontaneous  or  automatic. 

Graphic  Record  of  the  Contraction. — For  experimental  pur- 
poses narrow  transverse  sections  of  the  stomach  of  the  frog  or  the 
entire  bladder  muscle  of  the  cat,  excised  or  in  situ,  according  to  the 
method  of  Dr.  Colin  C.  Stewart,  may  be  employed.  If  kept  moist, 
they  will  retain  their  irritabihty  for  some  hours.  The  changes  of 
form  may  be  recorded  with  the  usual  muscle  lever.  When  thus  pre- 
pared, the  muscle  may  exhibit  for  several  hours  a  series  of  pulsa- 
tions, rhythmic  in  character.     With  spontaneously  acting  mammahan 


I02 


TEXT-BOOK  OF  PHYSIOLOGY. 


muscle  the  contraction  and  relaxation  periods  are  of  equal  duration. 
With  the  amphibian  muscle  they  are  of  unequal  duration,  as  a  rule. 
In  both  classes  of  animals  the  character  of  the  record,  a  succession 
of  large  and  small  contractions,  would  indicate  that  the  general 
rhythmic  movement  is  compounded  of  two  or  three  secondary 
rhythms  which  differ  in  rate  and  character.  A  single  pulsation  may 
be  recorded  by  stimulating  the  bladder  muscle  with  the  induced  or  the 
make  and  break  of  the  constant  current.  A  curve  of  such  a  contrac- 
tion is  shown  in  Fig.  41.  The  contraction  takes  place  more  rapidly 
than  the  relaxation;  the  two  phases  occupying  five  and  thirty- five 
seconds  respectively.  The  latent  period  covered  0.25  second.  With 
other  muscles  the  time  relations  are  slightly  different.  Tetanization 
of  the  bladder  muscle  of  the  cat  occurred  when 
the  stimuh  succeeded  each  other  with  a  certain 
rapidity;  the  interval  between  stimuli  approxi- 
mating a  period  somewhat  less  than  two  sec- 
onds. This  muscle  responds  to  variations  in 
temperature,  to  strength  of  stimulus,  to  the 
load,  in  a  manner  similar  to,  if  not  identical 
with,  the  skeletal  muscle  . 

The  Function  of  the  Visceral  Muscle. — 
In  a  general  way  it  may  be  said  that  the  vis- 
ceral muscle  determines  and  regulates  the  pas- 
sage through  the  viscus  or  organ  of  the  material 
contained  within  it.  The  food  in  the  stomach 
and  intestines  is  subjected  to  a  churning  pro- 
cess by  the  muscles,  in  consequence  of  which 
the  digestive  fluids  are  more  thoroughly  incor- 
porated and  their  characteristic  action  in- 
creased. At  the  same  time  the  food  is  carried 
through  the  canal,  the  absorption  of  the  nutri- 
tive material  promoted,  and  the  indigestible 
residue  removed  from  the  body.  The  blood 
is  delivered  in  larger  or  smaller  volumes  ac- 
cording to  the  needs  of  the .  tissues  through  a  relaxation  or  contrac- 
tion of  the  muscle-fibers  of  the  blood-vessels.  The  urine  is  forced 
through  the  ureter  and  from  the  bladder  by  the  contraction  of  their 
respective  muscles.  The  mode  of  action  of  the  individual  muscles 
will  be  described  in  successive  chapters. 

Ciliary  Movement. — The  free  surface  of  the  epithehum  cover- 
ing the  mucous  membrane  in  certain  regions  of  the  body  is  charac- 
terized by  the  presence  of  delicate  filamentous  processes  termed 
cilia.  (See  Fig.  42.)  Cihated  epithelium  is  found  in  man  and 
mammals  generally,  in  the  nose,  Eustachian  tube,  larynx,  with  the 
exception  of  the  vocal  membranes,  trachea  and  bronchial  tubes  as 


ii,r;Mininir»ii»iii[rnriiiiir;rN..ii 

Fig.  41. — The  Curve 
OF  Contraction 
OF  THE  Bladder 
Muscle  at  Body- 
temperature  in 
Response  to  a 
Single  Induction 
Current.  The 
time  is  indicated 
in  seconds. — 
(Slewart.) 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  103 

far  as  the  pulmonary  lobules,  Fallopian  tubes,  uterus,  and  epididymis. 
The  lumen  of  the  central  canal  of  the  spinal  cord  and  the  cavities  of 
the  brain  are  lined,  especially  in  childhood,  by  cells  provided  with 
similar  ciHa.  Cihated  epithelium  is  also  found  in  all  classes  of  ani- 
mals, and  especially  in  the  invertebrates. 

The  ciha  found  in  the  human  body  vary  in  length  from  0.003  ^^• 
to  0.005  ^^^^-  They  are  apparently  structureless  and  colorless,  and 
appear  to  have  their  origin  in  and  to  be  a  prolongation  of  a  trans- 
parent material  on  the  outer  surface  of  the  cell  material.  The 
number  of  ciha  present  on  the  surface  of  any  individual  cell  varies 
approximately  from  five  to  twenty-five.  When  cihated  epithehal  cells, 
freshly  removed  from  the  mucous  membrane  and  moistened  with 
normal  sahne,  are  examined  with  the  microscope,  it  will  be  found 
that  the  ciha  are  in  continuous  and  rapid  vibratile  movement,  so 
much  so  that  the  individual  cihum  cannot  be  distinguished.  In 
time,  however,  their  vitahty  declines  and  the  rapidity  of  movement 
diminishes.  When  the  movement  of  the  individual  cihum  falls  to 
about  eight  or  ten  per  second,  its  character 
can  be  readily  determined.  It  will  then  be 
seen  that  the  movement  is,  as  a  rule,  alter- 
nately a  backward  and  a  forward  one,  the 
cihum  lowering  and  then  raising  itself,  the 
latter  taking  place  more  quickly  and  ener- 
getically than  the  former.  As  the  cihum 
raises  itself  it  becomes  somewhat  flexed  in 
a  direction  corresponding  to  that  of  the 
general  movement.  The  movement,  how-  fig.  42.— Ciliated  Epi- 
ever,  varies  in  character  in  different  situa-  thelium. 

tions  and  in  different  animals.     The  cause 

of  the  movements  and  the  mechanism  of  their  coordination  are 
unknown.  They  are,  as  far  as  known,  independent  of  the  nervous 
system.  The  force  of  ciliary  motion  is  very  great.  A  load  of  twenty 
grams  can  be  supported  and  carried  forward  by  the  cilia  on  the 
mucous  membrane  of  the  mouth  and  esophagus  of  the  frog.  The 
activity  of  the  ciha  is  associated  with  the  nutrition  of  the  cell  of 
which  they  are  a  part  and  rises  and  falls  with  it.  Experimentally 
it  has  been  found  that  the  rate  and  energy  of  the  movement  are 
greatest  at  a  temperature  of  about  35°  to  40°  C,  especially  if  they 
are  bathed  with  normal  saline,  rendered  slightly  alkahne.  Low 
temperatures,  acids,  alkahes,  carbon  dioxid,  etc.,  retard  the  move- 
ment. 

■  The  function  of  the  ciha,  though  not  always  apparent,  is  asso- 
ciated with  the  function  of  the  passages  in  which  they  are  found.  As 
the  surfaces  of  these  passages  are  swept  by  a  current  of  considerable 
power,  it  is  probable  that  they  assist  in  the  passage  of  the  materials 


I04  TEXT-BOOK  OF  PHYSIOLOGY. 

which  ordinarily  traverse  them.  Mucus  and  particles  of  dust  are 
carried  upward  through  the  air-passages;  the  ovarian  cell  is  carried 
from  the  ovary  toward  the  uterus;  the  spermatozoa,  as  well  as  the 
fluid  in  which  they  are  contained,  are  carried  forward  through  the 
epididymis  ducts. 


CHAPTER  VII. 
THE  GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE. 

The  nerve-tissue,  which  unites  and  coordinates  the  various 
organs  and  tissues  of  the  body  and  brings  the  individual  into  relation- 
ship with  the  external  world,  is  arranged  in  two  systems,  termed 
the  encephalospinal  or  cerebrospinal  and  the  sympathetic. 

The  encephalospinal  system  consists  of: 

1.  The  brain  and  spinal  cord,  contained  within  the  cavities  of  the 

cranium  and  the  spinal  column  respectively,  and 

2.  The  cranial  and  spinal  nerves. 

The  sympathetic  system  consists  of: 

1.  A  double  chain  of  ganglia  situated  on  each  side  of  the  spinal  column 

and  extending  from  the  base  of  the  skull  to  the  tip  of  the  coccyx. 

2.  Various  collections  of  ganglia  situated  in  the  head,  face,  thorax, 

abdomen,  and  pelvis.  All  these  ganglia  are  united  by  an  elab- 
orate system  of  intercommunicating  nerves,  many  of  which  are 
connected  with  the  cerebrospinal  system. 

HISTOLOGY  OF  NERVE-TISSUE. 

The  Neuron. — The  nerve-tissue  has  been  resolved  by  the  in- 
vestigations of  modern  histologists  into  a  single  morphologic  unit,  to 
which  the  term  neuron  has  been  apphed.  The  entire  nerve  system 
has  been  shown  to  be  but  an  aggregate  of  an  infinite  number  of 
neurons,  each  of  which  is  histologically  distinct  and  independent. 
Though  having  a  common  origin,  as  shown  by  embryologic  investi- 
gations, they  have  acquired  a  variety  of  forms  in  different  parts  of  the 
nervous  system  in  the  course  of  development.  The  old  conception 
that  the  nerve  system  consisted  of  two  distinct  histologic  elements, 
nerve-cells  and  nerve-fibers,  which  differed  not  only  in  their  mode  of 
origin,  but  also  in  their  properties,  their  relation  to  each  other,  and 
their  functions,  has  been  entirely  disproved. 

The  neuron,  or  neurologic  unit,  is  histologically  a  nerve-cell,  the 
surface  of  which  presents  a  greater  or  less  number  of  processes  in 
varying  degrees  of  differentiation.  As  represented  in  figure  43,  A,  the 
neuron  may  be  said  to  consist  of:  (i)  The  nerve-cell,  neurocyte,  or 
corpus;  (2)  the  axon,  or  nerve  process;  (3)  the  end-tufts,  or  terminal 
branches.  Though  these  three  main  histologic  features  are  every- 
where recognizable,  they  exhibit  a  variety  of  secondary  features  in 

105 


io6 


TEXT-BOOK  OF  PHYSIOLOGY. 


different  situations  in  accordance  with  peculiarities  of  function. 
Though  the  nerve-cell  and  the  nerve-fiber  are  but  part  of  the  same 
neuron,  it  is  convenient  at  present  to  describe  them  separately. 

The  Nerve-cell. — The  nerve-cell,  or  body  of  the  neuron,  presents 
a  variety  of  shapes  and  sizes  in  different  portions  of  the  nervous 
system.  Originally  ovoid  in  shape,  it  has  acquired,  in  course  of  de- 
velopment, pecuharities  of  form  which  are  described  as  pyramidal, 
stellate,  pear-shaped,  spindle-shaped,  etc.  The  size  of  the  cell 
varies  considerably,  the  smallest  having  a  diameter  of  not  more  than 
■g-Q^Q-jj  of  an  inch,  the  largest  not  more  than  -^}y^  of  an  inch.     Each  cell 


1^" 


Ferminal 
nches. 


Xeurilemma. 


.  Xerve-cell. 


Terminal 
branches. 


Fig.  43. — A.  Efferent  neuron 


Afferent  neuron. 


consists  of  granular,  striated  protoplasm,  containing  a  distinct  ves- 
icular nucleus  and  a  well-defined  nucleolus.  A  cell  membrane  has 
not  been  observed.  From  the  surface  of  the  adult  cell  portions  of 
the  protoplasm  are  projected  in  various  directions,  which  portions, 
rapidly  dividing  and  subdividing,  form  a  series  of  branches,  termed 
dendrites  or  dendrons.  In  some  situations  the  ultimate  branches  of 
the  dendrites  present  short  lateral  processes,  known  as  lateral  buds, 
or  gemmules,  which  impart  to  the  branches  a  feathery  appearance. 
This  characteristic  is  common  to  the  cells  of  the  cortex  of  the  cere- 
brum and  of  the  cerebellum.     The  ultimate  branches  of  the  den- 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  107 

drites,  though  forming  an  intricate  feltwork,  never  anastomose  with 
one  another  nor  unite  with  dendrites  of  adjoining  cells.  According 
to  the  number  of  axons,  nerve-cells  are  classified  as  monaxonic, 
diaxonic,  polyaxonic.  Most  of  the  cells  of  the  nervous  system  of  the 
higher  vertebrates  are  monaxonic.  In  the  gangha  of  the  posterior 
or  dorsal  roots  of  the  spinal  and  cranial  nerves,  however,  they  are 
diaxonic.  In  this  situation  the  axons,  emerging  from  opposite  poles 
of  the  cell,  either  remain  separate  and  pursue  opposite  directions,  or 
unite  to  form  a  common  stem,  which  subsequently  divides  into  two 
branches,  w^hich  then  pursue  opposite  directions.  (See  Fig.  43,  B.) 
The  nerve-cell  maintains  its  own  nutrition,  and  presides  over  that  of 
the  dendrites  and  the  axon  as  well.  If  the  latter  be  separated  in  any 
part  of  its  course  from  the  cell,  it  speedily  degenerates  and  dies. 

The  axon,  or  nerve  process,  arises  from  a  cone-shaped  projection 
from  the  surface  of  the  cell,  and  is  the  first  outgrowth  from  its  pro- 
toplasm. At  a  short  distance  from  its  origin  it  becomes  markedly 
differentiated  from  the  dendrites  which  subsequently  develop.  It 
is  characterized  by  a  sharp,  regular  outhne,  a  uniform  diameter,  and 
a  hyahne  appearance.  In  structure,  the  axon  appears  to  consist  of 
fine  fibrillae  embedded  in  a  clear,  protoplasmic  substance.  Shafer 
advocates  the  view  that  the  fibrillag  are  exceedingly  fine  tubes  filled 
with  fluid.  The  axon  varies  in  length  from  a  few  milhmeters  to  one 
meter.  In  the  former  instance  the  axon,  at  a  short  distance  from  its 
origin,  divides  into  a  number  of  branches,  which  form  an  intricate 
feltwork  in  the  neighborhood  of  the  cell.  In  the  latter  instance 
the  axon  continues  for  an  indefinite  distance  as  an  individual  struc- 
ture. In  its  course,  however,  especially  in  the  brain  and  spinal  cord, 
it  gives  off  a  number  of  collateral  branches,  which  possess  all  its  his- 
tologic features.  The  long  axons  serve  to  bring  the  body  of  the  cell 
into  direct  relation  with  peripheral  organs,  or  with  more  or  less  re- 
mote portions  of  the  nerve  system,  thus  constituting  association  or 
commissural  fibers. 

The  more  or  less  elongated  axon  becomes  invested,  as  a  rule,  at  a 
short  distance  from  the  cell  with  nucleated  oblong  cells,  which  subse- 
quently become  modified  and  constitute  the  medullary  or  myehn 
sheath.  This  is  invested  by  a  thin,  cellular  membrane — the  neu- 
rilemma. These  three  structures  thus  constitute  what  is  known  as  a 
medidlated  nerve-fiber.  In  the  brain  and  spinal  cord  the  outer 
sheath,  however,  is  frequently  absent.  In  the  sympathetic  system  the 
myelin  is  also  frequently  absent,  though  the  axon  is  inclosed  by  the 
neurilemma,  thus  constituting  a  non-mediillated  nerve-fiber. 

The  end-tii}ts  or  terminal  organs  are  formed  by  the  sphtting  of  the 
axon  into  a  number  of  filaments,  which  remain  independent  of  one 
another  and  are  free  from  the  medullary  investment.  The  histologic 
pecuHarities  of  the  terminal  organs  vary  in  different  situations,  and  in 


io8  TEXT-BOOK  OF  PHYSIOLOGY. 

many  instances  are  quite  complex  and  characteristic.  In  peripheral 
organs,  as  muscles,  glands,  blood-vessels,  skin,  mucous  membrane, 
the  tufts  are  in  direct  histologic  and  physiologic  connection  with  their 
cellular  elements.  In  the  brain  and  spinal  cord  the  tufts  are  in  more 
or  less  intimate  relation  with  the  dendrites  of  adjacent  neurons. 

The  neurons  in  their  totahty  constitute  the  neuron  or  nerve 
tissue.  From  the  fact  that  they  are  arranged  both  serially  and  col- 
laterally into  a  regular  and  connected  whole,  they  collectively  con- 
stitute a  system  known  as  the  neuron  or  nerve  system. 

Neurons,  moreover,  are  grouped  into  more  or  less  completely 
organized  masses,  termed  organs,  which  in  accordance  with  their 
locations  may  for  convenience  be  divided  into  central  and  peripheral 
organs.  The  central  organs  of  the  nerve  system  consist  of  the 
encephalon  or  brain  and  the  spinal  cord;  the  peripheral  organs 
consist  of  the  cranial  nerves,  the  spinal  nerves,  the  sympathetic 
ganglia  and  their  branches. 

Nerve-fibers. — The  nerve-fibers  which  constitute  by  far  the 
larger  part  of  both  the  peripheral  and  central  organs  of  the  nerve 
system,  are  simply  the  axonic  processes  of  neurons  with  their  second- 
ary investments,  the  myelin  and  neurilemma;  according  as  they 
possess  or  do  not  possess  the  medullary  sheath,  they  may  be  divided 
into  two  groups — viz.,  medullated  and  non-meduUated  libers. 

Medullated  Nerve-fibers. — These  consist  for  the  most  part  of 
three  distinct  structures: 

1.  An  external  investing  sheath,  tubular  in  shape,  termed  the  neuri- 

lemma. 

2.  An  intermediate  semifluid  substance — the  medulla  or  myelin, 

3.  An  internal  dark  thread — the  axis-cylinder. 

The  neurilemma  is  a  thin,  transparent,  homogeneous  membrane 
closely  adherent  to  the  medulla.  Owing  to  its  colorless  appearance, 
it  can  be  seen  only  with  difficulty  in  fresh  tissue.  When  treated 
with  various  reagents,  it  becomes  distinct.  Physically,  it  is  quite 
resistant  and  elastic.  Its  function  is  doubtless  that  of  a  protective 
agent  to  the  structures  within. 

The  medulla,  myelin,  or  white  substance  0}  Schwann  completely 
fills  the  neurilemma  and  closely  invests  the  axis-cyhnder  or  axon.  In 
fresh  tissue  the  medulla  is  clear,  homogeneous,  semifluid,  and  highly 
refracting.  In  composition  it  is  oleaginous.  When  the  nerve  is 
treated  with  various  reagents  which  alter  its  composition,  the  medulla 
becomes  opaque  and  imparts  a  white,  glistening  appearance.  The 
function  of  the  medulla  is  quite  unknown. 

At  intervals  of  about  seventy-five  times  its  diameter  the  medul- 
lated nerve-fiber  undergoes  a  remarkable  diminution  in  size,  due  to 
an  interruption  of  the  medullary  substance,  so  that  the  neurilemma 
lies  directly  on  the  axis-cylinder.     These  constrictions,  or  7iodes  0} 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE. 


109 


Ranvier,  taking  their  name  from  their  discoverer,  occur  at  regular 
intervals  along  the  course  of  the  nerve,  separating  it  into  a  series  of 
segments.  The  portion  between  the  nodes  is  termed  the  internodal 
segment.  It  has  been  suggested  that  in  consequence  of  the  absence 
of  the  myehn  at  these  nodes,  a  free  exchange  of  nutritive  material 
and  decomposition  products  can  take  place  between  the  axis-cyhnder 
and  the  surrounding  plasma. 

The  axis-cylinder,  or  axon,  the  direct  outgrowth  of  the  nerve-cell, 
is  the  most  essential  element  of  the  nerve-fiber,  as  it  alone  is  uni- 
formly continuous  throughout.  In  the  natural  condition  it  is  trans- 
parent and  invisible;  but  when  treated  with  proper  reagents,  it 
presents  itself  as  a  pale,  granular,  flattened  band,  more  or  less  solid 
and   somewhat   elastic.     It   is   albuminous   in   composition.     With 


Fig.  44. 


^^^^^S^^^r^^jf 


-Transverse   Section   of   a   Nerve    (Median),     ep.  Epineurium.     pe. 
Perineurium,     ed.  Endoneurium. — (Landois  and  Stirling.) 


high  magnification  the  axis  presents  a  longitudinal  striation,  indicating 
a  fibrillar  structure.  The  hbrillse  appear  to  be  embedded  in  an 
intervening  semifluid  substance,  the  neuroplasm. 

Non-medullated  Nerve-fibers. — These  consist,  for  the  most 
part,  only  of  the  axis- cylinder,  though  in  some  portions  of  the  nerve 
system  a  neurilemma  is  also  present.  Though  much  less  abundant 
than  the  former  variety,  they  are  distributed  largely  throughout  the 
nerve  system,  but  are  particularly  abundant  in  the  sympathetic. 
Owing  to  the  absence  of  a  medulla,  they  present  a  rather  pale  or 
grayish  appearance. 

Sympathetic  Ganglia. — A  sympathetic  gangHon  consists  essen- 
tially of  a  connective-tissue  capsule  with  an  interior  framework. 


no  TEXT-BOOK  OF  PHYSIOLOGY. 

The  meshes  of  this  framework  contain  nerve-cells  provided  v^ith 
dendrites  and  branching  axons.  The  majority  of  the  axons  are  non- 
medullated.  In  all  instances,  with  the  exception  of  the  ganglion  cells 
of  the  heart,  the  axons  are  distributed  to  nonstriated  muscle  tissue 
and  to  the  epithehum  of  glands. 

The  nerve-cells  of  the  ganglia  are  also  in  histologic  connection 
with  the  terminal  branches  of  fine  medullated  nerve-fibers  which 
leave  the  spinal  cord  by  way  of  the  nerve-trunks.  These  nerve- 
fibers  are  designated  pre- ganglionic  fibers,  while  those  emerging  from 
the  cells  are  designated  post- ganglionic  fibers. 

The  Peripheral  Organs  of  the  Nerve  System. — These  consist 
of  the  cranial  and  spinal  nerves  and  the  sympathetic  gangha.  Each 
nerve  consists  of  a  variable  number  of  nerve-fibers  united  into  firm 
bundles  by  connective  tissue  which  supports  blood-vessels  and  lym- 
phatics.    The  bundles  are  technically  known  as  nerve-trunks  or  nerves. 

The  nerve-trunks  connect  the  brain  and  cord  with  all  the  re- 
maining structures  of  the  body.  Each  nerve  is  invested  by  a  thick 
layer  of  lamellated  connective  tissue,  known  as  the  epineurium. 
A  transverse  section  of  a  nerve  shows  (see  Fig.  44)  that  it  is  made 
up  of  a  number  of  small  bundles  of  fibers,  each  of  which  possesses 
a  separate  investment  of  connective  tissue — the  perineurium.  With- 
in this  membrane  the  nerve-fibers  are  supported  by  a  fine  stroma — 
the  endoneurium.  After  pursuing  a  longer  or  shorter  course,  the 
nerve-trunk  gives  off  branches,  which  interlace  very  freely  with 
neighboring  branches,  forming  plexuses,  the  fibers  of  which  are 
distributed  to  associated  organs  and  regions  of  the  body.  From 
their  origin  to  their  termination,  however,  nerve-fibers  retain  their 
individuality,  and  never  become  blended  with  adjoining  fibers. 

As  nerves  pass  from  their  origin  to  their  peripheral  terminations, 
they  give  off  a  number  of  branches,  each  of  which  becomes  invested 
with  a  lamellated  sheath — an  offshoot  from  that  investing  the  parent 
trunk.  This  division  of  nerve-bundles  and  sheath  continues  through- 
out all  the  branchings  down  to  the  ultimate  nerve-fibers,  each  of 
which  is  surrounded  by  a  sheath  of  its  own,  consisting  of  a  single 
layer  of  endothelial  cells.  This  delicate  transparent  membrane,  the 
sheath  of  Henle,  is  separated  from  the  nerve-fiber  by  a  considerable 
space,  in  which  is  contained  lymph  destined  for  the  nutrition  of  the 
fiber.  Near  their  ultimate  terminations  the  nerve-fibers  themselves 
undergo  division,  so  that  a  single  fiber  may  give  origin  to  a  number 
of  branches,  each  of  which  contains  a  portion  of  the  parent  axis- 
cyhnder  and  myelin. 

Blood-supply. — Nerves  being  parts  of  living  cells  require  for 
the  maintenance  of  their  nutrition  a  certain  amount  of  blood.  This 
is  furnished  by  the  blood-vessels  ramifying  in  and  supported  by  the 
connective-tissue  framework.     Here  as  elsewhere  there  is  a  constant 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  m 

exchange,  through  the  capillary  wall  and  the  neurilemma,  of  nutritive 
material  to  the  nerve  proper  and  of  waste  materials  to  the  blood. 

.The  Chemic  Composition  and  Metabolism. — Chemic  analysis 
of  nerve-tissue  has  shown  the  presence  of  water,  proteids  (two  glob- 
ulins and  a  nucleo-proteid),  neurokeratin  and  nuclein,  two  phos- 
phorized  bodies  (protagon  and  lecithin),  several  cerebrosides  (nitro- 
gen-holding bodies  of  a  glucoside  character,  as  shown  by  their  yielding 
the  reducing  carbohydrate  galactose),  inorganic  salts,  and  a  series  of 
nitrogen-holding  bodies  such  as  creatin,  xanthin,  urea,  leucin,  etc.  As 
to  the  metabolism  that  is  taking  place  in  nerve-cells  and  fibers, 
practically  nothing  definite  is  known.  That  such  changes,  how- 
ever, are  taking  place  would  be  indicated  first  by  the  blood-supply, 
and  second  by  the  fact  that  withdrawal  of  the  blood-supply  is  followed 
by  a  loss  of  irritability.  The  metabolism  of  the  central  organs  of  the 
nerve  system  is  more  active  and  extensive.  In  this  situation  any  with- 
drawal of  blood  from  compression  or  occlusion  of  blood-vessels  is 
followed  by  impairment  of  nutrition  and  loss  of  function. 


THE  RELATION  OF  THE  PERIPHERAL  ORGANS  TO  THE  CENTRAL 
ORGANS  OF  THE  NERVE  SYSTEM. 

Spinal  Nerves. — The  nerves  in  connection  with  the  spinal  cord 
are  thirty-one  in  number  o;i  each  side.  If  traced  toward  the  spinal 
column,  it  will  be  found  that  the  nerve-trunk  passes  through  an 
intervertebral  foramen.  Near  the  outer  Hmits  of  the  foramina  each 
nerve-trunk  divides  into  two  branches,  generally  termed  roots,  one 
of  which,  curving  shghtly  forward  and  upward,  enters  the  spinal 
cord  on  its  anterior  or  ventral  surface,  while  the  other,  curving  back- 
ward and  upward,  enters  the  spinal  cord  on  its  posterior  or  dorsal 
surface.  The  former  is  termed  the  anterior  or  ventral  root;  the  latter, 
the  posterior  or  dorsal  root.  Each  dorsal  root  presents  near  its  union 
with  the  ventral  root  a  small  ovoid  grayish  enlargement  known  as 
a  ganghon.  Both  roots  previous  to  entering  the  cord  subdivide  into 
from  four  to  six  fascicuh. 

A  microscopic  examination  of  a  cross-section  of  the  spinal  cord 
shows  that  the  fibers  of  the  ventral  roots  can  be  traced  directly  into 
the  body  of  the  nerve-cells  in  the  anterior  horns  of  the  gray  matter. 
The  fibers  of  the  dorsal  roots  are  not  so  easily  traced,  for  they  diverge 
in  several  directions  shortly  after  entering  the  cord.  In  their  course 
they  give  off  collateral  branches  which,  in  common  with  the  main 
fiber,  end  in  tufts  which  become  associated  with  nerve-cells  in  both 
the  anterior  and  posterior  horns  of  the  gray  matter. 

Cranial  Nerves. — The  nerves  in  connection  with  the  base  of  the 
brain  are  known  as  cranial  nerves;  some  of  these  nerves  present  a 
similar  gangHonic  enlargement,  and  therefore  may  be  regarded  as 


112 


TEXT-BOOK  OF  PHYSIOLOGY. 


dorsal  nerves,  while  others  may  be  regarded  as  ventral  nerves.  Their 
relations  within  the  medulla  oblongata  are  similar  to  those  within 
the  spinal  cord. 

Efferent  and  Afferent  Nerves. — Nerves  are  channels  of  com- 
munication between  the  brain  and  spinal  cord,  on  the  one  hand,  and 
the  muscles,  glands,  blood-vessels,  skin,  mucous  membrane,  viscera, 
etc.,  on  the  other.  Some  of  the  nerve-fibers  serve  for  the  transmission 
of  nerve  energy  from  the  brain  and  spinal  cord  to  certain  peripheral 
organs,  and  so  increase  or  retard  their  activities;  others  serve  for  the 
transmission  of  nerve  energy  from  certain  peripheral  organs  to  the 
brain  and  spinal  cord  which  gives  rise  to  sensation  or  other  modes  of 
nerve  activity.  The  former  are  termed  efferent  or  centrifugal,  the 
latter  afferent  or  centripetal  nerves.  Experimentally  it  has  been  de- 
termined that  the  anterior  or  ventral  roots  contain  all  the  efferent 
fibers,  the  posterior  or  dorsal  roots  all  the  afferent  fibers. 


THE  PERIPHERAL  ENDINGS  OF  NERVES. 

The  efferent  nerves  as  they  approach  their  ultimate  terminations 

lose  both  the  neurilemma  and  myehn  sheath.     The  axon  or  axis- 

cyhnder  then  divides  into  a  number  of  branches  which  become 

directly  and  intimately  associated  with  tissue-cells.     The  particular 


Nerve-fiber 
bundle. 


Fig.  45. — Motor  Nerve-endings  of  Intercostal  Muscle-fibers  of  a  Rabbit. 

X    ISO.— {Sidhr.) 


mode  of  termination  varies  in  different  situations.    These  terminations 

are  generally  spoken  of  as  end-organs,  terminal  organs,  or  end-tufts. 

In  the  skeletal  muscle  the  nerve-fiber  loses  both  neurilemma  and 

myehn  sheath  at  the  point  where  it  comes  in  contact  with  the  muscle- 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  113 

fiber.  After  penetrating  the  sarcolemma,  the  axon  or  axis-cyhnder 
divides  into  a  number  of  small  branches  which  appear  to  be  embedded 
in  a  relatively  large  mass  of  sarcoplasm  and  nuclei,  the  whole  form- 
ing the  so-called  "motor  plate."  Each  muscle-fiber  possesses  one 
such  plate  or  end-organ  in  mammalia,  several  in  the  frog. 

In  the  visceral  muscle  the  terminal  nerve-fibers  derived  from 
sympathetic  or  peripheral  neurons  are  primarily  non-meduUated, 
The  axons  divide  and  subdivide  and  form  plexuses  which  surround 
the  muscle-cell  bundles.  Fine  fibers  from  the  plexuses  are  given  off 
which  ultimately  come  into  relation  with  each  individual  cell,  on  the 
surface  of  which  they  terminate  in  the  form  of  one  or  more  granular 
masses. 

In  the  glands,  taking  as  an  illustration  the  parotid  and  mammary 
glands,  the  nerve-fibers,  also  derived  from  sympathetic  or  peripheral 
neurons,  pass  into  the  body  of  the  gland  and  ultimately  reach  the 
acini,  on  the  outer  surface  of  which  they  ramify  and  form  a  plexus. 
From  this  plexus  fine  fibers  penetrate  the  acinus  wall  and  end  on 
the  gland-cell.  The  fibers  pre- 
sent a  varicose  appearance  (Fig. 
46). 

The  afferent  nerves  as  they 
approach  their  ultimate  termina- 
tions   undergo   similar   changes. 
The  end-tufts  become  associated,      ^^^     46.-Terminations   of   Nerve- 
in  some  situations,  with  special-  fibers  in  the  Gland-cells.     A. 

ized   end-organs    which   are   ex-  Cell  of  the  parotid  gland  of  a  rabbit 

,  ,  iV,         i.-  B.    Cells    of    the    mammary  gland 

tremely  complex;  e.  g.,  the  retina  ^j  ^  ^^t  in  gesuiion.-{Doyon  a,id 

in  the  eye,  the  organ  of  Corti  in  Moral.) 

the  ear,  the  taste-beakers  in  the 
tongue,  the  olfactory  cells  in  the  nose. 

In  the  skin  and  mucous  membranes  the  mode  of  termination 
varies  considerably.     The  following  are  some  of  the  principal  modes : 

1.  Free  endings  in  the  epithelium. 

2.  Tactile  cells  of  Merkel. 

3.  Tactile  corpuscles  in  the  papillae  of  the  true  skin. 

4.  Pacinian  corpuscles  found  attached  to  the  nerves  of  the  hand  and 

feet,  to  the  intercostal  nerves,  and  to  nerves  in  other  situations. 

5.  End-bulbs  of  Krause  in  the  conjunctiva,  chtoris,  penis,  etc. 

(A  consideration  of  these  end-organs  will  be  found  in  the  chapters 
devoted  to  the  organs  of  which  they  form  a  part.) 

In  the  skeletal  muscles  afferent  fibers  become  associated  with  small 
spindle-shaped  structures  known  as  muscle-spindles  or  neuromuscle 
end-organs.  These  spindles  vary  in  length  from  i  mm.  to  4  mm. 
They  consist  of  a  capsule  of  fibrous  tissue  containing  from  five  to 
twenty  muscle-fibers.     After  penetrating  the  several  layers  of  the 


114 


TEXT-BOOK  OF  PHYSIOLOGY. 


Posterior 


GoAfflian^ 


capsule,  the  nerve-fibers  lose  the  neurilemma  and  myelin  sheaths. 
The  axons  or  axis-cyhnders  then  divide  into  several  long  narrow 
branches  which  wind  themselves  in  a  spiral  manner  around  the  con- 
tained muscle-fiber  and  terminate  in  small  oval-shaped  discs.  Similar 
endings  have  been  observed  in  the  tendons  of  muscles. 

Development  and  Nutrition  of  Nerves. — The  efferent  nerve- 
fibers,  which  constitute  some 
of  the  cranial  nerves  and  all 
the  ventral  roots  of  the  spinal 
nerves,  have  their  origin  in 
cells  located  in  the  gray  mat- 
ter beneath  the  aqueduct  of 
Sylvius,  beneath  the  floor  of 
the  fourth  ventricle,  and  in 
the  anterior  horns  of  the  gray 
matter  of  the  spinal  cord. 
These  cells  are  the  modified 
descendants  of  independent, 
oval,  pear-shaped  cells — the 
neuroblasts — which  migrate 
from  the  medullary  tube.  As 
they  approach  the  surface 
of  the  cord  their  axons  arc 
directed  toward  the  ventral 
surface,  which  eventually  they 
pierce.  Emerging  from  the 
cord,  the  axons  continue  to  grow,  and  become  invested  with  the 
myehn  sheath  and  neurilemma,  thus  constituting  the  ventral  roots. 

The  afferent  nerve-fibers,  which  constitute  some  of  the  cranial 
nerves  and  all  the  dorsal  roots  of  the  spinal  nerves,  develop  outside 
of  the  central  nervous  system  and  only  subsequently  become  con- 
nected with  it.  (See  Fig.  47.)  At  the  time  of  the  closure  of  the 
medullary  tube  a  band  or  ridge  of  epithelial  tissue  develops  near  the 
dorsal  surface,  which,  becoming  segmented,  moves  outward  and  forms 
the  rudimentary  spinal  ganglia.  The  cells  in  this  situation  develop 
two  axons,  one  from  each  end  of  the  cell,  which  pass  in  opposite 
directions,  one  toward  the  spinal  cord,  the  other  toward  the  per- 
iphery. In  the  adult  condition  the  two  axons  shift  their  position, 
unite,  and  form  a  T-shaped  process,  after  which  a  division  into  two 
branches  again  takes  place.  In  the  ganglia  of  all  the  sensori-cranial 
and  sensori-spinal  nerves  the  cells  have  this  histologic  peculiarity. 
The  efferent  fibers  are  therefore  to  be  regarded  as  outgrowths 
from  the  nerve-cells  in  the  ventral  horns  of  the  gray  matter,  and  serve 
to  bring  the  cells  into  anatomic  and  physiologic  relationship  directly 
with  the  skeletal  muscles  and  indirectly,  through  the  intermediation 


Anferivr 
£006 


Fig.  47. — Diagram  Showing  the  Mode  of 
Origin  of  the  Ventral  and  Dorsal 
Roots. — {Edinger,  after  His.) 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE. 


115 


of  ganglia  (see  sympathetic  nervous  system),  with  visceral  muscles 
and  glands. 

The  afferent  fibers  are  to  be  regarded  as  outgrowths  from  the 
cells  of  the  dorsal  nerve  gangha,  and  serve  to  bring  the  skin,  mucous 
membrane,  and  certain  visceral  structures  into  relation  with  special- 
ized centers  in  the  central  nervous  system. 

Nerve  Degeneration. — If  any  one  of  the  cranial  or  spinal  nerves 
be  divided  in  any  portion  of  its  course,  the  part  in  connection  with 
the  periphery  in  a  short  time  exhibits  certain  structural  changes,  to 
which  the  term  degeneration  is  appHed.  The  portion  in  connection 
with  the  brain  or  cord  retains  its  normal  condition.  The  degenerative 
process  begins  simultaneously  throughout  the  entire  course  of  the 
nerve,  and  consists  in  a  disintegration  and  reduction  of  the  medulla 
and  axis-cyhnder  into  nuclei,  drops  of  myehn,  and  fat,  which  in  time 
disappear  through  absorption,  leaving  the  neurilemma  intact.  Coin- 
cident with  these  structural  changes  there  is  a  progressive  alteration 


Fig.  48. — DEGENEitA.TiON  OF  Spinal  Nerves  and  Nerve-roots  after  Section. 
A.  Section  of  nerve-trunk  beyond  the  ganglion.  B.  Section  of  anterior  root. 
C.  Section  of  posterior  root.  D.  E.xcision  of  ganglion,  a.  Anterior  root.  p. 
Posterior  root.     g.  Ganglion.  — (Dalton.) 


and  diminution  in  the  excitabihty  of  the  nerve.  Inasmuch  as  the 
central  portion  of  the  nerve,  which  retains  its  connection  with  the 
nerve-cell,  remains  histologically  normal,  it  has  been  assumed  that 
the  nerve-cells  exert  over  the  entire  course  of  the  nerve-fibers  a 
nutritive  or  a  trophic  influence.  This  idea  has  been  greatly  strength- 
ened since  the  discovery  that  the  axis-cyhnder,  or  the  axon,  has  its 
origin  in  and  is  a  direct  outgrowth  of  the  cell.  When  separated 
from  the  parent  cell,  the  fiber  appears  to  be  incapable  in  itself  of 
maintaining  its  nutrition. 

The  relation  of  the  nerve-cells  to  the  nerve-fibers,  in  reference  to 
their  nutrition,  is  demonstrated  by  the  results  which  follow  section 
of  the  ventral  and  dorsal  roots  of  the  spinal  nerves.  If  the  anterior 
root  alone  be  divided,  the  degenerative  process  is  confined  to  the 
peripheral  portion,  the  central  portion  remaining  normal.     If  the 


ii6  TEXT-BOOK  OF  PHYSIOLOGY. 

posterior  root  be  divided  on  the  peripheral  side  of  the  ganghon,  de- 
generation takes  place  only  in  the  peripheral  portion  of  the  nerve. 
(See  Fig.  48.)  If  the  root  be  divided  between  the  ganglion  and  the 
cord,  degeneration  takes  place  only  in  the  central  portion  of  the  rooi. 
From  these  facts  it  is  evident  that  the  trophic  centers  for  the  ventral 
and  dorsal  roots  lie  in  the  spinal  cord  and  spinal  nerve  gangha, 
respectively,  or,  in  other  words,  in  the  cells  of  which  they  are  an 
integral  part.  The  structural  changes  which  nerves  undergo  after 
separation  from  their  centers  are  degenerative  in  character,  and  the 
process  is  usually  spoken  of,  after  its  discoverer,  as  the  Wallerian 
degeneration. 

When  the  degeneration  of  the  efferent  nerves  is  completed,  the 
structures  to  which  they  are  distributed,  especially  the  muscles,  un- 
dergo an  atrophic  or  fatty  degeneration,  with  a  change  or  loss  of  their 
irritability.  This  is,  apparently,  not  to  be  attributed  merely  to  in- 
activity, but  rather  to  a  loss  of  nerve  influences,  inasmuch  as  inactivity 
merely  leads  to  atrophy  and  not  to  degeneration. 


CLASSIFICATION  OF  NERVES. 

The  efferent  nerves  may  be  classified,  in  accordance  with  the 
characteristic  form  of  activity  to  which  they  give  rise,  into  several 
groups,  as  follows : 

1.  Muscle  or  motor  nerves,  those  which  convey  nerve  energy  or  nerve 

impulses  to  muscles  and  excite  them  to  activity. 

2.  Gland  or  secretor  nerves,  those  which  convey  nerve  impulses  to 

glands,  and  cause  the  formation  and  discharge  of  the  secretion 
peculiar  to  the  gland. 

3.  Vascular  or  vaso-motor  nerves,  those  which  convey  nerve  impulses 

to  blood-vessels,  and  cause,  either  by  stimulation  or  inhibition  of 
the  mechanism  of  their  walls,  a  contraction  (vaso-constrictors) 
or  dilatation  (vaso-dilatators)  of  the  vessel. 

4.  Inhibitor  nerves,  those  conveying  nerve  impulses   that  cause  a 

slowing  or  complete  cessation  of  the  rhythmic  action  of  organs. 

5.  Accelerator  nerves,  those  conveying  impulses  that  cause  an  increase 

in  the  rhythmic  action  of  certain  organs. 

The  efferent  nerves  have  been  somewhat  differently  classified  by 
Gaskell  as  follows: 

1.  Nerves  to  skeletal  muscles. 

2.  Nerves  to  vascular  muscles. 

(a)   Vaso-motor,  i.   e.,   vaso-constrictors;  accelerators  and  aug- 

mentors  of  the  heart. 
(6)   Vaso-inhibitor,   i.  e.,  vaso-dilatators;  and   inhibitors  of   the 

heart. 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  117 

3.  Nerves  to  visceral  muscles. 

{a)   Viscero- motor, 
(b)   Viscero-inhibitor. 

4.  Nerves  to  glands. 

The  afferent  nerves  may  also  be  classified,  in  accordance  with 
the  character  of  the  sensations  or  other  modes  of  nerve  activity  to 
which  they  give  rise,  into  several  groups,  as  follows : 

1.  Sensorijacient  nerves,  those  conveying  nerve  impulses  that  give 

rise  in  the  brain  to  conscious  sensations.  They  may  be  sub- 
divided into — 

(a)  Nerves  of  special  sense — e.  g.,  olfactory,  optic,  auditory,  gusta- 
tory, tactile,  thermal,  pain,  pressure,  muscle — giving  rise  to 
correspondingly  named  sensations. 

(b)  Nerves  of  general  sense — e.  g.,  the  visceral  afferent  nerves — 

those  which  give  rise  normally  to  vague  and  scarcely  percept- 
ible sensations,  such  as  the  general  sensations  of  well-being 
or  discomfort,  hunger,  thirst,  fatigue,  sex,  want  of  air,  etc. 

2.  Reflex  nerves,  those  which  convey  nerve  impulses  to  the  nerve- 

centers  and  cause  a  discharge  and  transmission  of  nerve  impulses 
outward  through  efferent  nerves  to  muscles,  glands,  or  blood- 
vessels, and  thus  influence  their  activity.  It  is  quite  probable 
that  one  and  the  same  nerve  may  subserve  both  sense  and 
reflex  action,  owing  to  the  collateral  branches  which  are  given 
off  from  the  posterior  roots  as  they  ascend  the  posterior  column 
of  the  cord. 

3.  Inhibitor  nerves,  those  which  are  capable  reflexly  of  retarding  or 

inhibiting  the  activity  of  either  nerve-centers  or  peripheral 
organs. 

PHYSIOLOGIC  PROPERTIES  OF  NERVES. 

Nerve  Irritability  or  Excitability  and  Conductivity. — These 
terms  are  employed  to  express  that  condition  of  a  nerve  which  enables 
it|to  develop  and  to  conduct  nerve  impulses  from  the  center  to  the 
periphery,  or  from  the  periphery  to  the  center,  in  response  to  the  action 
of  stimuli.  A  nerve  is  said  to  be  excitable  or  irritable  so  long  as  it 
possesses  these  capabilities  or  properties.  For  the  manifestation  of 
these  properties  the  nerve  must  retain  a  state  of  physical  and  chemic 
integrity;  it  must  undergo  no  change  in  structure  or  chemic  composi- 
tion. The  irritability  of  an  efferent  nerve  is  demonstrated  by  the 
contraction  of  a  muscle,  by  the  secretion  of  a  gland,  or  by  a  change 
in  the  cahber  of  a  blood-vessel,  whenever  a  corresponding  nerve  is 
stimulated.  The  irritability  of  an  afferent  nerve  is  demonstrated  by 
the  production  of  a  sensation  or  a  reflex  action  whenever  it  is  stimu- 
lated. The  irritability  of  nerves  continues  for  a  certain  period  of 
time  after  separation  from  the  nerve-centers  and  even  after  the  death 


ii8  TEXT-BOOK  OF  PHYSIOLOGY. 

of  the  animal,  the  time  varying  in  different  classes  of  animals.  In 
the  warm-blooded  animals,  in  which  the  nutritive  changes  take  place 
with  great  rapidity,  the  irritabihty  soon  disappears — a  result  due  to 
disintegrative  changes  in  the  nerve,  caused  by  the  withdrawal  of  the 
blood-supply  and  other  non-physiologic  conditions.  In  cold-blooded 
animals,  on  the  contrary,  in  which  the  nutritive  changes  take  place 
relatively  slowly,  the  irritabihty  lasts,  under  favorable  conditions,  for 
a  considerable  time.  Other  tissues  besides  nerves  possess  irritability, 
that  is,  the  property  of  responding  to  the  action  of  stimuli — e.  g., 
glands  and  muscles,  which  respond  by  the  production  of  a  secretion 
or  a  contraction. 

Independence  of  Tissue  Irritability. — The  irritability  of  nerves 
is  distinct  and  independent  of  the  irritability  of  muscles  and  glands, 
as  shown  by  the  fact  that  it  persists  in  each  a  variable  length  of  time 
after  their  histologic  connections  have  been  impaired  or  destroyed  by 
the  introduction  of  various  chemic  agents  into  the  circulation.  Curara, 
for  example,  induces  a  state  of  complete  paralysis  by  modifying  or 
depressing  the  conductivity  of  the  end-organs  of  the  nerves  just  where 
they  come  in  contact  with  the  muscles,  without  impairing  the  irrita- 
bility of  either  nerve-trunks  or  muscles.  Atropin  induces  complete 
suspension  of  gland  activity  by  impairing  the  terminal  organs  of  the 
secretor  nerves  just  where  they  come  into  relation  with  the  gland- 
cells,  without  destroying  the  irritability  of  either  gland-cell  or  nerve. 

Nerve  Stimuli. — Nerves  do  not  possess  the  power  of  spon- 
taneously generating  and  propagating  nerve  impulses;  they  can  be 
aroused  to  activity  only  by  the  action  of  an  external  stimulus.  In 
the  physiologic  condition  the  stimuli  capable  of  throwing  the  nerve 
into  an  active  condition  act  for  the  most  part  on  either  the  central  or 
peripheral  end  of  the  nerve.  In  the  case  of  motor  nerves  the  stimulus 
to  the  excitation,  originating  in  some  molecular  disturbance  in  the 
nerve-cells,  acts  upon  the  nerve-fibers  in  connection  with  them.  In 
the  case  of  sensor  or  afferent  nerves  the  stimuH  act  upon  the  peculiar 
end -organs  with  which  the  sensor  nerves  are  in  connection,  which  in 
turn  excite  the  nerve-fibers.  Experimentally,  it  can  be  demonstrated 
that  nerves  can  be  excited  by  a  sufficiently  powerful  stimulus  applied 
in  any  part  of  their  extent. 

Nerves  respond  to  stimulation  according  to  their  habitual  func- 
tion; thus,  stimulation  of  a  sensor  nerve,  if  sufficiently  strong,  re- 
sults in  the  sensation  of  pain;  of  the  optic  nerve,  in  the  sensation  of 
light;  of  a  motor  nerve,  in  contraction  of  the  muscle  to  which  it  is 
distributed;  of  a  secretor  nerve,  in  the  activity  of  the  related  gland, 
etc.  It  is,  therefore,  evident  that  peculiarity  of  nerve  function  de- 
])ends  neither  upon  any  special  construction  or  activity  of  the  nerve 
itself  nor  upon  the  nature  of  the  stimulus,  but  entirely  upon  the  pecu- 
liarities of  its  central  and  peripheral  end-organs. 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  119 

Nerve  stimuli  may  be  divided  into — 

1.  General  stimuli,  comprising  those  agents  which  are  capable  of 

exciting  a  nerve  in  any  part  of  its  course. 

2.  Special  stimuli,  comprising  those  agents  which  act  upon  nerves 

only  through  the  intermediation  of  the  end-organs. 

The  end-organs  are  speciahzed  highly  irritable  structures  placed 
between  the  nerve-fibers  and  the  surface.  They  are  especially 
adapted  for  the  reception  of  special  stimuli  and  for  the  liberation  of 
energy,  which  in  turn  excites  the  nerve-fiber  to  activity. 

General  stimuli: 

1.  Mechanic:  Sharp  taps,  sudden  pressure,  cutting,  etc. 

2.  Thermic:  Sudden  application  of  heated  object. 

3.  Chemic:  Contact  of  various  substances  which  alter  their  chemic 

composition  quickly,  e.  g.,  strong  acids  or  alkalies,  sol.  sodium 
chlorid  15  per  cent.,  sugar,  urea,  etc. 

4.  Electric:  Either  the  constant  or  induced  current. 
Special  stimuli: 

For  afferent  nerves — 

1.  Light  or  ethereal  vibrations  acting  upon  the  end-organs  of  the 

optic  nerve  in  the  retina. 

2.  Sound  or  atmospheric  undulations  acting  upon  the  end-organs  of 

the  auditor}^  nerve. 

3.  Heat  or  vibrations  of  the  air  acting  upon  the  end-organs  in  the  skin. 

4.  Chemic  agencies  acting  upon  the  end-organs  of  the  olfactory  and 

gustatory  nerves. 

For  efferent  nerves — 

A  molecular  disturbance  in  the  central  nerve-cells  from  w^hich 
they  arise,  the  nature  of  which  is  unknown. 

Nature  of  the  Nerve  Impulse. — As  to  the  nature  of  the  nerve 
impulse  generated  by  any  of  the  foregoing  stimuli,  either  general  or 
special,  but  little  is  known.  It  has  been  supposed  to  partake  of  the 
nature  of  a  molecular  disturbance,  a  combination  of  physical  and 
chemic  processes  attended  by  the  liberation  of  energy,  w^hich  propa- 
gates itself'  from  molecule  to  molecule.  The  passage  of  the  nerve 
impulse  is  accompanied  by  changes  of  electric  tension,  the  extent  of 
which  is  an  indication  of  the  intensity  of  the  molecular  disturbance. 
Judging  from  the  deflections  of  the  galvanometer  needle  it  is  probable 
that  when  the  nerve  impulse  makes  its  appearance  at  any  given  point 
it  is  at  first  feeble,  but  soon  reaches  a  maximum  development,  after 
which  it  speedily  declines  and  disappears.  It  may,  therefore,  be 
graphically  represented  as  a  wave-like  movement  with  a  definite 
length  and  time  duration.  Under  strictly  physiologic  conditions  the 
nerve  impulse  passes  in  one  direction  only;  in  efferent  nerves  from 
the  center  to  the  periphery,  in  afferent  nerves  from  the  periphery  to 
the  center.     Experimentally,  however,  it  can  be  demonstrated  that 


TEXT-BQOK  OF  PHYSIOLOGY. 


when  a  nerve  impulse  is  aroused  in  the  course  of  a  nerve  by  an  ade- 
quate stimulus  it  travels  equally  well  in  both  directions  from  the  point 
of  stimulation.  When  once  started,  the  impulse  is  coniined  to  the 
single  fiber  and  does  not  diffuse  itself  to  fibers  adjacent  to  it  in  the 
same  nerve-trunk. 

Rapidity  of  Conduction  of  the  Nerve  Impulse. — The  passage 
of  a  nerve  impulse,  either  from  the  brain  to  the  periphery  or  in  the 
reverse  direction,  requires  an  appreciable  period  of  time.  The 
velocity  with  which  the  impulse  travels  in  human  sensory  nerves  has 
been  estimated  at  about  50  meters  a  second,  and  for  motor  nerves  at 
from  28  to  T,T,  meters  a  second.  The  rate  of  movement  is,  however, 
somewhat  modified  by  temperature,  cold  lessening  and  heat  increas- 
ing the  rapidity;  it  is  also  modified  by  electric  conditions,  by  the 
action  of  drugs,  the  strength  of  the  stimulus,  etc. 
The  rate  of  transmission  through  the  spinal  cord 
is  considerably  slower  than  in  nerves,  the  average 
velocity  for  voluntary  motor  impulses  being  only  1 1 
meters  a  second,  for  sensory  impulses  12  meters, 
and  for  tactile  impulses  40  meters  a  second. 

Nerve  Fatigue  .^ — Inasmuch  as  nerves  are 
parts  of  living  cells,  the  seat  of  nutritive  changes, 
it  might  be  supposed  that  the  passage  of  nerve 
impulses  would  be  attended  by  the  disruption  of 
energy-holding  compounds,  the  production  of 
waste  products,  the  liberation  of  heat,  and  in 
time  by  the  phenomena  of  fatigue.  Though  it  is 
probable  that  changes  of  this  character  occur, 
yet  no  reliable  experimental  data  have  been  ob- 
tained which  afford  a  clue  as  to  the  nature  or 
extent  of  any  such  changes.  Stimulation  of  motor 
nerves  with  the  induced  electric  current  for  four 
hours  appears  to  be  without  influence  either  on 
the  intensity  of  the  nerve  impulse  or  the  rate 
of  its  conduction. 
Identity  of  Efferent  and  Afferent  Nerves  and  Nerve  Impulses. 
— Notwithstanding  the  classification  of  nerve-fibers  based  on  dift'er- 
ences  of  physiologic  actions,  there  are  no  characters,  either  histologic 
or  chemic,  which  serve  to  distinguish  them  from  one  another.  More- 
over, as  the  nerve  impulse  is  conducted  through  a  nerve-fiber  equally 
well  in  both  directions,  as  determined  by  experiments,  it  is  probable 
that  it  does  not  differ  in  character  in  the  two  classes  of  nerves.  That 
the  efferent  fibers  conduct  the  nerve  impulses  from  the  nerve-centers 
to  the  periphery,  and  the  aft'erent  nerves  from  the  periphery  to  the 
centers,  is  because  of  the  fact  that  they  receive  their  stimulus  physio- 
logically only  in  the  centers  or  at  the  periphery.     The  fundamental 


Fig.  49.  — Nerve- 
muscle  Prep- 
aration OF  A 
Frog.  F.  Fe- 
mur. S.  Sciatic 
nerve.  I.  Tendo 
Achillis. — (Lan- 
dois  and  Stir- 
ling.) 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  121 

reason  for  difference  of  effects  produced  by  stimulation  of  dift'erent 
nerves  is  the  character  of  the  organ  to  which  the  nerve  impulse  is 
conducted.  A  nerve  is  merely  the  transmitter  of  the  nerve  im- 
pulse, which  if  conducted  to  a  muscle  excites  contraction;  to  a 
gland,  secretion;  to  a  blood-vessel,  variation  in  caliber;  to  special 
areas  in   the   brain,  sensations  of  hght,  sound,  pain,  etc. 

Electric  Excitation  of  Nerves. — For  the  purpose  of  studying 
the  physiologic  activities  of  nerves  it  has  been  found  convenient 
to  employ  the  nerve-muscle  preparation  (the  gastrocnemius  muscle 
and  sciatic  nerve)  and  to  use  as  a  stimulus  the  induced  electric 
current.  (See  Fig.  49.)  When  kept  moist,  this  preparation  is 
extremely  sensitive  to  either  the  galvanic  or  the  induced  current. 

Though  the  development  and  conduction  of  a  nerve  impulse  may 
be  demonstrated  by  the  deflection  of  the  galvanometer  needle  or  the 
movement  of  the  mercury  in  the  capillary  electrometer,  it  is  more 
conveniently  demonstrated  by  the  contraction  of  a  muscle,  the  vigor 
of  which,  within  Hmits,  may  be  taken  as  a  measure  of  the  intensity 
of  the  impulse.  The  preparation  should  be  enclosed  in  a  moist 
chamber  and  the  nerve  connected  with  the  inductorium  through 
the  intervention  of  non-polarizable  electrodes.  The  muscle  may  be 
attached  to  the  muscle-lever  and  its  contractions  recorded. 

A  single  shock  of  an  induced  current  develops,  it  is  beheved, 
a  single  nerve  impulse  followed  by  a  single  muscle  contraction.  A 
minimal  contraction  following  a  minimal  electric  stimulus  presupposes 
the  development  of  a  nerve  impulse  of  low  intensity.  Within  certain 
limits  a  maximal  contraction  following  a  maximal  electric  stimulus 
presupposes  the  development  of  a  nerve  impulse  of  high  intensity. 
Intermediate  contractions  indicate  nerve  impulses  of  corresponding 
intensity. 

Tetanization  of  a  muscle  indicates  that  the  nerve  impulses  arrive 
at  the  muscle  with  a  frequency  so  great  that  the  muscle  does  not 
succeed  in  relaxing  from  the  effect  of  one  stimulus  before  the  next 
arrives.  Incomplete  as  well  as  complete  tetanus  may  be  developed 
by  gradually  increasing  the  frequency  of  the  stimulus.  The  character 
of  the  contraction  caused  by  indirect  stimulation — i.  e.,  though  the 
nerve — does  not  differ  in  any  essential  respect  from  that  due  to  direct 
stimulation. 


ELECTRIC  PHENOMENA  OF  NERVES. 

Electric  Currents  from  Injured  Nerves. — It  was  discovered 
by  du  Bois-Reymond  that  electric  currents  can  be  obtained  from 
nerves  as  well  as  from  muscles,  and  that  the  electric  properties  of 
the  former  correspond  in  most  respects  to  those  of  the  latter.  The 
laws  governing  the  development  and  mode  of  action  of  the  currents 


TEXT-BOOK  OF  PHYSIOLOGY. 


derived  from  muscles  are  equally  applicable  to  the  currents  derived 
from  nerves. 

A  nerve-cylinder  obtained  by  making  two  transverse  sections  of 
any  given  nerve  presents,  as  in  the  case  of  muscles,  a  natural  and 
tv\^o  artificial  transverse  surfaces.  A  line  drawn  around  the  cylinder 
at  a  point  lying  midway  between  the  two  end  surfaces  constitutes 
the  equator.  From  such  a  cyHnder  strong  currents  are  obtained 
when  the  natural  longitudinal  surface   and    the  transverse  surface 

are  connected  with  the  electrodes  of 
the  galvanometer  circuit.  The  strength 
of  the  current  thus  obtained  will  di- 
minish or  increase  according  as  the 
electrode  on  the  longitudinal  surface  is 
removed  from  or  brought  near  to  the 
equator.  If  two  symmetric  points  on 
the  longitudinal  surface  equidistant 
from  the  equator  are  united,  no  cur- 
rent is  obtainable.  When  asymmetric 
points  on  the  longitudinal  surface  are 
connected,  weak  currents  are  obtained, 
in  which  case  the  point  lying  nearer 
the  equator  becomes  positive  to  the 
point  more  distant,  which  becomes 
negative.  From  these  facts  it  is  evi- 
dent that  all  points  on  the  longitudinal 
surface  are  electrically  positive  to  the 
transverse  surface  and  that  the  point 
of  greatest  positive  tension  is  situated 
near  the  equator  (Fig.  50). 

The  electromotive  force  of  the 
nerve-current  varies  in  strength  with 
the  length  and  thickness  of  the  nerve. 
The  strongest  current  obtained  from 
the  nerve  of  the  frog  is  equal  to  the 
0.002  of  a  Daniell  cell;  that  obtained 
from  the  nerve  of  the  rabbit,  0.026  of  a  Daniell.  The  existence  of 
the  nerve,  its  strength,  duration,  etc.,  depend  largely  on  the  mainte- 
nance of  physiologic  conditions.  All  influences  which  impair  the 
nutrition  of  the  nerve  diminish  the  current.  With  the  death  of  the 
nerve  all  electric  phenomena  disappear. 

Negative  Variation  of  the  Nerve  Current. — During  the  pas- 
sage of  the  nerve  impulse  the  resting  nerve  current,  or  the  demarca- 
tion current,  diminishes  more  or  less  completely  in  intensity,  undergoes 
a  negative  variation,  as  shown  by  the  return  of  the  galvanometer 
needle,  due  to  a  change  in  its  electromotive  condition  or  to  a  diminu- 


FiG.  50. — Diagram  to  Illus- 
trate THE  Currents  in 
Nerves.  The  arrowheads 
indicate  the  direction;  the 
thickness  of  the  lines  indicates 
the  strength  of  the  currents. — 
{Landois  and  Stirling.) 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  123 

tion  of  the  difference  in  potential  between  the  positive  longitudinal 
and  negative  transverse  sections.  This  negative  variation  of  the  de- 
marcation current  is  observed  equally  well  from  either  the  central  or 
peripheral  end  of  the  nerve.  If  the  two  ends  of  the  nerve  are  con- 
nected with  galvanometers  and  the  nerve  stimulated  in  the  middle, 
the  demarcation  currents  simultaneously  undergo  a  negative  variation. 
This  may  be  taken  as  a  proof  that  the  excitation  process  propagates 
itself  equally  well  in  both  directions.  The  negative  variation  is  inti- 
mately connected  with  changes  in  the  molecular  condition  of  the  nerve 
and  is  not  due  to  any  extraneous  electric  or  other  influence.  And 
du  Bois-Reymond  was  also  enabled  to  obtain  a  negative  variation  of 
the  current  in  the  nerves  of  a  Hving  frog  which  were  yet  in  connection 
with  the  spinal  cord.  In  this  experiment  the  sciatic  nerve  was  divided 
at  the  knee  and  freed  from  its  connections  up  to  the  spinal  column;  the 
transverse  and  longitudinal  surfaces  were  then  placed  in  connection 
with  the  electrodes  of  the  galvanometer  wires  and  the  current  per- 
mitted to  influence  the  needle.  The  animal  was  then  subjected  to 
the  action  of  strychnin.  Upon  the  appearance  of  the  muscle 
spasms  the  needle  was  observed  to  swing  backward  toward  the  zero 
point  to  the  extent  of  from  i  to  4  degrees,  and  upon  the  cessation  of 
the  spasms  to  return  to  its  previous  position.  In  an  experiment  of 
this  nature  it  is  obvious  that  the  negative  variation  was  the  result 
of  a  physiologic  stimulation  of  the  nerve  arising  within  the  spinal 
cord. 

The  question  also  here  arises  as  to  whether  the  negative  variation 
is  due  to  a  steady,  continuous  decrease  of  the  natural  current,  or 
whether  it  is  due  to  successive  and  rapidly  following  variations  in  its 
intensity,  similar  to  that  observed  in  muscles.  Though  this  cannot 
be  demonstrated  Avith  the  physiologic  rheoscope,  as  was  the  case  with 
the  muscle,  there  can  be  no  doubt,  both  from  experimentation  and 
analogy,  that  the  latter  supposition  is  the  correct  one.  It  has  been 
shown  that  when  non-polarizable  electrodes  connected  with  Siemen's 
telephone  are  placed  in  connection  with  the  longitudinal  and  trans- 
verse sections  of  a  nerve,  low,  sonorous  vibrations  are  perceived 
during  tetanic  stimulation, — a  proof  that  the  active  state  of  the  nerve 
is  connected  with  the  production  of  discontinuous  electric  currents. 
The  oscillations  of  the  mercurial  column  of  the  capillary  electrom- 
eter also  reveal  similar  electric  changes.  It  was  also  demonstrated 
by  Bernstein  with  a  specially  devised  apparatus,  the  repeating  rheo- 
tome,  that  the  negative  variation  is  composed  of  a  large  number  of 
single  variations  which  succeed  each  other  in  rapid  succession  and 
summarize  themselves  in  their  effect  on  the  needle. 

Electric  Currents  from  Uninjured  Nerves. — The  pre-existence 
of  electric  currents  in  living  and  wholly  uninjured  nerves  while  at 
rest  has  also  been  denied  by  Hermann,  who  regards  all  portions  of 


124  TEXT-BOOK  OF  PHYSIOLOGY. 

the  nerve  as  isoelectric,  any  difference  of  potential  being  the  result  of 
some  injury  to  its  surface. 

Action  Currents. — For  reasons  to  be  stated  below,  it  is  very  diffi- 
cult to  determine  the  presence  of  diphasic  action  currents  during  the 
passage  of  an  excitatory  impulse  through  the  nerve-fiber.  The  so- 
called  negative  variation  of  the  resting  nerve  current, — the  demarca- 
tion current, — which  is  occasioned  by  tetanic  stimulation,  Hermann 
regards  as  the  expression  of  an  action  current  which  flows  in  the  nerve 
in  a  direction  opposite  to  the  demarcation  current.  The  origin  of  this 
action  current  is  to  be  sought  for  in  the  continuous  negativity  of  that 
portion  of  the  longitudinal  surface  of  the  nerve  in  contact  with  the 
diverting  electrode,  while  the  dying  substance  of  the  transverse  surface 
takes  no  part  in  the  excitation.  This  tetanic  action  current,  or  nega- 
tive variation,  was  discovered  by  du  Bois-Reymond,  and  Bernstein 
later  succeeded  in  obtaining  this  action  current  during  the  passage 
of  a  single  excitation  process.  That  the  return  of  the  galvanometer 
needle  toward  the  zero  point  is  not  due  to  an  annulment  of  the  demar- 
cation current  itself,  but  to  the  appearance  of  an  action  current,  is 
shown  by  the  fact  that  if  the  former  be  compensated  by  a  battery 
current  until  the  needle  rests  on  the  zero  point  the  appearance  of  the 
latter  current  will  cause  the  needle  to  swing  in  a  direction  the  opposite 
of  that  caused  by  the  demarcation  current.  The  negative  variation 
and  action  current  may  therefore  be  regarded  as  one  and  the  same 
thing.  It  is  the  expression  of  the  change  the  nerve  is  undergoing 
during  the  passage  of  the  nerve  impulse.  The  rapidity  with  which 
the  negative  variation  or  action  current  travels,  the  variation  in  its 
intensity  from  moment  to  moment,  the  time  required  for  it  to  pass 
a  given  point,  would  express  the  change  in  the  nerve  to  which  the 
term  nerve  impulse  is  given.  From  experiments  made  with  the 
differential  rheotome,  Bernstein  calculated  that  the  speed  of  the 
negative  variation  is  about  28  meters  a  second;  that  it  is  at  first 
feeble,  soon  rises  to  a  maximum,  and  then  decHnes;  that  it  requires 
0.0006  to  0.0008  of  a  second  to  pass  a  given  point.  From  these  data 
it  is  evident  that  the  negative  variation  or  action  current  has  a  space 
value  of  about  18  mm.  Transferring  these  statements  to  the  nerve 
impulse,  it  may  be  said  that  it  is  a  molecular  disturbance,  traveling 
at  the  rate  of  about  28  meters  a  second,  is  wave-like  in  character,  the 
wave  being  18  millimeters  in  length,  and  occupying  from  0.0006  to 
0.0008  of  a  second  in  passing  any  given  point. 

Absence  of  Diphasic  Action  Currents. — When  any  two  points  on 
the  longitudinal  surface  which  do  not  exhibit  a  current  are  connected 
with  the  galvanometer  and  a  single  wave  of  excitation  passes  beneath 
the  electrodes,  it  might  be  expected  that,  as  in  the  case  of  the  muscle, 
a  diphasic  action  current  would  be  observed,  from  the  fact  that  the 
portions  of  the  nerve  beneath  the  electrodes  become  alternately  neg- 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  125 

ative  with  reference-  to  all  the  rest  of  the  nerve.  This,  however,  is 
not  the  case,  the  absence  of  the  two  opposing  phases  of  the  action 
current  being  explained  on  the  supposition  that  the  negativity  of  the 
two  led-off  points  is  of  equal  amount,  and  that,  owing  to  the  great 
rapidity  with  which  the  excitation  wave  travels,  the  two  phases  fall 
together  too  closely  in  time  to  alternately  influence  the  galvanometer 
needle.  During  stimulation  of  the  nerve,  when  two  currentless  or  iso- 
electric points  are  connected,  there  is  also  an  absence  of  the  action 
current,  as  was  observed  first  by  du  Bois-Reymond,  and  which  is  to 
be  explained  on  similar  grounds.  It  is  true  that  an  apparent  action 
current  is  sometimes  seen  when  the  stimulating  current  is  very  power- 
ful or  the  seat  of  stimulation  too  near  the  diverting  electrodes.  This, 
however,  must  be  attributed  to  an  electrotonic  state  of  the  nerve. 

The  Effects  of  a  Galvanic  Current  on  a  Nerve.— When  a  con- 
stant galvanic  current  of  medium  strength  is  made  to  pass  through  a 
portion  of  a  nerve,  several  distinct  effects  are  produced: 

1.  The  development  0}  a  nerve  impulse  at  the  moment  the  current 
enters  and  at  the  moment  the  current  leaves  the  nerve,  i.  e.,  at  the 
moment  the  circuit  is  made  and  at  the  moment  it  is  broken.  The 
development  of  the  nerve  impulse  is  made  evident  by  the  contraction 
of  the  muscle  if  the  nerve-muscle  preparation  be  used.  If  the  current 
be  either  very  weak,  or  very  strong,  the  muscle  contraction  may  not 
always  take  place. 

2.  The  development  of  electric  currents  on  each  side  of  the  positive 
pole  or  anode,  and  the  negative  pole  or  cathode  (see  Fig.  51),  which 
can  be  led  off  by  means  of  wires  into  a  galvanometer  circuit  from 
either  the  artificial  transverse  and  longitudinal  surfaces,  or  from  any 
two  points  on  the  longitudinal  surface  as  shown  by  the  deflection 
of  the  galvanometer  needle.  The  direction  of  these  electric  cur- 
rents in  the  nerve  coincides  with  that  of  the  galvanic  or  "polarizing 
current."  The  "natural  nerve  currents,"  the  currents  of  injury  or 
demarcation  currents,  as  they  are  variously  termed,  are  at  the  same 
time  increased  and  decreased  at  opposite  extremities  of  the  nerve 
according  to  the  direction  of  the  polarizing  current. 

To  this  changed  condition  of  the  electromotive  forces  in  a  nerve 
the  term  electrotonus  was  given  (du  Bois-Reymond).  The  currents 
themselves  are  known  as  electrotonic  currents;  from  their  relation 
to  the  anode  and  cathode,  they  are  termed  anelectrotonic  and  cat- 
electrotonic  currents.  The  condition  of  the  nerve  around  the  poles 
both  in  the  intra-polar  and  extra-polar  regions  is  known  as  an- 
electrotonus  and  catelectrotonus. 

The  electrotonic  currents  vary  considerably  in  strength  and  ex- 
tent, according  to  the  intensity  of  the  polarizing  current,  increasing 
steadily  with  the  intensity  of  the  latter  up  to  the  point  at  which  the 
polarizing  current  begins  to  destroy  the  physical  and  chemic  integrity 


126  TEXT-BOOK  OF  PHYSIOLOGY. 

of  the  nerve.  The  electrotonic  currents  are  strongest  in  the  imme- 
diate neighborhood  of  the  electrodes,  but  gradually  diminish  in  strength 
as  the  distance  between  the  polarized  and  led-off  portions  is  increased. 
The  distance  to  which  the  electrotonic  currents  extend  along  the  nerve 
will  depend  very  largely  upon  the  strength  of  the  polarizing  current, 
though  it  is  conditioned  by  the  physical  state  of  the  nerve;  for  if  it  be 
ligated  or  injured  beyond  the  polarized  portion,  the  electrotonic  cur- 
rents are  abolished.  The  electrotonic  currents  have  no  necessary 
connection  with  the  natural  nerve  currents,  nor  are  they  to  be  regarded 
as  branchings  of  the  galvanic  current.  They  are  in  all  probabihty  of 
artificial  origin,  due  to  an  inner  positive  and  negative  polarization  of 
the  nerve  which  extends  for  a  variable  distance  on  each  side  of  the 
poles,  and  due  to  the  action  of  the  polarizing  or  the  galvanic  current. 
3.  An  alteration  in  the  excitability  and  conductivity  of  the  nerve 
in  the  neighborhood  of  the  poles,  whereby  the  results  of  nerve  stimu- 
lation— that  is,  muscle  contraction,  sensation,  and  inhibition — are 
increased  or  decreased  according  to  the  strength  and  direction  of  the 


^- 


POLARIZING  J 
o'j?  CURRENT  •/ 
+  ^  1 i^^ 


'j^£Y  -^   /^  L  GALVANOMETER 


anelectrotonic  katelectroton ic 

current5  currents 

Fig.  51. — Electrotonic  Currents. 

current.  To  this  condition  the  term  electrotonus  was  also  given 
(Pfiiiger).  This  word  has  thus  been  employed  to  express  two  distinct 
series  of  effects  exhibited  by  a  nerve  through  a  portion  of  which  a  con- 
stant galvanic  current  is  passing.  It  appears  desirable,  for  the  sake  of 
clearness,  to  limit  the  term  electrotonus  to  the  electric  or  electrotonic 
currents  which  can  be  led  off  from  either  extremity  of  the  nerve,  and 
to  apply  to  the  modifications  of  irritability  which  accompany  electro- 
tonus the  expression,  electrotonic  alteration  of  excitability  and  con- 
ductivity. 

During  the  passage  of  the  current  the  excitabihty  of  the  intra- 
polar  as  well  as  the  extra-polar  regions  undergoes  a  change  which, 
as  shown  on  examination,  is  found  to  be  diminished  in  the  neigh- 
borhood of  the  anode  or  positive  pole  and  increased  in  the  neigh- 
borhood of  the  cathode  or  negative  pole.  These  alterations  in  the 
excitability  are  most  marked  in  the  immediate  vicinity  of  the  elec- 
trodes, though  they  extend  for  some  distance  into  both  the  extra- 
polar  and  intra-polar  regions,   though  with  gradually  diminishing 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  127 

intensity,  until  they  finally  disappear.  Between  the  electrodes  there 
is  a  point  where  the  excitability  is  unchanged  and  known  as  the 
neutral  or  indifferent  point  (Fig.  52).  The  extent  to  which  the  ex- 
citability is  modified  as  well  as  the  position  of  the  neutral  point  will 
depend  largely  on  the  strength  of  the  polarizing  or  galvanic  current. 


,,-T-^ 


Fig.  52. — Scheme  of  the  Electrotonic  Excitability. — {Landois  and  Stirling.) 


The  electrotonic  alterations  of  excitability  and  conductivity  can 
be  experimentally  demonstrated  on  the  muscle-nerve  preparation  in 


the  following  manner: 


I.  With  a  descending  current  of  medium  strength.     Previous  to  the 
closure  of  the  polarizing  current,  the  nerve  is  stimulated  first 


^\ 


+ 


ANODE 


\  REGION    OF 
VNCREASED  EXCITABILITY 


KATHODE 


SECONDARY  COIL 

Fig.  53. — Diagram  Showing  the  Region  of  Increased  Excitability  Caused  by 
THE  Passage  of  a  Galvanic  Current,  Stimulation  of  which  Gives  Rise 
TO  Incrj;ased  Contraction. 


in  the  extra-polar  anodic  region  and  the  extra-polar  cathodic 
region  with  an  induction  shock  of  medium  intensity  and  the 
height  of  the  contraction  recorded.  On  repeating  the  stimulation 
ajter  closure  of  the  polarizing  current  the  contraction  resulting 
from  stimulation  of  the  anodic  region  will  be  enfeebled  or  mav  be 


128 


TEXT-BOOK  OF  PHYSIOLOGY. 


entirely  wanting,  while  the  contraction  from  stimulation  of  the 
cathodic  region  will  be  decidedly  increased.  (See  Fig.  53.) 
With  an  ascending  current  of  the  same  strength.  After  prelimi- 
nary testing  of  the  excitability  and  the  subsequent  closure  of 
the  polarizing  current,  it  will  be  found  that  stimulation  of  the 
extrapolar  anodic  region  will  provoke  a  much  less  energetic 
contraction  or  perhaps  none  at  all.  Stimulation  of  the  extra- 
cathodic  region,  though  of  increased  excitability,  as  shown  by 
the  previous  experiment,  may  also  fail  to  provoke  a  contraction, 
owing  to  the  diminished  conductivity  of  the  region  in  the  neighbor- 
hood of  the  anode.  The  impulse  on  reaching  this  region  is 
blocked  in  its  passage.  A  similar  if  not  more  marked  decrease 
in  the  conductivity  may  be  developed  in  the  region  of  the  cathode 
if  the  current  strength  be  veiy  great.     (See  Fig.  54.) 


REGION    OF 
DECREASED    EXCITABILITY 


Fig.  54. — Diagram  Showing  the  Region  of  Decreased  Excitability  Caused 
BY  THE  Passage  of  a  Galvanic  Current,  Stimulation  of  which  Gives 
Rise  to  Decreased  Contraction. 


The  Law  of  Contraction;  Polar  Stimulation.— It  was  stated 
in  a  previous  paragraph  that  when  a  galvanic  current  of  medium 
strength  is  made  to  enter  a  nerve,  and  when  it  is  withdrawn  from  the 
nerve,  there  is  a  contraction  of  its  related  muscle.  These  are  generally 
known  as  the  make  and  break  effects.  During  the  actual  passage 
of  the  current  no  effect  is  observed  so  long  as  its  strength  remains 
uniform.  Any  sudden  variation  in  the  strength  of  the  curjrent  at 
once  arouses  the  nerve  to  activity,  as  shown  by  a  muscle  contraction. 

The  muscle  response  to  the  make  and  break  of  the  constant  current 
is  more  or  less  variable  unless  the  direction  of  the  current  as  well  as 
its  strength  be  taken  into  consideration.  If  the  current  is  made  to 
flow  from  the  central  toward  the  peripheral  end  of  the  nerve  it  is 
termed  a  direct,  descending,  or  centrifugal  current;  if  it  is  made  to 
flow  in  the  reverse  direction,  it  is  termed  an  indirect,  ascending,  or 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE. 


129 


centripetal  current.     The  strength  of  the  current  is  determined  and 
regulated  by  means  of  a  rheocord. 

The  make  and  break  of  currents  of  different  but  known  strengths 
and  directions  give  rise  to  contractions  which  occur  with  more  or  less 
regularity.  The  order  in  which  they  occur  under  these  varying 
conditions  of  experimentation  has  been  determined  and  tabulated 
as  follows  by  Pfiuger,  and  is  termed  the  law  0}  contraction: 


Ascending  Current. 

Descending  Cdrrent. 

Make. 

Break. 

Make. 

Break. 

Weak,  

Medium, 

Strong, 

Contraction. 
Contraction. 
Rest. 

Rest. 

Contraction. 

Contraction. 

Contraction. 
Contraction. 
Contraction. 

Rest. 

Contraction. 
Rest    or    weak 
contraction. 

The  results  as  above  tabulated  are  sometimes  compHcated  on  the 
opening  of  the  circuit  by  a  series  of  irregular  pulsations  of  the  muscle, 
an  apparent  tetanus,  and  long  known  as  the  opening  tetanus  of 
Ritter,  which  is  attributed  to  rapid  changes  in  the  irritabihty  of  the 
nerve,  in  the  region  of  the  anode.  A  similar  tetanic  contraction  of  the 
muscle  is  sometimes  observed  on  the  closure  of  the  circuit  due  to 
continued  excitation  in  the  region  of  the  cathode.  This  is  known 
as  the  closing  tetanus  of  Wundt.  All  the  phenomena  of  the  law  of 
contraction  were  explained  by  Pfliiger  on  the  assumption  that  the 
current  stimulates  the  nerve  only  at  the  one  electrode,  at  the  cathode 
on  closing,  and  at  the  anode  on  opening;  or,  in  other  words,  by  the 
appearance  of  catelectrotonus  or  by  the  disappearance  of  anelectro- 
tonus,  both  conditions  being  attended  by  a  rise  of  excitabihty — not, 
however,  by  the  opposite  changes.  It  is  further  assumed  that  the 
appearance  of  catelectrotonus  is  more  effective  as  a  stimulus  than  the 
disappearance  of  anelectrotonus.  For  these  reasons  the  term  polar 
stimulation  is  generally  employed  in  discussing  the  make  and  break 
effects  of  the  galvanic  current.  The  law  of  contraction  may  then  be 
explained  as  follows:  Very  feeble  currents,  either  ascending  or  de- 
scending, produce  contraction  only  upon  the  closure  of  the  circuit,  the 
sudden  increase  0}  the  excitability  in  the  catelectrotonic  area  being 
alone  sufficient  to  generate  an  impulse.  The  contraction  which 
follows  the  closing  of  the  weak  ascending  current  depends  upon  the 
fact  that  the  decrease  of  excitability  and  conductivity  at  the  anode  is 
insufficient  to  interfere  with  the  conduction  of  the  cathodal  stimulus. 
Medium  currents,  either  ascending  or  descending,  produce  contrac- 
tion both  on  closing  and  opening  the  circuit.  The  appearance  of 
catelectrotonus  and  the  disappearance  of  anelectrotonus  are  both 
sufficiently  powerful  to  generate  an  impulse  without,  however,  seri- 
ously impairing  the  conductivity  of  the  nerve. 


130  TEXT-BOOK  OF  PHYSIOLOGY. 

Very  strong  currents  produce  contraction  only  upon  the  opening 
of  the  ascending  and  closure  of  the  descending  currents,  or  upon  the 
passage  of  the  excitability  in  the  former  from  the  marked  aneleclro- 
ionic  decrease  to  the  normal  condition,  and  in  the  latter  from  the  nor- 
mal to  that  of  catelectrotonic  increase.  The  absence  of  contraction 
upon  the  closure  of  the  ascending  current  is  dependent  upon  the 
blocking  of  the  cathodal  stimulus  by  the  decrease  of  the  excitability 
and  conductivity  at  the  anode.  With  the  opening  of  the  descending 
current  the  disappearance  of  anelectrotonus  should  also  be  followed 
by  contraction,  which  would  indeed  be  the  case  if  the  stimulus  so 
generated  was  not  blocked  by  the  decrease  of  the  conductivity  at  the 
cathode. 

The  order  in  which  the  contractions  occur  may  be  tabulated  as 
follows : 

With  Ascending  Current.  With  Descending  Current. 

Weak, I.  K.  C.  C*  _.  K.  C.  C. 

Medium, 2.  K.  C.  C  A.  O.  C.f  K.  C.  C.  A.  O.  C. 

Strong, 3.        -_  A.  O.  C.  K.  C.  C.  A.  O.  C.(?) 

Polar  Stimulation  of  Human  Nerves. — The  preceding  state- 
ments as  to  changes  in  the  excitabihty  caused  by  the  passage  of  a 
constant  current,  as  well  as  to  the  law  of  contraction,  are  based  en- 
tirely on  experiments  made  with  the  isolated  nerve  of  the  frog.  It 
is  probable,  however,  that  the  same  phenomena  would  have  been 
observed  had  the  nerve  of  a  mammal  been  used  and  its  excitability 
been  maintained. 

If  the  electrodes  connected  with  the  wires  of  a  sufficiently  strong 
galvanic  battery  be  applied  to  the  skin  over  the  course  of  a  superficially 
lying  nerve,  e.  g.,  the  brachial,  it  will  be  found  that  there  occurs  on 
the  closure  of  the  circuit  an  increase  in  the  excitability  in  the  extra- 
polar  anelectrotonic  region  and  a  decrease  in  the  excitability  in  the 
extra-polar  catelectrotonic  region,  as  shown  by  stimulating  the  nerve 
in  the  extra-polar  regions  with  the  induced  current — results  which  are 
in  apparent  contradiction  to  those  obtained  with  the  isolated  nerve. 
This  want  of  accordance  in  the  results  of  the  two  classes  of  experi- 
ments arises  from  a  failure  to  recognize  the  fact  that  the  physiologic 
anode  and  cathode  do  not  coincide  with  the  physical  anode  and 
cathode. 

It  has  been  experimentally  demonstrated  that  owing  to  the  large 
amount  of  readily  conducting  tissue  by  which  the  nerve  is  surrounded, 
the  current  density,  though  great  immediately  under  the  electrode, 
quickly  decreases  at  a  short  distance  from  it,  so  that  for  the  nerve  it 
becomes  almost  nil.  The  current,  therefore,  shortly  after  entering, 
again  leaves  the  nerve  at  various  points  which  become  physiologic 


*  K.  C.  C,  cathodal  closing  contraction,      f  A.  O.  C,  anodal  opening  contraction. 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  131 

cathodes.  Stimulation  of  this  physiologic  cathode  with  the  induced 
current  gives  rise,  therefore,  to  the  phenomenon  of  increased  excita- 
bility in  the  region  of  the  anode.  If,  however,  the  galvanic  and 
stimulating  current  be  combined  in  one  circuit  and  both  be  applied 
to  the  same  tract  of  nerve,  results  will  be  obtained  which  harmonize 
with  those  obtained  with  the  frog's  nerve. 

The  changes  in  the  excitability  of  a  nerve  of  a  living  man  and  the 
contractions  which  follow  the  closing  and  opening  of  the  constant 
current  have  been  thoroughly  studied  by  Waller  and  de  Watteville. 
These  observers  employed  a  method  similar  to  that  of  Erb,  conjoin- 
ing in  one  circuit  the  testing  and  polarizing  currents.  By  the  graphic 
method  they  recorded  first  the  contraction  produced  by  an  induc- 
tion shock  alone;  and,  secondly,  the  contraction  produced  by  the 
same  stimulus  under  the  influence  of  the  polarizing  current.  As  a 
result  of  many  experiments,  they  also  demonstrated  an  increase  of 


Fig.  55. — .\node  of  Battery.  Polar 
region  of  nerve  is  anodic.  Peri- 
polar region  of  nerve  is  cathodic. 


Fig.  56.— Cathode  of  Battery.  Polar 
region  of  nerve  is  cathodic.  Peri- 
polar region  of  nerve  is  anodic. — 
(Waller.) 


the  excitability  in  the  polar  region  when  it  is  cathodic,  and  a  decrease 
when  it  is  anodic.  Following  the  suggestion  of  Helmholtz,  that  the 
current  density  quickly  decreases  with  the  distance  from  the  elec- 
trodes, they  recognize,  at  the  point  of  entrance  and  exit  of  the  current 
from  the  nerve,  two  regions — a  polar,  having  the  same  sign  as  the 
electrode,  and  a  peripolar,  having  the  opposite  sign  (Figs.  55  and  56). 
The  peripolar  regions  also  experience  similar  alterations  of  excita- 
bility, though  less  in  degree,  according  as  they  are  cathodic  or  anodic. 
As  it  is  impossible  to  confine  the  current  to  the  tnmk  of  the  nerve 
when  surrounded  by  living  tissues,  as  is  easily  the  case  when  experi- 
menting with  the  frog's  nerves,  it  is  incorrect  to  speak  of  either 
ascending  or  descending  currents.  Waller,*  who  has  thoroughly 
studied  the  electrotonic  effects  of  the  galvanic  current  from  this  point 
of  view,  sums  up  his  conclusions  in  the  following  words:  "We  must 
apply  one  electrode  only  to  the  nerve  and  attend  to  its  effects  alone, 


*"  Human  Physiology-,"  p.  363,  1891. 


132  TEXT-BOOK  OF  PHYSIOLOGY. 

completing  the  circuit  through  a  second  electrode,  which  is  applied 
according  to  convenience  to  some  other  part  of  the  body. 

"Confining  our  attention  to  the  first  electrode,  let  us  see  what 
will  happen  according  as  it  is  anode  or  cathode  of  a  galvanic  current 
(Figs.  55  and  56).  If  this  electrode  be  the  anode  of  a  current,  the 
latter  enters  the  nerve  by  a  series  of  points  and  leaves  it  by  a  second 
series  of  points;  the  former,  or  proximal  series  of  points,  collectively 
constitutes  the  polar  zone  or  region;  the  latter,  or  distal  series  of 
points,  collectively  constitutes  the  peripolar  zone  or  region.  In  such 
case  the  polar  region  is  the  seat  of  entrance  of  current  into  the  nerve — 
i.  e.,  is  anodic;  the  peripolar  region  is  the  seat  of  exit  of  current  from 
the  nerve — i.  e.,  is  cathodic.  If,  on  the  contrary,  the  electrode  under 
observation  be  the  cathode  of  a  current,  the  latter  enters  the  nerve 
by  a  series  of  points  which  collectively  constitute  a  'peripolar'  region, 
and  it  leaves  the  nerve  by  a  series  of  points  which  collectively  con- 
stitute a  'polar'  region.  The  current,  at  its  entrance  into  the  body, 
diffuses  widely,  and  at  its  exit  it  concentrates;  its  'density'  is  greatest 
close  to  the  electrode,  and,  the  greater  the  distance  of  any  point  from 
the  electrode,  the  less  the  current  density  at  that  point;  hence  it  is 
obvious  that  the  current  density  is  greater  in  the  polar  than  in  the 
peripolar  region.  These  conditions  having  been  recognized,  we  may 
apply  to  them  the  principles  learned  by  study  of  frogs'  nerves  under 
simpler  conditions.  Seeing  that,  with  either  pole  of  the  battery, 
whether  anode  or  cathode,  the  nerve  has  in  each  case  points  of  en- 
trance (constituting  a  collective  anode)  and  points  of  exit  to  the  cur- 
rent (constituting  a  collective  cathode),  and  admitting  as  proved  that 
make  excitation  is  cathodic,  break  excitation  anodic,  we  may,  with  a 
sufficiently  strong  current,  expect  to  obtain  a  contraction  at  make 
and  at  break  with  either  anode  or  cathode  applied  to  the  nerve;  and 
we  do  so,  in  fact.  When  the  cathode  is  applied,  and  the  current  is 
made  and  broken,  we  obtain  a  cathodic  make  contraction  and  a  cathodic 
break  contraction;  when  the  anode  is  applied,  and  the  current  is  made 
and  broken,  we  obtain  an  anodic  make  contraction  and  an  anodic 
break  contraction.  These  four  contractions  are,  however,  of  very 
different  strengths;  the  cathodic  make  contraction  is  by  far  the 
strongest;  the  cathodic  break  contraction  is  by  far  the  weakest;  the 
cathodic  make  contraction  is  stronger  than  the  anodic  make  con- 
traction; the  anodic  break  contraction  is  stronger  than  the  cathodic 
break  contraction.  Or,  otherwise  regarded,  if,  instead  of  comparing 
the  contractions  obtained  with  a  sufficiently  strong  current,  we  ob- 
serve the  order  of  their  appearance  with  currents  gradually  increased 
from  weak  to  strong,  we  shall  find  that  the  cathodic  make  contraction 
appears  first,  that  the  cathodic  break  contraction  appears  last,  and 
the  formula  of  contraction  for  man  reads  as  follows: 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE. 


133 


"Weak  current, K.  C.  C. 

Medium  current, K.  C.  C  A.  C.  C.  A.  O.  C. 

Strong  current, K.  C.  C.  A.  C.  C.  A.  O.  C. 


K.  O.  C. 


The  constant  or  the  galvanic  current  is  frequently  used  for  thera- 
peutic and  diagnostic  purposes.  In  accordance  with  the  statements 
above  quoted,  one  electrode  should  be  appHed  to  the  part  to  be  in- 
vestigated, the  other  to  some  indifferent  region.  The  electrode  con- 
veying the  current  to  or  from  this  part  should  be  of  a  size  sufficient 
to  locahze  the  current  and  to  increase  its  density.  It  was  discovered 
by  Duchenne  that  there  are  certain  points  all  over  the  body  stimula- 
tion of  which  is  more  quickly  followed  by  muscle  contraction  than 


M.  biceps  brachii. 

M.  brach.  anticus. 


N.  medianus 


M.  pronator  teres. 

M.  flex,  digitor.  coinraun.  profund. 

M.  flex,  carpi  radialis. 

M.  flex,  digitor.  sublim. 

M.  flex.  dig.  subl.  (dig.  ind.  et  min.) 
\M.  ttex.poU.  lougus 
N.  med- 


N.ulnaris. 


M.  flexor  carpi  ulnaris. 


N.  ulnaris. 


Fig.  57. — Motor  Points  of  the  Median  and  Ulnar  Nerves,  with  the  Muscles 
Supplied  by  Them. — {Landois  and  Stirling) 


Others.  It  was  subsequently  discovered  by  Remak  that  these  points 
coincide  with  the  entrance  of  the  nerve  into  the  muscle.  It  is  to 
these  motor  points  that  the  one  electrode  should  be  appUed.  The 
position  of  some  of  these  points  on  the  forearm  is  shown  in  Fig. 

57- 

Reactions  of  Degeneration. — In  consequence  of  the  degen- 
eration and  changes  in  irritabihty  which  occur  in  nerves  when  separ- 
ated from  their  centers  and  in  muscles  when  separated  from  their 
related  nerves,  either  experimentally  or  as  the  result  of  disease,  the 
response  of  these  structures  to  the  induced,  and  the  make  and  break 
of  the  constant  current,  differs  from  that  observed  in  the  physiologic 
condition.     The  facts  observed  under  the  apphcation  of  these  two 


134  TEXT-BOOK  OF  PHYSIOLOGY. 

forms  of  electricity  are  of  importance  in  the  diagnosis  and  thera- 
peutics of  the  precedent  lesions.  The  principal  difference  of  behavior 
is  observed  in  the  muscles,  which  exhibit  diminished  or  abohshed 
excitabihty  to  the  induced  current,  while  at  the  same  time  manifesting 
an  increased  excitability  to  the  constant  current;  so  much  so  is  this  the 
case  that  a  closing  contraction  is  just  as  likely  to  occur  at  the  positive 
as  at  the  negative  pole.  This  peculiarity  of  the  muscle  response  is 
termed  the  reaction  of  degeneration.  The  synchronous  diminished 
excitability  of  the  nerves  is  the  same  for  either  current.  The  term 
"partial  reaction  of  degeneration"  is  used  when  there  is  a  normal 
reaction  of  the  nerves,  with  the  degenerative  reaction  of  the  muscles. 
This  condition  is  observed  in  progressive  muscular  atrophy. 

Reflex  Action.^ — Inasmuch  as  many  of  the  muscle  movements  of 
the  body,  as  well  as  the  formation  and  discharge  of  secretions  from 
glands,  variations  in  the  cahber  of  blood-vessels,  inhibition  and 
acceleration  in  the  activity  of  various  organs,  are  the  result  of  stimu- 
lations of  the  terminal  organs  of  afferent  nerves,  they  are  termed,  for 
convenience,  reflex  actions,  and,  as  they  take  place  for  the  most  part 
through  the  spinal  cord  and  medulla  oblongata  and  independently 
of  the  brain  or  of  volitional  influences,  they  are  also  termed  involun- 
tary actions.  A  reflex  action,  therefore,  may  be  defined  as  an  action 
which  takes  place  independent  of  volition  and  in  response  to  per- 
ipheral stimulation.  As  many  of  the  processes  to  be  described  in 
succeeding  chapters  are  of  this  character,  requiring  for  their  per- 
formance the  cooperation  of  several  organs  and  tissues  associated 
through  the  intermediation  of  the  nerve  system,  it  seems  advisable  to 
consider  briefly,  in  this  connection,  the  parts  involved  in  a  reflex 
action,  as  well  as  their  mode  of  action.  As  shown  in  Fig.  58,  the 
necessary  structures  are  as  follows : 

1.  A  sentient  surface,  skin,  mucous  membrane,  sense-organ,  etc. 

2.  An  afferent  nerve-fiber  and  cell. 

3.  An  emissive  cell,  from  which  arises  — 

4.  An  efferent  nerve,  distributed  to  a  responsive  organ,  as 

5.  Muscle,  gland,  blood-vessel,  etc. 

Such  a  combination  of  structures  constitutes  a  reflex  mechanism 
or  arc,  the  nerve  portion  of  which  is  composed  of  but  two  neurons — 
an  afl"erent  and  an  efferent.  An  arc  of  this  simplicity  would  of  neces- 
sity subserve  but  a  simple  movement.  The  majority  of  reflex  activ- 
ities, however,  are  extremely  complex,  and  involve  the  cooperation 
and  coordination  of  a  number  of  nerve  centers  situated  at  different 
levels  of  the  spinal  cord  on  the  same  and  opposite  side,  and  of  re- 
sponsive organs  frequently  situated  at  distances  more  or  less  remote 
from  one  another.  This  implies  that  a  number  of  neurons  are 
associated  in  function.  The  afferent  neurons  are  brought  into  re- 
lation with  the  dendrites  of  the  efferent  neurons  by  the  end-tufts  of 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE. 


135 


the  collateral  branches,  which  may  extend  for  some  distance  up  and 
down  the  cord  before  passing  into  the  various  segments. 

For  the  excitation  of  a  reflex  action  it  is  essential  that  the 
stimulus  applied  to  the  sentient  surface  be  of  an  intensity  sufficient 
to  develop  in  the  terminals  of  the  afferent  nerve  a  series  of  nerve 
impulses,  which,  traveling  inward,  will  be  distributed  to  and  re- 
ceived by  the  dendrites  of  the  emissive  or  motor  cell.  With  the 
reception  of  these  im- 
pulses there  is  apparently  a 
a  disturbance  of  the  L-cS^y  pf^  ~ 
equihbrium  of  its  molec-  '~^ 
ules,  a  liberation  of  en- 
ergy, and,  in  conse- 
quence, a  transmission 
outward  of  impulses 
through  the  efferent 
nerve  to  muscle,  gland, 
or  blood-vessel,  separ- 
ately or  collectively,  with 
the  production  of  muscu- 
lar contraction,  glandular 
secretion,  vascular  dila- 
tation or  contraction,  etc. 

The  reflex  actions  take  place,  for  the  most  part,  through  the  spinal 
cord  and  medulla  oblongata,  which,  in  virtue  of  their  contained 
centers,  coordinate  the  various  organs  and  tissues  concerned  in  the 
performance  of  the  organic  functions.  The  movements  of  mastica- 
tion; the  secretion  of  saHva;  the  muscular,  glandular,  and  vascular 
phenomena  of  gastric  and  intestinal  digestion;  the  vascular  and 
respiratory  movements;  the  mechanism  of  micturition,  etc.,  are  illus- 
trations of  reflex  activitv. 


.  58. — Diagram  Showing  Structures  Con- 
nected WITH  Reflex  Actions.  A.  Trans- 
verse section  of  spinal  cord  with  centers  in  the 
anterior  horn  of  the  gray  matter  for  muscles 
m,  glands,  g,  and  blood-vessels,  h.  ej.n.  Efferent 
nerves  which  convey  nerve  impulses  to  these 
organs.  5.  Sensory  surface,  af.n.  Afferent 
nerve  conveying  nerve  impulses  to  the  centers 
in  the  spinal  cord. 


CHAPTER  VIII. 

FOODS. 

^The  functional  activity  of  every  organ  and  tissue  of  the  body  is 
accompanied  by  a  more  or  less  active  disintegration  of  the  living 
material,  the  bioplasm,  of  which  it  is  composed.  The  complex  and 
highly  unstable  molecules  of  this  living  material  are  continually 
undergoing  disruption  and  falhng  into  less  complex  and  more  stable 
compounds;  these,  through  oxidative  processes,  are  eventually  re- 
duced through  a  series  of  descending  chemic  stages  to  a  small  number 
of  simpler  compounds  w^hioh,  being  of  no  further  value  to  the  organ- 
ism, are  ehminated  by  the  various  ehminating  or  excretory  organs, 
the  lungs,  skin,  kidney,  liver.  Among  these  excreted  compounds  the 
more  important  are  urea,  uric  acid,  and  carbon  dioxid.  Many  other 
compounds,  organic  as  well  as  inorganic,  are  also  eliminated  from 
the  body  in  the  various  excretions,  though  they  are  present  in  but 
small  amounts.  Coincident  with  this  disintegration  of  living  material 
there  is  a  transformation  of  its  potential  into  kinetic  energy,* which 
manifests  itself  for  the  most  part  as  heat  and  mechanic  motion. 

In  order  that  the  organs  and  tissues  may  continue  in  the  per- 
formance of  their  functions,  it  is  essential  that  they  be  supplied  with 
nutritive  materials  similar  to  those  which  enter  into  their  own  com- 
position: viz.,  proteids,  fat,  carbohydrates,  water,  and  inorganic 
salts.  These  compounds,  though  originally  derived  from  the  food, 
are  immediately  derived  from  the  blood  as  it  flows  through  the  capil- 
lary blood-vessels.  The  blood  is  therefore  to  be  regarded  as  a  reser- 
voir of  nutritive  material  in  a  condition  to  be  absorbed  and  trans- 
formed into  utilizable  and  living  material.  Inasmuch  as  the  mate- 
rials lost  to  the  body  daily,  through-  disintegration  and  oxidation, 
though  considerable,  are  supplied  by  the  blood,  it  is  evident  that  this 
fluid  would  diminish  rapidly  in  volume,  with  a  corresponding  decline 
in  functional  activity,  were  it  not  restored  by  the  introduction  into 
the  body  of  new  material  in  the  food.  With  the  diminution  of  the 
volume  of  the  blood  and  an  insufficient  supply  to  the  tissues,  there 
arise  the  sensations  of  hunger  and  thirst,  which  lead  to  the  consump- 
tion of  food  and  the  subsequent  restoration  of  the  physiologic  condi- 
tion of  the  tissues.  These  two  sensations  are  also  partially  dependent 
on  the  empty  condition  of  the  stomach  and  the  dryness  of  the  mucous 
membrane  of  the  mouth  and  throat. 

136 


FOODS.  137 

The  foods  which  are  consumed  daily  in  response  to  the  sensations 
of  hunger  and  thirst  are  complex  in  composition  and  contain,  though 
in  varying  amounts,  proteids,  fats,  carbohydrates,  water,  and  inor- 
ganic salts,  which,  in  contradistinction  to  foods,  are  termed  food 
principles  or  nutritive  principles.  In  these  compounds  is  also  to  be 
found  the  potential  energy  necessary  to  maintain  the  dynamic  equi- 
librium of  the  body  and  which  will  become  manifest  as  heat  and 
mechanic  motion  in  the  transformations  of  the  material  underlying 
the  nutritive  processes. 

The  animal  body  may  be  therefore  regarded  as  a  machine  capable 
each  day  of  performing  a  certain  amount  of  work  by  the  expendi- 
ture of  a  definite  amount  of  energy.  In  the  performance  of  its  work, 
whether  it  be  the  raising  of  weights  against  gravity,  the  overcoming 
of  friction,  cohesion,  or  elasticity,  the  machine  suffers  disintegration 
and  loses  a  portion  of  its  available  energy.  Unlike  other  machines, 
however,  it  possesses  the  power,  within  limits,  of  self-renewal,  self- 
adjustment,  when  supplied  with  foods  in  proper  quantity  and  quality. 


QUANTITIES  OF  FOOD  PRINCIPLES  REQUIRED  DAILY. 

In  order  that  the  body  may  continue  in  the  performance  of  its 
work  and  yet  retain  a  given  weight,  it  is  essential  that  the  loss  to  the 
body  daily  shall  be  exactly  compensated  by  the  introduction  and 
assimilation  of  a  corresponding  amount  of  food  principles.  If  this 
condition  is  realized,  the  body  neither  gains  nor  loses,  but  remains  in 
a  condition  of  nutritive  equilibrium.  The  determination  of  the  exact 
quantities  of  the  different  food  principles  required  daily  and  their 
ratio  one  to  another  is  made  from  an  examination  of  the  quantity  and 
composition  of  the  daily  excretions.  Since  the  proteids  disintegrated 
are  represented  in  the  excretions  by  urea  and  similar  nitrogen-holding 
compounds  and  the  fats  and  carbohydrates  by  carbon  dioxid,  it 
becomes  possible  to  determine  from  them  the  quantities  required  to 
restore  equihbrium  under  any  given  condition.  But  as  the  activity 
of  the  nutritive  changes  will  vary  in  accordance  with  climatic  condi- 
tions, work  done,  etc.,  and  as  the  excreted  products  will  vary  in  the 
same  ratio,  it  is  obvious  that  the  required  amounts  of  food  will  vary 
in  accordance  with  these  varying  conditions,  if  equilibrium  is  to  be 
maintained. 

Various  estimates  have  been  made  by  different  investigators  as 
to  the  amounts  of  the  excreted  products  and  the  food  principles  re- 
quired daily,  which,  though  differing  to  some  extent,  have,  neverthe- 
less, an  average  nutritive  and  energy-producing  value.  The  follow- 
ing table  shows  the  diet  scale  of  Vierordt  and  the  excretions  to  which 
it  would  give  rise.  As  the  income  and  outgo  practically  balance, 
there  would  be  no  change  in  the  weight. 


138  TEXT-BOOK  OF  PHYSIOLOGY. 

COMPARISON  OF  THE  INCOME  AND  OUTGO. 


Incomi;. 


Proteid, 

Fat, 

Carbohydrates,  -. 

Salts, 

Water, j     2 

Oxygen, 


Grams. 

Ounces. 

120 

4-25 

90 

3-17 

330 

11.64 

32 

I-I3 

2818 

99-30 

756 
4146 

26.66 

146.13 

Outgo. 


Water, 

Urea, 

Feces,  dry, 

Salts, 

Carbon  dioxid, 

Water  formed  in  body, 


Gram.s. 

Ounces. 

2818 

99-3° 

40 

1.40 

38 

1.60 

32 

1-13 

922 

32-37 

296 

IO-33 

4146 

146.15 

Other  estimates  as  to  the  amounts  of  the  organic  food  principles 
required  daily  are  as  follows : 

Ranke.  Voit.  Moleschott.  Atwater.  Hultgren. 

Grams.  Grams.  Grams.  Grams.  Grams. 

Proteid, 100  118  130  125  134 

Fat,   100  56  84  125  79 

Starch, 250       500       550       400       522 

In  arranging  tables  showing  the  relation  between  the  income  and 
the  outgo,  it  is  generally  customary  to  state  merely  the  amounts  by 
weight  of  the  nitrogen  and  carbon  each  contains.  This  method 
furnishes  sufficiently  accurate  information  regarding  the  metabolism 
of  the  body,  for  the  reason  that  the  nitrogen  represents  the  proteid, 
and  the  carbon,  with  the  exception  of  that  contained  in  the  proteid, 
the  fat  and  carbohydrates  which  have  undergone  disintegration  or 
metabolism. 

The  following  balance  table,  as  given  by  Ranke,  shows  the  rela- 
tion of  the  nitrogen  to  the  carbon  in  the  average  mixed  diet  and  in  the 
excretions  of  a  man  weighing  70  kilograms,  in  a  condition  of  nutritive 
equilibrium : 


Income. 

Grams. 

100 

N. 

c. 

Proteid, . 

15-5 

53-0 
79.0 
93 -o 

Fat, .   . 

Carbohydrates,  — 

250 

15-5 

225.0 

Outgo. 


Grams 


Urea,  _  _ . 
Uric  acid. 
Feces,  --. 
CO,,  --. 


10.84 
208.00 

225.00 


FOODS.  139 

From  the  above  it  will  be  observed  that  the  daily  discharge  for 
each  kilogram  of  body-weight  is  0.21  gram  nitrogen  and  3.03  grams 
of  carbon;  the  relation  of  the  two  being  -^  =  14.5.  On  a  diet  in 
which  there  is  an  excess  of  either  proteid  or  carbohydrates  this  ratio 
necessarily  changes. 


CLASSIFICATION  OF  FOOD  PRINCIPLES. 

Though  the  food  principles  are  grouped  as  proteids,  fats,  carbo- 
hydrates, etc.,  the  members  of  each  group  differ  somewhat  in  chemic 
composition,  digestibility,  and  nutritive  value.  These  groups  are  as 
follows: 

I.  Peoteids. 

Principle.  Where  found. 

Myosin, . Flesh  of  animals. 

Albumin,  vitellin, White  of  egg,  yolk  of  egg. 

Caseinogen, Milk. 

Serum-albumin,   fibrin, Blood  contained  in  meat. 

Glutin, Grain  of  wheat  and  other  cereals. 

Vegetable  albumin, Soft-growing  vegetables. 

Legumin, Peas,  beans,  lentils,  etc. 

2.  Fats. 

Animal  fats, In  adipose  tissue  of  animals. 

Vegetable  oils, In  seeds,  grains,  nuts,  fruits,  and  other 

vegetable  tissues. 

3.   Carbohydr.^tes. 

Dextrose  or  grape-sugar )  j^  ^^.^^_ 

Levulose  or  fruit-sugar, I 

Lactose  or  milk-sugar, Milk. 

Saccharose  or  cane-sugar, Sugar-cane,  beet  roots. 

Maltose, Malt  and  malted  foods. 

q        ,  I  Cereals,  tuberous  roots,  and  legumin- 

,                                                    <j^      ^^g  plants. 
Glycogen, Liver,  muscles. 

4.  Inorganic. 

Water, 

Sodium  and  potassium  chlorid, f  -.  1        n        •      1        j  .  ui 

e  J-  /^     •  J        ,  .  In    nearlv    all    animal    and   vegetable 

Sodium,    potassium,    and    calcium  }      t    (\     ' 

phosphates  and  carbonates, \ 

Iron, ' 

5.  Vegetable  Acids. 

Citric,  tartaric    acetic,  malic, In  fruit  and  vegetables. 

6.  Accessory  Foods. 
Coffee    Tea,  Cocoa,  Alcohol. 


I40  TEXT-BOOK  OF  PHYSIOLOGY. 

Disposition  of  Food. — The  protcid  principles  of  the  food,  after 
undergoing  digestion  and  conversion  into  peptones,  are  absorbed 
into  the  blood.  During  the  act  of  absorption  they  are  transformed 
into  the  form  of  proteids  characteristic  of  blood.  After  being  dis- 
tributed by  the  blood-stream  to  the  tissues,  they  are  brought  into 
relation  with  the  living  cells.  The  disposition  made  of  the  proteid 
material  by  the  bioplasm  of  the  cell  has  not  been  definitely  deter- 
mined. According  to  Voit,  of  the  proteid  thus  brought  into  contact 
with  the  living  tissues,  only  a  small  percentage  is  utilized  and  assimi- 
lated for  tissue  repair.  This  he  terms  tissue  or  organ  proteid.  The 
remaining  large  percentage  circulating  in  the  interstices  of  the  tissues, 
though  not  forming  an  integral  part  of  them,  is  acted  on  directly  by 
them,  merely  in  virtue  of  contact — split  up,  oxidized,  and  reduced  to 
simpler  compounds.     This  he  terms  circulating  proteid. 

According  to  Pfliiger  and  others,  this  view  is  not  tenable.  Pfliiger 
asserts  that,  as  material  changes  or  metaboHsm  can  only  take  place 
within  living  cells,  all  the  proteid  must  first  be  assimilated  and  organ- 
ized by  the  cells  before  it  can  undergo  metabolic  changes.  Metab- 
olism by  contact  action  is  denied,  and  the  division  of  proteids  into 
organ  and  circulating  proteid  is  not  justifiable. 

In  the  process  of  metabolism  the  proteid  suffers  disintegration,  giv- 
ing rise  through  oxidation  to  some  carbon-holding  compound,  possibly 
fat,  and  to  some  nitrogen-holding  compounds,  which  eventually  give 
rise  to  urea.  The  intermediate  stages,  though  not  definitely  known,  are 
possibly  represented  by  glycin,  creatin,  ammonium  carbamate,  etc. 
The  disintegration  of  the  proteids  is  attended  by  the  disengagement  of 
heat,  thus  contributing  to  the  general  store  of  the  energy  of  the  body. 

The  fat  principles,  after  digestion,  are  absorbed  by  the  lymphatic 
vessels  and  discharged  by  the  thoracic  duct  into  the  blood,  from  which 
they  rapidly  disappear.  Though  it  is  possible  that  a  portion  of  the 
fat  enters  directly  into  the  formation  of  the  living  material,  it  is  gener- 
ally believed  that  it  is  at  once  oxidized  and  reduced  to  carbon  dioxid 
and  water  with  the  liberation  of  energy.  The  natural  supposition 
that  a  portion  of  the  ingested  fat  was  directly  stored  up  in  the  cells  of 
the  areolar  connective  tissue,  thus  giving  rise  to  adipose  tissue,  has 
been  a  subject  of  much  controversy,  though  modern  experimentation 
renders  this  very  probable.  The  body-fat,  under  physiologic  con- 
ditions, is  also  a  product  of  the  metabohc  activity  of  connective-tissue 
cells  and  is  a  derivative  of  both  proteids  and  carbohydrates. 

The  carbohydrate  principles,  after  digestion,  are  absorbed  into 
the  blood  as  dextrose.  This  compound  is  then  stored  up  in  the  liver 
and  muscles  as  glycogen.  The  intermediate  stages  which  glycogen 
passes  through  before  it  is  reduced  to  carbon  dioxid  and  water  are 
only  imperfectly  known.  Though  a  large  part  of  the  carbohydrate 
material  is  at  once  oxidized,  it  is  now  well  established  that  another 


FOODS.  141 

portion  contributes  to  the  formation  of,  if  it  is  not  directly  converted 
into,  fat.  As  the  carbohydrates  form  a  large  portion  of  the  food,  they 
contribute  materially  to  the  production  of  energy. 

The  inorganic  principles,  though  not  playing  apparently  as  active 
a  part  in  the  metabohsm  of  the  body  as  the  organic,  are  nevertheless 
essential  to  its  physiologic  activity. 

Water  is  promptly  absorbed  after  ingestion  and  becomes  a  part 
of  the  circulating  fluids — blood  and  lymph.  In  the  digestive  appa- 
ratus it  favors  the  occurrence  of  those  chemic  changes  in  the  food 
necessary  for  their  absorption,  it  promotes  absorption  of  the  food, 
holds  various  constituents  of  the  blood  and  other  fluids  in  solution, 
hastens  the  general  metabohsm  of  the  body,  holds  in  solution  various 
products  of  metabohc  activity,  and,  leaving  the  body  through  the 
excretory  organs,  promotes  their  elimination. 

Sodium  chlorid  is  absorbed  into  the  blood  and,  unless  taken  in 
excess,  is  utihzed  in  replacing  that  M^hich  is  lost  to  the  organism  daily. 
The  exact  role  which  sodium  chlorid  plays  in  the  nutritive  process 
is  unknown;  but,  as  it  is  present  as  a  necessary  constituent  in  all  the 
fluids  and  sohds  of  the  body,  and  as  it  is  instinctively  employed  as 
a  condiment,  it  may  be  assumed  to  have  a  more  or  less  important, 
function. 

When  taken  as  a  condiment,  it  imparts  sapidity  to  the  food  and 
excites  the  flow  of  the  digestive  fluids;  it  ultimately  furnishes  the 
chlorin  for  the  hydrochloric  acid  of  the  gastric  juice.  Judging  from 
the  impairment  of  the  nutrition  as  observed  in  animals  after  depriva- 
tion of  salt  for  a  long  period  of  time,  it  favorably  influences  the  growth 
and  functional  activity  of  all  tissues. 

It  is  well  known  that  herbivorous  animals,  races  of  men  as  well  as 
individuals  who  live  largely  on  vegetable  foods,  require  a  larger  addi- 
tional amount  of  sodium  chlorid  than  carnivorous  animals,  or  human 
beings  who  live  largely  on  animal  foods,  even  though  the  two  classes 
of  foods  contain  relatively  the  same  amounts.  The  explanation  is 
that  the  vegetable  foods  contain  potassium  salts  which,  meeting  in 
the  blood  with  sodium  chlorid,  undergo  decomposition  into  potassium 
chlorid  and  sodium  carbonate  or  phosphate,  all  of  which,  when  in 
excess,  are  at  once  ehminated  by  the  kidneys.  The  blood,  therefore, 
becomes  poorer  in  sodium  chlorid,  one  of  its  necessary  constituents. 

Potassium  phosphate  and  carbonate  are  also  essential  to  the 
normal  composition  of  the  sohds  and  fluids.  They  impart  a  certain 
degree  of  alkahnity  to  the  blood  and  lymph,  one  of  the  conditions 
necessary  to  the  hfe  and  activity  of  the  tissue-cells  bathed  by  them. 
When  administered  in  small  doses,  they  increase  the  force  of  the 
heart,  raise  the  arterial  pressure,  and  increase  the  activity  of  the 
circulation. 

Calcium  phosphate  and  carbonate  are  partly  utilized  in  maintain- 


142  TEXT-BOOK  OF  PHYSIOLOGY. 

ing  the  solidity  of  the  bones  and  teeth,  replacing  the  amount  metab- 
ohzed  daily.  Inasmuch  as  the  metabolism  of  these  two  tissues  is 
slight,  there  is  not  much  need  in  the  adult  for  lime  as  an  article  of 
food.  In  young  animals  lime  is  essential  to  the  solidification  and 
development  of  bone.  When  deprived  of  it,  the  skeleton  undergoes 
a  defective  development  similar  to  the  pathologic  condition  known 
as  rickets.  Lime  is  present  in  milk  to  the  extent  of  0.15  per  cent.,  as 
well  as  in  eggs  and  peas  in  relatively  large  quantities. 

Iro7i  is  contained  in  both  animal  and  vegetable  foods,  not,  how- 
ever, in  the  form  of  inorganic  iron,  nor  in  the  form  of  an  organic  salt, 
but  as  a  compound  with  nuclein,  thus  forming  an  integral  part  of  the 
proteid  molecule.  After  absorption  the  iron  is  utilized  in  the  forma- 
tion of  the  coloring-matter  of  the  blood-corpuscles — hemoglobin. 
The  organic  compounds  of  iron  and  the  nucleins  have  been  termed 
hematogens.  The  amount  of  iron  ingested  has  been  estimated  at 
from  10  to  90  milhgrams,  the  larger  part  of  which  is  eliminated  in 
the  feces.  The  relatively  small  part  ehminated  by  the  kidneys  and 
liver  is  usually  taken  as  the  amount  metabolized,  though  it  is  probable 
that  this  is  not  wholly  true,  as  there  is  evidence  that  iron  can  be  re- 
tained in  the  body  and  utihzed  again  in  the  formation  of  new  hemo- 
globin. Contrary  to  what  might  be  expected,  milk  contains  but  a 
very  small  quantity  of  iron,  not  more  than  3  or  4  milligrams  in  1000 
grams  (human  milk) — an  amount  insufficient  for  the  development  of 
the  necessary  hemoglobin.  This  is  compensated  for,  however,  by 
the  accumulation  of  iron  in  the  liver  during  intrauterine  life.  Ac- 
cording to  Bunge,  the  liver  of  a  newly  born  rabbit  contains  as  much 
as  18.2  milhgrams  per  100  grams  of  body-weight,  while  at  the  end 
of  twenty- four  days  it  only  contains  3.2  milligrams  per  100  grams  of 
body- weight. 

Vegetable  acids  increase  the  secretions  of  the  alimentary  canal,  and 
are  apt,  in  large  amounts,  to  produce  flatulence  and  diarrhea.  After 
entering  into  combination  with  bases  to  form  salts,  they  stimulate  the 
action  of  the  kidneys  and  promote  a  greater  elimination  of  all  the 
urinary  constituents.  In  some  unknown  way  they  influence  nutrition ; 
when  deprived  of  these  acids,  the  individual  becomes  scorbutic. 

The  accessory  foods, — coffee,  tea,  and  cocoa, — when  taken  in 
moderation  have  a  stimulating  influence  on  the  nervous  system,  as 
shown  by  the  removal  of  both  mental  and  physical  fatigue,  by  an 
increased  capacity  for  sustained  mental  work,  by  the  persistent  wake- 
fulness among  those  unaccustomed  to  their  use.  Coffee  more  especially 
increases  the  frequency  and  force  of  the  heart-beat,  raises  the  arterial 
pressure,  and  hastens  the  general  blood-flow.  It  has  no  influenc 
either  in  the  way  of  increasing  or  decreasing  proteid  metabohsme 

Tea  frequently  acts  as  an  astringent  on  the  alimentary  canal  on 
account  of  the  tannin  which  passes  into  the  water  when  the  infusion 


FOODS.  143 

is  made.  Inasmuch  as  tannin  also  coagulates  peptones,  the  excessive 
use  of  tea  as  a  beverage  is  apt  to  derange  the  digestive  organs  and 
the  general  process  of  digestion. 

Cocoa  is  more  nutritive  than  either  coffee  or  tea,  on  account  of  the 
large  amount  of  fat  and  proteid  it  contains.  It  is,  hov^Tver,  less 
stimulating. 

The  active  principles  in  coffee,  tea,  and  cocoa,  and  to  which  their 
effects  are  to  be  attributed,  are  caffeifi,  thein,  and  theobro?m?i  respec- 
tively. These  alkaloids  are  chemically  closely  related  one  to  the 
other  and  to  the  compound  xanthin.  They  are  present  in  the  coffee 
seeds,  the  tea  leaves,  and  the  cocoa  bean  to  the  extent  of  1.7  per  cent., 
1.4  per  cent.,  and  1.6  per  cent,  respectively.  When  prepared  as  a 
beverage,  however,  there  is  three  times  as  much  caffein  in  coffee 
as  thein  in  tea. 

Alcohol  when  taken  in  small  quantities  stimulates  the  digestive 
glands, to  increased  activity  and  thus  promotes  digestive  power.  Its 
absorption  into  the  blood  is  followed  by  increased  action  of  the  heart, 
dilatation  of  the  cutaneous  blood-vessels,  a  sensation  of  warmth, 
and  an  excitation  of  the  brain.  In  large  quantities  it  acts  as  a  paralyz- 
ant, depressing  more  especially  the  vaso-constrictor  nerve-centers  and 
certain  areas  of  the  brain,  as  shown  by  an  impairment  in  the  power  of 
sustained  attention,  clearness  of  judgment,  and  muscle  coordination. 

Alcohol  is  undoubtedly  oxidized  in  the  body,  as  only  about  2 
per  cent,  can  be  obtained  from  the  urine  and  expired  air.  It  thus 
contributes  to  the  store  of  the  body-energy.  As  to  whether  for  this 
reason  it  can  be  regarded  as  a  food, — that  is,  whether  it  can  be  sub- 
stituted in  part  at  least  for  fat  or  carbohydrate  material  without  im- 
pairing the  proteid  metabohsm, — is  at  present  a  subject  of  experimen- 
tation and  discussion.  According  to  some  investigators,  alcohol  does 
not  retard  proteid  metabohsm,  for  when  it  is  introduced  into  the  body 
in  amounts  equivalent  to  the  carbohydrates  withdrawn  from  the  food 
there  is  at  once  a  rise  in  the  amount  of  nitrogen  excreted.  Hence  it 
cannot  be  regarded  as  a  food.  According  to  other  investigators, 
alcohol  retards  or  protects  proteid  metabohsm  just  as  effectually  as 
an  equivalent  amount  of  starch  or  sugar.  Many  more  experiments 
are  required  to  decide  this  question.  When  taken  habitually  in  large 
quantities,  alcohol  deranges  the  activities  of  the  digestive  organs, 
lowers  the  body-temperature,  impairs  muscle  power,  lessens  the 
resistance  to  depressing  external  conditions,  diminishes  the  capacity 
for  sustained  mental  work,  and  leads  to  the  development  of  structural 
changes  in  the  connective  tissues  of  the  brain,  spinal  cord,  and  other 
organs.  In  zymotic  diseases  and  in  cases  of  depression  of  the  vital 
powers  it  is  most  useful  as  a  restorative  agent. 


144  TEXT-BOOK  OF  PHYSIOLOGY. 


THE  ENERGY  OR  HEAT  VALUE  OF  FOOD  PRINCIPLES. 

The  food  consumed  not  only  restores  the  material  metabolized 
and  discharged  from  the  body,  but  also  the  energy  which  has 
been  expended  as  heat  and  mechanic  motion.  The  food  principles 
are  products  of  the  constructive  processes  taking  place  in  the  vege- 
table world  during  the  period  of  growth  and  activity.  At  the  time 
of  their  formation  there  is  an  absorption  and  storing  of  the  sun's 
energy  which  then  exists  in  a  potential  condition.  During  the  metab- 
olism of  the  animal  body  these  compounds  are  reduced  through 
oxidation  to  relatively  simple  bodies,  such  as  carbon  dioxid,  water, 
urea,  etc.,  with  the  hberation  of  their  contained  energy.  All  of  the 
energy  of  the  body,  whatever  its  manifestations  may  be,  can  be  traced 
to  chemic  changes  going  on  in  the  tissues,  and  more  particularly  to 
those  changes  involved  in  the  oxidation  of  the  food  principles. 

The  amount  of  heat  or  energy  which  any  given  food  principle  will 
yield  can  be  determined  by  burning  a  definite  amount  {e.  g.,  i  gram) 
to  carbon  dioxid  and  water  and  ascertaining  the  extent  to  which  the 
heat  thus  liberated  will  raise  the  temperature  of  a  given  amount  of 
water  (e.  g.,  i  kilogram).  The  amount  of  heat  may  be  expressed  in 
gram  or  kilogram  degrees  or  calories,  a  gram  calorie  or  kilogram 
calorie  being  the  amount  of  heat  required  to  raise  the  temperature  of 
a  gram  or  a  kilogram  (looo  grams)  of  water  i°  C.  The  apparatus 
employed  for  this  purpose  is  termed  a  calorimeter,  and  consists 
essentially  of  a  closed  chamber  in  which  the  oxidation  takes  place, 
surrounded  by  a  water  jacket,  the  rise  in  temperature  of  the  water 
indicating  the  amount  of  heat  produced. 

The  results  obtained  by  investigators  employing  different  calorim- 
eters and  different  food  principles  of  the  same  group  vary,  though 
within  certain  limits:  e.  g.,  i  gram  of  casein  yields  5.867  kilogram 
calories;  i  gram  of  lean  beef,  5.656  calories;  i  gram  of  fat  yields 
9.353,  9.423,  9.686  calories;  i  gram  of  carbohydrate,  4.182,  4.479, 
etc.,  calories.  These  numbers  represent  the  physical  heat  values  of 
these  food  principles. 

In  the  human  body  as  determined  by  calorimetric  methods  the 
oxidation  of  the  food  principles  yields  practically  the  same  amount 
of  heat  they  yield  when  oxidized  outside  the  body,  with  the  excep- 
tion of  the  proteids,  which  are  oxidized  only  to  the  stage  of  urea.  As 
this  compound  is  capable  of  further  reduction  in  the  calorimeter  to 
carbon  dioxid  and  water  with  the  hberation  of  heat,  the  quantity  of 
heat  it  contains  must  therefore  be  deducted  from  the  calorimetric 
heat  value  of  the  proteid.  According  to  Rubner,  i  gram  of  urea  will 
yield  2.523  kilogram  calories.  As  the  urea  which  results  from  the 
oxidation  of  i  gram  of  proteid  is  about  ^  of  a  gram,  the  amount  of 
heat  to  be  deducted  from  the  heat  value  of  the  proteid  is  -J  of  2.523,  or 
0.841  calories.     It  has  also  been  shown  that  some  of  the  ingested 


FOODS.  145 

proteid  escapes  in  the  feces,  the  heat  value  of  which  must  also  be 
determined  and  deducted.  This  having  been  done,  the  physiologic 
heat  value  becomes  4.124  calories. 

The  following  estimates  give  approximately  the  number  of  kilo- 
gram calories  produced  when  the  food  is  burned  to  carbon  dioxid, 
water,  and  urea  in  the  body: 

I  gram  of  proteid  yields, 4.124  calories 

I       "       "  fat  "      9.353 

I       "       "  carbohydrate  yields, 4.116        " 

The  total  number  of  kilogram  calories  or  kilogram  degrees  of 
heat  yielded  by  any  of  the  previously  given  diet  scales  can  be  readily 
determined  by  multiplying  the  quantities  of  food  principles  con- 
sumed by  the  above-mentioned  factors.  The  diet  scale  of  Vierordt, 
for  example,  yields  the  following: 

120  grams  of  proteid  yields,    494.88  calories. 

90  "     "    fat  "        841.77 

330         "     "   starch         "        1358.28        " 

2694.93 

The  total  calories  obtained  from  other  diet  scales  would  be  as 
follows:  Ranke,  2335;  Voit,  3387;  Moleschott,  2984;  Atwater,  3331; 
Hukgren,  3436. 

Starvation. — The  relation  of  the  different  food  principles  to  the 
general  nutritive  process  becomes  more  apparent  from  an  examination 
of  the  excretions  from  the  body  during  the  process  of  starvation  com- 
bined with  an  examination  of  the  organs  and  tissues  after  death. 
If  an  animal  be  deprived  entirely  of  food,  a  dechne  in  body-weight 
at  once  sets  in,  which  continues  until  about  40  per  cent,  of  the  weight 
has  been  lost,  when  death  generally  ensues.  This  results  from  the 
fact  that  the  active  tissue  cells  consume,  for  the  purpose  of  maintain- 
ing the  normal  temperature  of  the  body,  not  only  their  own  reserve 
food  material,  but  that  of  the  less  active  or  storage  tissues  as  well; 
and,  in  consequence,  there  is  a  progressive  diminution  in  weight. 

The  phenomena  which  characterize  this  non-physiologic  con- 
dition are  as  follows:  hunger,  intense  thirst,  gastric  and  intestinal 
uneasiness  and  pain,  diminished  pulse-rate  and  respiration,  muscular 
weakness  and  emaciation,  a  lessening  in  the  amount  of  urine  and  its 
constituents,  diminished  exhalation  of  carbon  dioxid,  an  exhalation 
of  a  fetid  odor  from  the  body,  vertigo,  stupor,  dehrium,  at  times  con- 
vulsions, a  sudden  fall  in  body-temperature,  and  finally  death.  The 
duration  of  life  after  complete  deprivation  of  food  varies  from  eight  to 
thirteen  days  or  more,  though  this  period  can  be  prolonged  if  the  ani- 
mal be  supphed  with  water,  this  being  more  essential  under  the  cir- 
cumstances than  the  organic  materials  which  can  be  supphed  by  the 
organism  itself.  The  duration  of  the  starvation  period  will  vary  in 
accordance  with  the  previous  condition  of  the  animal  and  the  amount 
10 


146 


TEXT-BOOK  OF  PHYSIOLOGY. 


of  reserved  food  the  body  contains.  The  excretion  of  urea  dechnes 
very  rapidly  during  the  first  two  days — a  fact  which  has  been  attrib- 
uted to  a  rapid  consumption  of  the  surplus  proteid  food.  After 
this  period,  when  the  tissues  begin  to  metabolize  their  own  proteid, 
the  excretion  remains  fairly  constant  until  toward  the  close,  when  the 
amount  ehminated  falls  very  rapidly.  As  proteids  contain  about  16 
per  cent,  of  nitrogen,  i  part  of  nitrogen  equals  6.25  parts  of  proteid. 
Hence,  for  every  i  gram  of  nitrogen  or  2.14  grams  urea  excreted, 
it  may  be  assumed  that  6.25  grams  of  proteid  or,  according  to  Voit,  30 
grams  of  flesh  have  been  metabolized.  The  daily  excretion  of  urea, 
therefore,  indicates  the  extent  of  the  proteid  metaboHsm.  It  has 
been  observed  also  that  there  is  a  steady  diminution  in  the  excretion  of 
carbon  dioxid,  though  this  is  greatest  in  the  last  few  days.  As  fat 
contains  about  76  per  cent,  of  carbon,  i  part  of  carbon  equals  1.31 
parts  of  fat.  Hence,  for  every  i  gram  of  carbon  or  3.66  grams  carbon 
dioxid  excreted  it  may  be  assumed  that  1.31  grams  of  fat  have  been 
metabolized.  The  daily  excretion  of  carbon,  therefore,  indicates  the 
extent  of  fat  metabohsm.  The  carbohydrates  are  here  left  out  of 
consideration,  as  they  constitute  only  about  i  per  cent,  of  the  body- 
weight.  It  must  be  borne  in  mind,  however,  that  in  the  metabohsm 
of  proteid  a  certain  quantity  of  fat  is  produced  which  also  undergoes 
oxidation.  The  amount  of  the  carbon  or  the  fat  that  the  proteid 
would  give  rise  to,  as  previously  determined,  must  therefore  be  sub- 
tracted from  that  ehminated  by  the  lungs,  etc.,  in  order  to  determine 
the  amount  of  body-fat  metabohzed.  Observations  of  human  beings 
in  the  fasting  condition  show  that  for  a  period  of  ten  days  there  is  a 
daily  excretion  of  about  21  grams  of  urea,  equivalent  to  about  70 
grams  of  proteid.  This  amount,  however,  may  be  reduced  to  from 
50  to  60  per  cent,  if  the  individual  has  a  surplus  of  body-fat.  Human 
beings  under  similar  circumstances  may  lose  during  the  first  few  days 
200  grams  of  fat  daily. 

The  following  table  shows  the  excretion  of  nitrogen  and 
carbon  and  the  calculated  amounts  of  proteid  and  fat  metabolized 
from  an  experiment  made  by  Ranke  on  himself  during  a  fast  of 
twenty-four  hours,  beginning  twenty-four  hours  after  the  last  meal: 


Disintegration  of  Tissue. 
(Calculated.) 

Expenditure. 

Nitrogen. 

7.8 
0.0 

7.8 

Carbon. 
26.5 

157-5 
184.0 

Urea,  17  gm., \ 

Uric  acid,  0.2  gm., j 

Carbon  dioxid, 

Nitrogen. 

Carbon. 

Proteid,  50  gm., 

Fat,  199.6  gm.,_   _        _   _ 

7.2 
0.0 
7.2 

34 
180.6 

184.0 

FOODS. 


147 


Coincidently  with  these  losses  to  the  body  there  is  also  a  gradual 
loss  of  inorganic  salts,  and  toward  the  termination  of  the  period  a 
sudden  fall  in  temperature  of  several  degrees  centigrade,  in  conse- 
quence of  the  final  consumption  of  all  available  food,  when  death 
ensues,  in  all  probabihty,  from  a  cessation  in  the  action  of  the  heart. 

Postmortem  Appearances. — It  has  been  experimentally  determined 
that  animals  die  when  the  body-weight  has  declined  to  about  40  per 
cent.  Postmortem  examination  shows  that  the  loss  of  material, 
though  very  generally  distributed  throughout  the  body,  is  greatest 
in  organs  and  tissues  least  essential  to  life. 

The  results  of  an  analysis  of  the  organs  and  tissues  of  a  cat  after 
a  thirteen-day  period  of  starvation,  during  w^hich  the  animal  lost 
1 01 7  grams  in  weight,  are  given  in  the  following  table,  based  on  data 
furnished  by  Voit : 


Organ. 


Adipose  tissue,. 

Spleen, 

Liver, 

Testes, 

Muscles,   

Blood, 


Kidneys, 

Skin  and  hair,. 

Lungs, 

Intestines, 

Pancreas, 

Bones, 


Heart,  

Nervous  system, 


Actual  Loss 

Percentage. 

OF  Tissue. 

Grams. 

97 

267 

67 

6 

54 

49 

40 

I 

31 

429 

27 

37 

26 

7 

21 

89 

18 

3 

18 

21 

17 

I 

14 

55 

3 

0 

3 

I 

It  will  be  observed  from  this  table  that  the  adipose  tissue  suffers 
the  greatest  loss,  the  entire  amount  disappearing  with  the  exception 
of  a  small  portion  in  the  posterior  part  of  the  orbital  cavity  and 
around  the  kidneys.  The  muscles,  though  only  losing  31  per  cent, 
of  their  weight,  yet  furnish  429  grams  of  presumably  proteid  material, 
for  nutritive  purposes.  The  heart  and  nervous  system  experience 
but  slight  loss. 

Mixed  Diet. — The  chemic  composition  of  the  tissues,  taken  in 
connection  with  their  metaboUsm  during  starvation,  imphes  that  no 
one  article  of  food  is  sufficient  for  tissue  repair  and  heat  production; 
but  that  all  classes  of  foods — in  other  words,  a  mixed  diet — are 
essential  to  the  maintenance  of  a  normal  nutrition.  Experimental 
investigation  has  also  conclusively  established  this  fact.  Moreover, 
the  amounts  of  nitrogen  and  carbon  eliminated  daily,  and  the  ratio 


148  TEXT-BOOK  OF  PHYSIOLOGY. 

existing  between  them,  indicate  the  amounts  of  proteid,  fat,  and 
carbohydrate  which  are  required  to  cover  the  loss. 

Metabolism  on  a  Purely  Proteid  Diet. — Notwithstanding 
the  chemic  composition  of  the  proteids  and  the  possibihty  of  their 
giving  rise  to  both  fat  and  a  carbohydrate  during  their  metabohsm, 
it  has  been  found  extremely  difficult  to  maintain  the  normal  nutrition 
for  any  length  of  time  on  a  pure  proteid  or  fat-free  flesh  diet.  This, 
however,  has  been  accomplished  with  dogs.  It  was  found,  however, 
that,  in  order  to  maintain  the  equilibrium,  it  was  necessary  to  increase 
the  proteids  from  two  to  three  times  the  usual  amount.  Thus,  a  dog 
weighing  30  to  35  kilograms  required  from  1500  to  1800  grams  of 
flesh  daily  in  order  to  get  the  requisite  amount  of  carbon  to  prevent 
consumption  of  its  own  adipose  tissue.  Under  similar  circumstances, 
a  human  being  weighing  70  kilograms  would  require  more  than  2000 
grams  of  lean  beef — an  amount  which,  from  the  nature  of  the  digestive 
apparatus,  it  would  be  practically  impossible  to  digest  and  assimilate 
for  any  length  of  time.  Even  the  shght  habitual  excess  beyond  the 
amount  normally  required  is  imperfectly  assimilated  and  gives  rise 
to  the  production  of  nitrogen-holding  compounds  which,  on  account 
of  the  difficulty  with  which  they  are  eliminated  by  the  kidneys,  ac- 
cumulate within  the  body  and  develop  the  gouty  diathesis,  with  all 
its  protean  manifestations. 

Metabolism  on  a  Fat  and  Carbohydrate  Diet. — As  nitrogen 
is  an  indispensable  constituent  of  the  tissues,  it  is  evident  that  neither 
fat  nor  carbohydrates  can  maintain  nutritive  equihbrium  except  for 
very  short  periods.  On  such  a  diet  the  tissues  consume  their 
own  proteids,  as  shown  by  the  continuous  excretion  of  urea,  though 
the  amount  is  less  than  during  starvation.  An  excess  of  fat  retards 
the  metabolism  of  proteids.  The  same  holds  true  for  the  carbohy- 
drates. 

Thus,  in  any  well-arranged  dietary  there  should  be  a  combina- 
tion of  proteids,  fats,  and  carbohydrates  in  amounts  sufficient  to 
maintain  nutritive  equilibrium;  in  other  words,  to  repair  the  loss  of 
tissue  and  to  furnish  the  requisite  amount  of  heat  in  accordance  with 
work  done,  as  well  as  with  climatic  and  seasonal  variations. 


COMPOSITION  OF  FOODS. 

The  food  principles  essential  to  the  maintenance  of  the  nutrition 
of  the  body  are  contained  in  varying  proportions  in  compound  sub- 
stances termed  foods;  e.  g.,  meat,  milk,  wheat,  potatoes,  etc.  Their 
nutritive  value  depends  partly  on  the  amounts  of  their  contained 
food  principles  and  partly  on  their  digestibihty.  The  dietary  of 
civihzed  man  embraces  foods  derived  from  both  the  animal  and 
vegetable  worlds. 


FOODS. 


149 


Composition  of  Animal  Foods. — The  following  table  shows  the 
average  percentage  composition  of  various  kinds  of  meats,  cow's 
milk,  and  eggs: 


In  100  Parts. 


Beef.       Veal. 


Water, '  76.25 

Proteid, 20.24 

Fat, 1.68 

Carbohydrates, 0.50 

Salts, i  1.38 


77.82 

19.86 

0.82 

0.80 

0.70 


Mut- 
ton. 


75-59 
17.11 

5-47 
0.60 
1.23 


Pork.      Fowl. 


72.57 

19-31 

5.82 

0.60 

1.70 


70.80 

22  70 

4.10 

1.20 

r.2o 


Fish. 


79-30 

18.30 

0.70 

0.90 

0.80 


Cow's 
Milk. 


86.87 
4-75 
3-50 
4.00 
0.17 


Eggs. 


73-67 
12-55 
12. II 

0-5S 
1-13 


Meats. — It  will  be  observed  from  these  analyses  that  the  meats 
contain  from  18  to  20  per  cent,  of  a  proteid  which  belongs  in  virtue 
of  its  chemic  relations  to  the  group  of  globulins.  In  the  Hving  con- 
dition this  body,  known  as  myosinogen,  is  in  a  semi-fluid  condition, 
but  shortly  after  death  undergoes  coagulation,  giving  rise  to  sohd 
myosin  and  a  soluble  albumin.  There  are  also  present  in  meat  small 
percentages  of  other  forms  of  proteid;  e.  g.,  myoalbumin,  myoglob- 
uhn,  paramyosinogen,  etc.  After  being  subjected  to  the  cooking 
process,  meats  contain  the  albuminoid  body  gelatin,  a  product  of  the 
transformation  of  the  proteids  of  the  connective  tissue. 

The  percentage  of  fat,  contained  within  the  meat  substance, 
is  very  small  except  in  mutton  and  pork,  where  it  rises  to  5.4  per  cent, 
and  5.8  per  cent,  respectively.  The  fat-globules  in  these  meats  are 
packed  closely  between  the  muscle-fibers,  and  prevent  the  easy 
entrance  of  the  digestive  fluids,  and  hence  they  are  more  difficult  of 
digestion  than  beef. 

The  carbohydrates  vary  from  0.5  to  i  per  cent.,  and  are  represented 
by  glycogen.  The  principal  inorganic  salts  are  potassium  phosphate 
and  sodium  chlorid. 

Cooking,  when  properly  done,  not  only  makes  the  meat  more 
palatable  and  appetizing  from  the  development  of  agreeable  flavors, 
but  converts  the  connective  tissue,  which,  in  old  animals  especially, 
is  tough  and  resisting,  into  gelatin,  thus  rendering  it  more  easy  of 
mastication  and  digestion.  At  the  same  time  parasitic  organisms, 
such  as  the  embryonic  forms  of  tenia  or  tapeworm,  trichina 
spiralis,  as  well  as  bacterial  growths,  which  frequently  infest  the 
bodies  of  animals,  are  destroyed  and  made  harmless. 

Milk  is  the  natural  food  of  the  young  of  all  mammals,  and  is 
usually  regarded  as  typical  on  account  of  the  ratio  existing  among 
its  nutritive  principles.  The  analysis  given  above  is  that  of  cow's 
milk.  Examined  microscopically,  milk  is  seen  to  consist  of  a  clear 
fluid,  the  milk  plasma,  holding  in  suspension  an  enormous  number  of 
small,  highly  refractive  oil  globules,  which  measure  on  the  average 


I50 


TEXT-BOOK  OF  PHYSIOLOGY. 


about  TTridd  ^^  3,n  inch  in  diameter.  Each  globule  is  supposed  by 
some  observers  to  be  surrounded  by  a  thin  albuminous  envelope, 
which  enables  it  to  maintain  the  discrete  form.  Others  deny  the 
existence  of  such  a  membrane.  The  chief  proteid  constituent  of 
milk,  caseinogen,  is  held  in  solution  by  the  presence  of  phosphate  of 
lime.  On  the  addition  of  acetic  acid  or  sodium  chlorid  up  to  the 
point  of  saturation  the  caseinogen  is  precipitated  as  such  and  may  be 
collected  by  appropriate  chemic  methods.  When  taken  into  the 
stomach,  caseinogen  is  coagulated;  that  is,  it  is  separated  into  casein 
or  tyrein  and  a  small  quantity  of  a  new  soluble  proteid.  This  change 
is  brought  about  by  the  presence  in  the  gastric  juice  of  a  special 
ferment  known  as  rennin  or  pexin. 

The  fat  of  milk  is  more  or  less  solid  at  ordinary  temperatures. 
It  is  a  combination  of  olein,  palmitin,  and  stearin,  with  a  small  quan- 
tity of  butyrin  and  caproin.  When  milk  is  allowed  to  stand  for  some 
time,  the  fat-globules  rise  to  the  surface  and  form  a  thick  layer  known 
as  cream.  When  subjected  to  the  churning  process,  the  fat-globules 
run  together  and  form  a  coherent  mass — butter. 

Lactose  is  the  particular  form  of  sugar  found  in  milk.  In  the 
presence  of  Bacillus  acidi  lactici,  the  lactose  is  decomposed  into 
lactic  acid  and  carbon  dioxid,  the  former  of  which  not  only  imparts 
a  sour  taste  to  the  milk,  but  causes  a  precipitation  of  the  caseinogen. 
The  chief  salt  found  in  milk  is  phosphate  of  lime,  and  this  is  the 
chief  source  of  this  agent  in  the  formation  of  bones. 

Eggs  are  also  to  be  regarded  as  complete  natural  foods,  inasmuch 
as  they  contain  all  the  necessary  food  principles.  The  analysis 
given  in  the  above  table  represents  the  composition  of  the  entire  egg. 
The  white  of  the  egg  contains  12  per  cent,  of  proteid  and  2  per  cent, 
of  fat.  The  yolk,  however,  contains  15  per  cent,  of  proteid  and  30 
per  cent,  of  fat. 

Composition  of  Cereal  Foods. — The  average  composition  of 
the  principal  cereals  is  shown  in  the  following  table : 


In  100  Parts. 


Water, 

Proteid, 

Fat, 

Carbohydrate, 

Cellulose, 

Salts, 


Wheat. 

Rye. 

Barley. 

Oats. 

Corn. 

Rice. 

13-56 

12.65 

13-77 

12.37 

13.10 

13.12 

12.3s 

12-55 

II. 14 

10.41 

9-85 

7.88 

1-75 

1.97 

2.16 

5-23 

4-57 

0.85 

67.Q0 

67-95 

64-93 

57-78 

68.42 

76-55 

2.63 

3.00 

5-31 

11.19 

2.50 

0-55 

1.81 

1.88 

2.69 

.3-02 

1.56 

1.05 

Buck- 
wheat. 


12.62 

10.02 

2.24 

64-43 
8.67 


That  the  cereals  are  most  important  and  useful  articles  of  diet  is 
evident  from  their  composition,  consisting,  as  they  do,  of  proteids 
and  carbohydrates  in  large  proportion.     Owing  to  the  cellulose  or 


FOODS. 


151 


woody  fiber  which  envelops  and  penetrates  the  grain,  they  are  some- 
what difficult  of  digestion.  A  section  of  a  grain  of  wheat  shows  the 
external  cellulose  envelope,  the  husk,  beneath  which  is  a  layer  of  large 
cells  containing  the  chief  proteid — the  gluten.  The  interior  of  the 
grain  consists  of  small  cavities,  the  walls  of  which  are  formed  of  cellu- 
lose and  which  contain  the  granules  of  starch,  fat,  small  quantities  of 
proteid,  and  inorganic  salts.  All  other  cereals  have  a  similar  structure. 

In  the  preparation  of  white  flour  from  wheat  it  is  customary  to 
remove  the  husk,  a  process  which  involves  the  removal  also  of  a  por- 
tion, if  not  all,  of  the  gluten  cells,  so  that  such  flour  contains  less  nitrog- 
enized  material  than  the  original  grain.  It  is  possible,  however,  in 
the  milling  of  wheat,  to  remove  only  the  husk  and  retain  the  gluten  in 
the  flour,  as  in  the  preparation  of  whole  wheat  flour. 

Bread  is  an  artificially  prepared  food  made  either  of  wheat  or 
rye.  Owing  to  the  fact  that  the  proteids  of  the  other  cereals  do  not 
possess  the  same  adhesive  properties  when  kneaded  with  water,  they 
can  not  be  used  for  bread-making  purposes.  In  the  making  of  bread, 
the  flour  is  kneaded  with  water  until  a  glutinous  mass — dough — is 
formed.  During  this  process,  salt,  sugar,  and  yeast  are  added.  It 
is  then  placed  in  a  temperature  of  about  100°  F.  In  the  presence  of 
heat  and  moisture  the  natural  ferment  of  the  flour — diastase — con- 
verts a  portion  of  the  starch  into  sugar,  which  in  turn  is  split  up  into 
carbon  dioxid  and  alcohol  by  the  yeast  plant.  The  bubbles  of 
carbon  dioxid,  becoming  entangled  in  the  dough,  cause  it  to  swell 
or  rise  and  subsequently  give  the  porous  or  spongy  character  to  the 
bread.  When  baked  at  a  temperature  of  400°  F.,  the  alcohol  is 
driven  off;  yeast  cells  and  other  organisms  are  destroyed;  the  starch, 
particularly  that  on  the  surface,  is  dextrinized.  Thus  prepared, 
white  bread  consists  of  water,  32  per  cent.;  proteid,  8.8  per  cent.; 
fat,  1.7  per  cent. ;  carbohydrate,  56.3  per  cent. ;  salts,  0.9  per  cent.  The 
principal  salts  are  potassium  and  magnesium  phosphate.  Whole 
wheat  bread  consists  of  water,  40  per  cent.;  proteid,  12.2  per  cent.; 
fat,  1.2  per  cent.;  carbohydrate,  43.5  percent.;  salts,  1.3  percent.; 
cellulose,  1.8  per  cent. 

Composition  of  Vegetable  Foods. — The  average  composition 
of  some  of  the  principal  vegetables  is  shown  in  the  following  table : 


In  100  Parts. 


Water,    

Proteid;    

Fat, 

Carbohydrates, . 

Cellulose, 

Salts, 


Pota- 

Tur- 

Toma- 

Aspa- 

Beans. 

Peas. 

toes. 

Beets. 

nips. 

toes. 

ragus. 

13-74 

14.99 

75-47 

82.20 

89.42 

96.30 

93-75 

23.21 

22.85 

1-95 

1.80 

1-35 

0.90 

1.79 

2.14 

1-79 

0.15 

0.30 

0.18 

0.50 

0.25 

53-67 

52-36 

20.69 

13.00 

7-36 

2.80 

2.63 

3-69 

5-43 

0.76 



0.94 

__ 

1.04 

3-SS 

2.58 

0.98 

1.60 

0-75 

0.40 

0.54 

Cab- 
bage. 


89.97 
1.89 
0.20 
4.87 
1.84 
1.23 


152  TEXT-BOOK  OF  PHYSIOLOGY. 

The  vegetable  foods,  as  a  class,  vary  considerably  in  nutritive 
value  and  digestibility,  the  latter  depending  on  the  amount  of  cellu- 
lose they  contain.  A  section  of  a  vegetable  shows  not  only  the  pres- 
ence of  an  external  cellulose  envelope,  but  also  an  inner  framework 
which  penetrates  its  substance  in  all  directions.  The  nutritive 
principles  are  contained  in  small  cavities,  the  walls  of  which  are 
formed  by  the  framework.  Nearly  all  vegetables  require  cooking 
before  being  eaten.  When  subjected  to  heat  and  moisture,  not  only 
is  the  texture  of  the  vegetable  softened  and  disintegrated,  but  the 
starch  grains  are  hydrated  and  partially  prepared  for  conversion 
into  dextrin  and  sugar.  At  the  same  time  various  savory  substances 
are  set  free,  which  make  the  food  more  palatable. 

Beans  and  peas  contain  large  quantities  of  a  proteid,  legumin, 
and  starch,  and  hence  are  especially  valuable  as  nutritive  foods. 
The  presence  of  the  cellulose  envelope,  especially  in  ripe  beans  and 
peas,  combined  with  rather  a  dense  texture,  renders  them  somewhat 
difficult  of  digestion.  Potatoes,  though  largely  employed  as  food, 
are  extremely  poor  in  proteids,  2  per  cent.,  and  carbohydrates,  20  per 
cent.  When  sufhciently  cooked  they  are  easily  digested,  owing  to 
the  small  amount  of  cellulose  they  contain. 

Green  vegetables, — e.  g.,  lettuce,  spinach,  tomatoes,  asparagus, 
onions,  etc.,  though  containing  food  principles  in  small  amounts, 
are,  nevertheless,  valuable  adjuncts  to  the  dietary,  for  the  reason 
that  they  contain  inorganic  as  well  as  organic  salts,  which  appear 
to  be  necessary  to  the  maintenance  of  the  normal  nutrition.  The 
want  of  green  vegetables  has  been  supposed  to  be  the  cause  of 
scurvy. 

Ripe  fruits,  grapes,  cherries,  apples,  pears,  peaches,  strawberries, 
lemons,  oranges,  etc.,  though  consumed  largely,  possess  but  little 
nutritive  value.  They  consist  largely  of  water,  75  to  85  per  cent., 
proteids  a  trace,  sugar  from  5  to  13  per  cent.,  organic  acids  (citric, 
malic,  tartaric),  pectose,  and  various  inorganic  salts. 

Relative  Value  of  Animal  and  Vegetable  Foods.— Though 
both  animal  and  vegetable  foods  contain  the  different  classes  of  food 
principles,  it  is  not  a  matter  of  entire  indifference  as  to  which  are 
consumed.  It  has  been  found  by  experiment  that  animal  proteids 
are  more  easily  and  completely  digested  and  absorbed  than  vegetable 
proteids;  that  cellulose  is  not  only  highly  indigestible,  but  by  its  pres- 
ence in  large  quantities  retards  the  digestive  process  and  impairs  the 
activity  of  the  entire  digestive  mechanism,  though  in  moderate  quan- 
tity it  undoubtedly  aids  digestion  indirectly  by  mechanically  pro- 
moting peristalsis.  The  following  table  shows  the  relative  diges- 
tibihty  of  the  two  classes  of  foods: 


FOODS. 


153 


Weigi 

3T  OF  Food. 

Vegetable. 

Animal. 

Digested. 

Undigested. 

Digested. 

Undigested. 

Of  100  parts 

Of  100  parts 

Of  100  parts 

hydrates,  . 

of  solids,  - 
of  proteid, 
of  fats  or 

75-S 
46.6 

90.3 

24-5 

534 
9-7 

89.9 
81.2 

96.9 

II. I 

carbo- 

18.8 
3-1 

Construction  of  Dietaries. — Inasmuch  as  neither  animal  nor 
vegetable  foods  contain  the  food  principles  in  proper  quantities  and 
proportions,  the  instinctive  choice  of  mankind  has  led  to  a  com- 
bination of  the  two  classes  of  foods.  From  the  analyses  tabulated 
above  it  becomes  comparatively  easy  to  construct  a  suitable  dietary, 
composed  of  different  articles  of  food,  in  which  the  food  principles 
shall  bear  the  proper  ratio  one  to  the  other — a  ratio  based  on  the  total 
quantity  of  nitrogen  (15  to  20  grams)  and  carbon  (225  to  300  grams) 
eliminated  from  the  body  daily. 

It  is  only  necessary,  therefore,  to  combine  two  or  more  foods, 
the  composition  of  which  is  known,  in  quantities  sufiEicient  to  furnish 
the  requisite  amount  of  nitrogen  and  carbon,  or  their  equivalents  in 
proteid,  fat,  and  carbohydrates.  As  illustrations  of  such  combina- 
tions the  following  examples  are  given: 

Food  Principles. 

Proteid, loo  gm. 

100  gm. 


'I 


N. 
15 


Foods. 

Meat, 225  gm., 

Bread, 450  gm.. 

Fats,    113  gm.,  J  lb. 

Potatoes, 450  gra.,   i  lb. 

Milk, 225  gm., 

Eggs, 113  gm.,  J  lb. 

Cheese,   56  gm.,  ^  lb 


^  lb 
I  lb. 


i  pint 


N. 

7-5  gm. 

S-5  gm. 

1-3  gm. 
1.7  gm. 
2.0  gm. 
3-0  gm. 
21.0 


Foods. 

Meat, 250  gm.,     8.8  oz 

Bread, 400  gm.,  14.2  oz.  J   "|  p  , 

Fat,  100  gm.,     3.5  oz.      \       \'~u7". 

Sugar,   70  gm.,     2.5  oz.      j  Carbohydrates, 250  gm.       ^- 

15 
C. 

34  gm. 
117  gm. 
84  gm. 
45  gm. 
20  gm. 
15  gm. 
20  gm. 

335 

(WaUer.) 
DAILY  RATION  OF  THE  UNITED  STATES  SOLDIER. 

Fresh  beef, 20  oz. 

or     pork,    12  oz. 

or     bacon,    12  oz. 

Flour,    18  oz. 

or  soft  bread, 18  oz. 

or  hard  bread, 16  oz. 

Potatoes, 160Z. 

or  potatoes  iii  and  tomatoes  45 16  oz. 

Beans  or  peas,    2.4  oz 

Rice  or  hominy, 1.6  oz. 

Coffee, 1.60Z. 

Sugar,    2.000Z. 

Vinegar, 0.32  gill 

Salt, 0.60  gill 


C. 

50 

75 

100 


225 


CHAPTER   IX. 
DIGESTION. 

Foods  are  heterogeneous  compounds  consisting  of  organic  and 
inorganic  nutritive  principles  associated  with  a  varying  amount  of 
non-nutritive  material,  such  as  the  dense  parts  of  the  connective 
tissue  of  the  animal  foods  and  the  woody  fiber  or  cellulose  of  the 
vegetable  foods.  Before  the  nutritive  principles  can  be  utilized  they 
must  be  dissociated  from  the  non-nutritive  material.  Even  when 
consumed  in  the  free  state,  the  food  principles  are  seldom  in  a  condi- 
tion to  be  absorbed  into  the  blood  and  assimilated  by  the  tissues. 
When  foods  are  consumed  in  their  natural  state  or  after  they  have 
been  subjected  to  the  cooking  process,  they  are  subjected  while  in 
the  food  canal  to  the  solvent  action  of  various  fluids  by  which  they 
are  disintegrated  and  reduced  to  the  hquid  condition.  The  nutri- 
tive principles  freed  from  their  combinations  are  changed  in  chemic 
composition  and  transformed  into  substances  capable  of  absorption. 
To  all  the  physical  and  chemic  changes  which  foods  undergo  in  the 
food  canal  the  term  digestion  has  been  given. 

The  digestive  apparatus  comprises  the  entire  ahmentary  or  food 
canal  and  its  various  appendages:  the  teeth,  the  tongue,  the  mouth, 
the  gastric  and  intestinal  glands,  the  pancreas,  and  the  Hver  (Fig.  59). 

The  canal  itself  is  a  musculo-membranous  tube  about  thirty-two 
feet  in  length,  and  extends  from  the  mouth  to  the  anus.  It  may  be 
subdivided  into  several  distinct  portions,  as  mouth,  pharynx,  esoph- 
agus, stomach,  small  and  large  intestines.  The  mouth  is  provided 
(i)  with  teeth,  by  which  the  food  is  divided,  (2)  with  the  tongue, 
and  (3)  with  glands,  by  which  a  solvent  fluid,  the  sahva,  is  secreted. 
The  glands,  though  situated  for  the  most  part  outside  the  mouth,  are 
connected  with  it  by  means  of  ducts.  Posteriorly  the  mouth  opens 
into  the  pharynx  or  throat,  a  somewhat  pyramidal-shaped  structure 
about  five  inches  in  length,  which  in  turn  is  followed  by  the  esoph- 
agus or  gullet,  a  tube  about  nine  inches  in  length.  As  the  esophagus 
passes  through  the  diaphragm  it  expands  into  the  stomach,  a  curved 
pyriform  organ,  which  serves  as  a  reservoir  for  the  reception  and 
retention  of  the  food  for  a  varying  length  of  time.  The  small  intes- 
tine is  that  portion  of  the  ahmentary  canal  extending  from  the  end 
of  the  stomach  to  the  beginning  of  the  large  intestine;  owing  to  its 
length,  about  twenty-two  feet,  it  .presents  a  very  convoluted  appear- 
ance in  the  abdominal  cavity.  Embedded  in  its  walls  are  the  intes- 
tinal glands  which  open  on  its  surface  and  secrete  the   intestinal 

154 


DIGESTION. 


155 


fluid.  In  the  upper  portion  of  the  small  intestine,  within  five  inches 
of  the  stomach,  there  are  generally  two  orifices,  the  terminations  of 
the  ducts  of  the  liver  and  pancreas,  organs  which  secrete  the  bile 
and  pancreatic  juice  respectively.  The  large  intestine  is  from  five 
to  six  feet  in  length  and  extends  from  the  end  of  the  small  intestine 
to  the  anus.     Its  walls  contain  a  large  number  of  glands. 


\^m- A  Sa/ivsry  6/ancf 


LBrg& 
Intestine 


Vermiform  /l/3per7(fix 


Fig.  59. — Diagram  of  the  Alimentary  Canal. — {Modified  from  Landois.) 

The  general  process  of  digestion  is  largely  accompHshed  by  the 
chemic  action  of  the  digestive  fluids :  the  saliva,  the  gastric,  intestinal, 
and  pancreatic  juices,  and  the  bile.  Though  taking  place  through- 
out a  large  portion  of  the  food  canal,  the  process  may  be  subdivided 
into  several  stages:  viz.,  mouth  digestion,  gastric  digestion,  and  in- 
testinal digestion. 


156  TEXT-BOOK  OF  PHYSIOLOGY. 

As  a  result  of  the  action  of  these  fluids  the  nutritive  principles  are 
prepared  for  absorption  into  the  blood;  the  non-nutritive  principles, 
along  with  certain  waste  products,  pass  into  the  large  intestine  to  be 
finally  extruded  from  the  body. 

MOUTH  DIGESTION. 

The  digestion  of  the  food  as  it  takes  place  in  the  mouth  comprises 
a  series  of  physical  and  chemic  changes,  the  result  of  the  action  of 
the  teeth,  the  tongue,  and  the  sahva.  The  mechanic  division  of  the 
food  and  the  incorporation  of  the  saliva  with  it  are  termed  respec- 
tively mastication  and  insalivation. 

MASTICATION. 

Mastication  is  the  mechanic  division  of  the  food,  and  is  accom- 
phshed  by  the  teeth  and  the  movements  of  the  lower  jaw  under  the 
influence  of  muscle  contractions.  Complete  mechanic  disintegration 
of  the  food  is  essential  to  its  subsequent  solution  and  chemic  trans- 
formation; for  when  finely  divided  it  presents  a  larger  surface  to  the 
action  of  the  digestive  fluids  and  thus  enables  them  to  exert  their 
respective  actions  more  effectively  and  in  a  shorter  period  of  time. 

The  Teeth. — In  man  passing  from  childhood  to  aduh  hfe  two 
sets  of  teeth  make  their  appearance.  The  first  set  constitute  the 
temporary,  deciduous,  or  milk  teeth;  the  second  set  constitute  the 
permanent  teeth,  which  should  last  with  proper  care  through  life  or 
to  an  advanced  age. 

The  temporary  teeth,  twenty  in  number,  ten  in  each  jaw,  though 
smaller  than  the  permanent  teeth,  have  the  same  general  conforma- 
tion. They  are  divided  into  four  incisors,  two  cuspids  or  canines, 
and  four  molars  for  each  jaw. 

The  permanent  teeth,  thirty- two  in  number,  sixteen  in  each  jaw, 
are  divided  into  four  incisors,  two  cuspids  or  canines,  four  bicuspids 
or  premolars,  and  six  molars  for  each  jaw. 

Each  tooth  may  be  said  to  consist  of  three  portions:  (i)  the 
crown,  or  that  portion  which  projects  above  the  gums;  (2)  the  root 
or  fang,  that  portion  embedded  in  the  alveolar  socket;  (3)  the  con- 
stricted portion  or  neck,  which  is  surrounded  by  the  free  margin  of 
the  gum.  The  teeth  are  firmly  secured  in  their  sockets  by  a  fibrous 
membrane,  the  peridental  membrane,  which  is  attached,  on  the  one 
hand,  to  the  alveolar  process,  and,  on  the  other,  to  the  cementum. 

A  vertical  section  of  a  tooth  shows  that  it  consists  of  three  distinct 
soUd  structures,  the  enamel,  the  dentine,  and  the  cementum,  which 
have  the  anatomic  relationship  as  represented  in  Fig.  60.  In  the 
center  of  the  dentine  there  is  a  cavity  the  general  shape  of  which  varies 
in  different  teeth,  and  which  is  occupied  during  the  Hving  condition  by 
the  tooth  pulp. 


DIGESTION. 


157 


pa. 


Microscopic  examination  of  the  tooth  reveals  the  presence  of 
irregular  stellate  spaces,  the  interglobular  spaces,  between  the  dentine 
and  the  cementum,  which  are  occupied  by  connective-tissue  cells. 
Clefts  of  varying  size  are  also  observed  at  the  junction  of  the  dentine 
and  the  enamel,  and  which  extend  for  some  distance  into  the  latter. 

The  enamel  is  composed  of  dense  hard  cyHnders  which,  on  account 
of  their  small  size  and  close  relationship,  appear  to  be  hexagonal  in 
shape.  These  cyhnders  are  held  together  by  cement  substance.  The 
free  border  of  the  enamel  is  covered,  in  early  Ufe  at  least,  by  a  thin 
membrane  known  as  the  cuticle  or 
membrane  of  Nasmyth. 

The  dentine  is  somewhat  less  dense 
than  the  enamel.  It  is  composed  of 
connective-tissue  fibers  embedded  in 
a  ground-substance,  both  of  which 
have  undergone  calcification  in  the 
course  of  development.  The  dentine 
is  penetrated  by  a  series  of  fine  canals, 
the  dentine  canals  or  tubules,  which 
begin  by  open  mouths  on  the  pulp 
side.  From  this  point  the  tubules 
pass  outward  to  the  cementum  and 
enamel,  where  their  terminal  branches 
communicate  with  and  terminate  in 
the  interglobular  spaces  and  clefts. 
In  their  course  the  tubules  give  off  a 
series  of  branches  which  communicate 
freely  with  one  another.  The  dentine 
bordering  the  tubule  is  somewhat  more 
dense  than  the  intertubular  portion 
and  constitutes  what  is  known  as  the 
dentinal  sheath  or  Neumann's  sheath. 

The  cementum  resembles  bone  be- 
cause it  contains  both  lacunae  and 
canaHcuH,  though  it  is,  as  a  rule, 
devoid  of  Haversian  canals. 

The  pulp  consists  of  a  framework 
of  connective  tissue  which  affords  support  to  blood-vessels  and  nerves, 
both  of  which  enter  the  pulp  chamber  through  a  small  foramen  at  the 
apex  of  the  root.  The  outer  surface  of  the  pulp  is  covered  with  a 
layer  of  large  spheric  cells,  the  odontoblasts.  Each  cell  presents  on 
its  inner  surface  short  processes  which  pass  into  the  pulp;  on  its  outer 
surface  it  presents  a  long  process  which  enters  a  dentine  tubule  and 
extends  as  far  as  its  ultimate  terminations.  Collectively  these  pro- 
cesses are  known  as  the  dentine  fibers.     Inasmuch  as  the  fibers  do 


V: 


fl-..RM. 


_C 


B  — 


Fig.  60. — Vertical  Section  of 
Tooth  in  Jaw.  E.  Enamel. 
D.  Dentine.  P.  M.  Periodontal 
membrane.  P.  C.  Pulp  cavity. 
C.  Cement.  B.  Bone  of  lower 
jaw.  V.  Vein.  a.  Artery.  N. 
Nerve. — {Stirling) 


158  TEXT-BOOK  OF  PHYSIOLOGY. 

not  completely  occupy  the  lumen  of  the  tubule,  it  is  probable  that 
there  is  a  free  circulation  of  lymph  from  the  pulp  chamber  through 
the  dentine  tubules  into  the  enamel  clefts,  into  the  interglobular 
spaces,  and  possibly  into  the  lacunae  of  the  cementum. 

The  peridental  membrane  is  composed  of  connective-tissue  fibers 
abundantly  suppHed  with  blood-vessels  and  nerves. 

Movements  of  the  Lower  Jaw. — The  lower  jaw  is  capable  of  a 
downward  and  upward,  an  antero-posterior,  and  a  lateral  move- 
ment, all  dependent  on  the  pecuhar  construction  of  the — 

Temporo-maxillary  Articulation. — This  articulation  is  formed  by 
the  anterior  portion  of  the  glenoid  cavity,  the  eminentia  articularis, 
and  the  condyle  of  the  inferior  maxilla,  all  of  which  are  united  by 
means  of  ligaments.  Situated  between  the  glenoid  cavity  and  the 
condyle  is  a  plate  of  fibro-cartilage  oval  in  shape  and  biconcave. 
This  cartilage  divides  the  joint  into  two  cavities, — one  above,  the 
other  below, — each  of  which  is  provided  with  a  synovial  membrane. 
The  function  of  the  cartilage  is  to  present  constantly  an  articulating 
surface  to  the  condyle  in  the  various  movements  of  the  lower  jaw, 
which  it  is  enabled  to  do  by  virtue  of  its  mobility. 

In  the  downward  movement  of  the  lower  jaw  each  condyle  ghdes 
forward,  carrying  with  it  the  interarticular  fibro-cartilage  the  upper 
concave  surface  of  which  is  applied  to  the  convex  surface  of  the 
eminentia  articularis.  In  the  upward  movement  of  the  jaw  both  the 
condyles  and  the  cartilages  pass  backward  and  resume  their  normal 
position.  The  movements  of  depression  and  elevation  are  made 
possible  by  the  transverse  direction  of  the  condyle.  In  the  carnivor- 
ous animals,  whose  food  requires  considerable  cutting,  these  move- 
ments are  especially  well  developed.  In  the  antero-posterior  move- 
ment the  jaw  moves  in  a  horizontal  direction  and  the  condyles  and 
the  articular  cartilages  glide  forward  and  backward  in  the  glenoid 
fossae.  In  the  rodent  animals  the  long  axis  of  the  condyle  runs 
in  the  antero-posterior  direction,  which  allows  of  a  considerable 
ghding  movement.  When  the  jaw  performs  a  lateral  movement,  the 
condyle  and  cartilage  of  one  side  remain  in  their  normal  position, 
while  the  opposite  condyle  and  cartilage  glide  forward  in  the  glenoid 
fossa,  directing  the  symphysis  of  the  jaw  to  the  opposite  side  of  the 
median  hne.  The  lateral  movements  are  well  exhibited  by  the 
herbivorous  animals,  in  which  they  are  quite  extensive,  and  made 
possible  by  the  small  size  of  the  condyle  and  the  large  extent  of 
articulating  surface.  In  man  the  structure  of  the  joint  is  such  as  to 
admit  of  all  these  possibilities,  and  the  lower  jaw  acquires  in  conse- 
quence great  freedom  of  movement. 

The  Functions  of  the  Muscles  of  Mastication. — The  move- 
ments of  the  lower  jaw  are  caused  by  the  action  of  numerous 
muscles,  which,  having  a  fixed  point  of  origin,  are  attached  to  various 


DIGESTION.  159 

points  on  its  surface.     The  muscles  concerned  in  the  movements 
of  mastication  are  presented  in  the  following  table : 

Anterior  belly  of  digastric  1  -r-.  i.u     i  •  j 

Mvlohvoid  I  Depress  the  lower  jaw  and  open 

r^     •  -u     -J  \      the  mouth. 

Geniohyoid  J 

T  .    "   1         .•         r  .  Elevate  the  lower  jaw  and  close 

Internal  portion  of  masseter  f       th  th 

Internal  pterygoid  j 

External  pterygoids  )   Draw  the  lower  jaw  forward  and 


External  portion  of  masseter  >  cause  the  lower  teeth  to  project 

Anterior  fibers  of  temporal  j  beyond  the  upper. 

Posterior  fibers  of  temporal 

Internal  portion  of  masseter  f  Draw  the  lower  jaw  back  to  its 


Digastric,   mylohyoid,  and  genio-   I       normal  position. 

hyoid  J 

Internal  pterygoids  \  Contracting  alternately,  draw  the 

External  pterygoids  J       jaw  to  the  opposite  side. 

Pterygoids,  external  and  interna      1   t>      .  •    .•  ^       c 

^  ■[  [   Produce   gnnding   movements   of 

M3er  J       the  lower  jaw. 

The  action  of  the  depressor  muscles  becomes  apparent  when  their 
points  of  origin  and  insertion  are  considered.  The  anterior  belly 
of  the  digastric,  the  mylohyoid,  the  geniohyoid  muscles,  agree  in 
having  a  similarity  of  origin — the  hyoid  bone — and  a  common  area 
of  insertion,  the  anterior  portion  of  the  inferior  maxillary.  Their 
anatomic  relation  is  such  that  their  combined  action  will  depress  the 
lower  jaw  and  open  the  mouth. 

The  elevator  muscles  arise  from  various  points  on  the  side  of 
the  head,  and  are  inserted  into  the  coronoid  process,  ramus,  and 
internal  surface  of  the  angle  of  the  lower  jaw.  When  the  mouth 
has  been  opened,  the  simultaneous  contraction  of  these  muscles 
elevates  the  jaw  and  closes  the  mouth  with  considerable  force.  The 
power  of  these  muscles  is  very  great,  and  depends  on  the  shortness 
and  thickness  of  the  muscle-bundles. 

The  action  of  the  rotator  muscles,  those  which  give  rise  to  the 
lateral  movements  of  the  jaw,  depends  in  hke  manner  on  their  origin 
and  insertion.  Arising  from  the  superior  maxillary  and  sphenoid 
bones,  they  are  inserted  into  the  neck  of  the  condyle  and  angle  of 
the  lower  jaw  respectively.  When  they  contract,  the  condyle  on  the 
corresponding  side  is  drawn  forward,  while  the  opposite  condyle 
remains  stationary.  As  a  result,  the  symphysis  of  the  jaw  is  directed 
to  the  opposite  side.  The  grinding  movements  of  the  jaw  are  pro- 
duced by  the  coordinated  action  of  all  the  groups  of  muscles  acting 
more  or  less  successively. 

For  the  proper  mastication  of  the  food  it  is  essential  that  it  be 
kept  between  the  opposing  surfaces  of  the  teeth.  This  is  accom- 
plished by  the  contraction  of  the  orbicularis  oris  and  buccinator 
muscles  from  without  and  the  tongue  muscles  from  within. 


i6o  TEXT-BOOK  OF  PHYSIOLOGY. 

The  Nerve  Mechanism  of  Mastication.* — The  movements  of 
mastication,  though  originating  in  efforts  of  the  will  and  under  its 
control,  are  for  the  most  part  of  an  automatic  or  reflex  character;  for 
when  once  initiated  by  a  voluntary  effort  they  continue  for  an  inde- 
finite period — so  long,  in  fact,  as  the  impressions  which  the  food  makes 
upon  the  afferent  nerves  are  received  by  the  nerve-centers  which 
regulate  and  control  them.  That  the  masticatory  movements  are  of 
this  reflex  nature  is  shown  by  the  fact  that  they  will  be  maintained 
even  though  the  voluntary  effort  which  called  them  forth  has  sub- 
sided and  the  attention  has  been  directed  to  some  entirely  different 
subject.  It  would  appear  that  all  that  is  necessary  under  such  con- 
ditions is  the  exciting  action  of  the  food  upon  the  periphery  of  the 
afferent  nerves  distributed  to  the  tongue  and  mouth. 

The  nerves  involved  in  this  reflex  are  shown  in  the  following  table : 

Afferent  Nerves.  Efferent  Nerves. 

1.  Lingual  branch  of  fifth  nerve.  i.  Inferior   maxillary   division   of   fifth 

nerve. 

2.  Glossopharyngeal.  2.  Hypoglossal  or  sublingual. 

3.  Facial  or  portio  dura. 

The  nerve-center  coordinating  the  movements  of  mastication  is 
situated  in  the  medulla  oblongata.  The  afferent  or  excitor  nerves 
which  receive  the  impressions  of  the  food  are  distributed  largely  to 
the  mucous  membrane  of  the  tongue.  When  these  impressions  are 
received  by  the  center  in  the  medulla  oblongata,  it  discharges  nerve 
impulses,  which,  passing  outward  through  motor  nerves,  excite  con- 
traction in  the  masticatory  muscles.  The  motor  nerves  innervating 
the  muscles  are:  (i)  The  small  root  of  the  fifth  nerve,  which,  after 
emerging  from  the  cavity  of  the  cranium  through  the  foramen  ovale, 
joins  the  inferior  maxillary  division  of  the  large  sensor  root.  It 
then  is  distributed  to  the  masseter,  temporal,  internal,  and  external 
pterygoids,  anterior  belly  of  the  digastric,  and  mylohyoid  muscles, 
and  controls  their  movements.  (2)  The  hypoglossal  nerve,  which, 
after  emerging  through  the  anterior  condyloid  foramen,  passes  down- 
ward and  forward  to  be  distributed  to  the  extrinsic  and  intrinsic 
muscles  of  the  tongue.  (3)  The  facial  or  portio  dura,  which,  after 
emerging  from  the  stylomastoid  foramen,  is  distributed  to  the  mus- 
cles of  the  face.  Irritation  of  any  one  of  these  nerves  produces  con- 
vulsive movements  in  the  muscles  to  which  it  is  distributed,  while 
their  division  is  followed  by  paralysis  of  these  muscles.  The  medulla 
not  only  generates  the  impulses  which  are  directly  responsible  for 
the  movements  of  mastication,  but  also  coordinates  them  in  such  a 
manner  that  the  movements  of  mastication  may  be  directed  toward 
the  accompHshment  of  a  definite  purpose. 

*By  this  term  is  meant  a  combination  of  nerves,  afferent  and  efferent,  and  nerve 
centers  which  when  stimulated  coordinates  the  actions  of  the  organs  with  which  it  is 
associated. 


DIGESTION. 


i6i 


INSALIVATION. 

Insalivation  is  the  incorporation  of  the  sahva  with  the  food,  and 
takes  place  for  the  most  part  during  mastication.  The  sahva  ordi- 
narily present  in  the  mouth  is  a  complex  fluid  composed  of  the  various 
secretions  of  the  parotid,  submaxillary,  and  subhngual  glands  and 
the  muciparous  folhcles  of  the  mouth,  which  collectively  constitute 
the  sahvary  apparatus  (Fig.  6i). 

The  parotid  gland  is  situated  in  front  of  and  partly  below  the 
external  ear,  where  it  is 
held  in  position  by  the 
fascia  and  skin.  From  the 
anterior  border  of  the  gland 
there  emerges  a  duct  (Sten- 
sen's),  which,  after  crossing 
the  masseter  muscle  to  its 
anterior  border,  turns  in- 
ward, pierces  the  buccin- 
ator, and  opens  on  the  sur- 
face of  the  cheek  opposite 
the  second  upper  molar 
tooth. 

The  submaxillary 
gland  is  situated  below  the 
jaw  in  the  anterior  part  of 
the  submaxillary  triangle. 
From  the  gland  there 
emerges  a  duct  (Wharton's) 
which  opens  into  the  mouth 
by  a  minute  orifice  on  the 
surface  of  a  papilla  by  the 
side  of  the  tongue. 

The  sublingual  gland 
is  situated  just  beneath  the 
mucous  membrane  in  the 
anterior  part  of  the  mouth, 
where  it  forms  a  projection 

between  the  gums  and  tongue.  The  posterior  part  of  the  gland 
gives  origin  to  a  duct  (the  duct  of  Rivinus,  described  also  by  Bartho- 
hn)  which  opens  into  the  mouth  with  or  very  near  to  the  duct  of 
Wharton.  The  anterior  part  of  the  gland  gives  origin  to  a  varying 
number  of  ducts  (Walthers)  which  open  separately  along  the  edge  of 
the  sublingual  plica  of  the  mucous  membrane. 

Histologic  Structure  of  the  Salivary  Glands.^ — In  their  uki- 
mate  structure  the  sahvary  glands  bear  a  close  resemblance  to  one 


Fig.  6i. — Salivary  Glands,  i,  2.  Parotid. 
3.  Duct  of  Steno.  4.  Submaxillary.  5. 
Sublingual.  6.  Mylohyoid  muscle.  7. 
Lingual  branch  of  the  fifth  nerve.  8.  Duct 
of  Wharton.  9.  Digastric  muscle.  10. 
Sternomastoid  muscle.  11.  External  jugu- 
lar vein.  12.  Facial  vein.  13.  Temporal 
vein.  14,  15.  Internal  jugular  vein.  16. 
Branch  of  the  cervical  plexus.  17.  Sub- 
lingual nerve. — {Le  Bon.) 


l62 


TEXT-BOOK  OF  PHYSIOLOGY. 


lixcrelory 
duct. 


another.  They  are  compound  tubulo-alveolar  glands  composed  of 
small  irregularly  shaped  lobules  united  by  areolar  tissue  and  con- 
nected by  branches  of  the  sahvary  ducts.  Each  lobule  is  made  up  of 
a  number  of  small  alveoli  or  acini  more  or  less  tubular  in  shape  which 
are  the  terminal  expansions  of  the  smallest  ducts.  (See  Fig.  62.) 
The  wall  of  the  acinus  is  formed  by  a  reticulated  basement  mem- 
brane, surrounded  externally  by  blood-vessels,  the  spaces  between 
which  constitute  lymph-spaces  or  channels.  The  inner  surface  of 
the  acinus  membrane  supports  a  single  layer  of  irregular  spheric 
or  polygonic  epithelial  cells.  The  ceHs  do  not  entirely  fill  up  the 
cavity  of  the  acinus,  but  leave  an  intercellular  space,  the  lumen, 
which  constitutes  the  beginning  of  the  duct  for  the  transmission  of 

the  secretion  to  the  mouth.  From 
each  acinus  there  passes  a  narrow 
intercalary  duct  lined  by  a  layer  of 
flattened  cells.  The  common  ex- 
cretory duct — formed  by  the  union 
of  the  intralobular  and  interlobular 
ducts — consists  also  of  a  basement 
membrane,  lined,  however,  by  tall 
columnar  epithelial  cells.  The  sali- 
vary glands  are  abundantly  supphed 
with  blood-vessels  and  nerves  which 
are  closely  related  to  their  activity. 

Based  partly  on  the  character 
of  their  secretions  and  partly  on 
the  microscopic  appearance  of  their 
secreting  cells,  the  salivary  glands 
have  been  divided  by  Heidenhain 
into  two  classes:  viz.,  serous  or  al- 
buminous, and  mucous  glands.  To 
the  first  class  belong  the  parotid,  a 
portion  of  the  submaxillary,  and  a  portion  of  the  glands  of  the 
tongue.  To  the  second  class  belong  a  portion  of  the  submaxillary 
gland,  the  subHngual,  a  portion  of  the  glands  of  the  tongue,  the  glands 
of  the  cheeks,  hps,  palate,  and  pharynx.  It  is  possible  that  a  single 
alveolus  of  any  gland  may  contain  both  albuminous  and  mucous 
cells. 

In  the  serous  glands  the  cells  are  more  or  less  spheric  in  shape, 
nucleated,  and  almost  completely  filled  with  dark  granular  material 
(Fig.  63).  In  the  mucous  glands  the  cells  are  large,  clear  in  appear- 
ance, and  loaded  with  a  highly  refracting  material  resembling  mucin 
(Fig.  64).  Between  the  basement  membrane  and  the  clear  cells  are 
to  be  found  in  the  acini  of  the  submaxillary  glands  small  crescentic 
shaped  cells  filled  with  granular  material  which  stains  deeply  with  vari- 


FiG.    62. — Scheme   of  the   Human 
Submaxillary  Gland. — {Stdhr.) 


DIGESTION. 


i6: 


ous  coloring-matters.  These  are  known  as  the  demilunes  of  Heiden- 
hain.  At  one  time  it  was  supposed  that  they  were  young  cells  des- 
tined to  take  the  place  of  the  clear  cells  which  were  beheved  to  be 
exhausted  and  to  have  undergone  disintegration.  At  the  present  time 
they  are  regarded  as  albuminous  or  serous  cells  which  exhibit  changes 
similar  to  the  cells  of  the  parotid  gland. 

The  glands  embedded  in  the  mucous  membrane  covering  the 
tongue,  lips,  cheek,  palate,  and  pharynx  are  for  the  most  part  lined 
with  epithelial  cells  which  contain  a  highly  refracting  matter  similar 
to,  if  not  identical  with,  that  found  in  the  cells  of  the  submaxillary 
and  sublingual  glands. 

Nerve-supply. — Experimental  research  has  demonstrated  that 
each  salivary  gland  receives  nerve-fibers  which  influence  the  produc- 
tion of  the  secretion  (secretor  nerves)  and  fibers  which  dilate  or  con- 
strict the  blood-vessels  (vaso-dilatator  and  vasoconstrictor  nerves). 


Fig.  63. — Section  of  Human  Paro- 
tid Gland  Including  Several 
Acini,  d.  Cut  intralobular  duct. 
— {Pier  sol.) 


Fig.  64.— Section  of  Hum.an  Sublin- 
gual Gland.  Among  the  clear 
cells  Lining  the  mucous  acini  are 
nests  {g,  g)  of  granular  elements 
which  constitute  the  demilunes  of 
Heidenhain. — {Pier  sol.) 


The  secretor  fibers  penetrate  the  basement  membrane  and  finally 
terminate  between  and  on  the  surface  of  the  secretory  cells.  The 
vaso-motor  fibers  terminate  between  and  on  the  muscle-cells  in  the 
walls  of  the  blood-vessels.  The  nerve-fibers  in  direct  relation  with 
the  cells  and  blood-vessels  of  the  parotid  gland  are  derived  from  the 
otic  ganglion.  The  cells  of  this  ganglion  are,  however,  invested  by 
the  terminal  branches  of  other  nerve-fibers  (preganglionic)  derived 
from  the  medulla  oblongata.  The  relation  of  these  preganghonic 
fibers  to  the  blood-vessels  and  cells  is  shown  by  the  increase  in 
secretion  and  a  change  in  the  caliber  of  the  vessels  when  they  are 
subjected  to  electric  stimulation.  The  nerve-fibers  which  are  in 
direct  relation  with  the  vessels  and  cells  of  the  submaxillary  and 
sublingual  glands  are  derived  from  the  submaxillary,  the  subhngual, 
and  the  superior  cervical  ganglia.  These  local  ganglion  cells  are 
also  closely  invested  by  the  terminal  branches  or  arborizations  of  the 


i64  TEXT-BOOK  OF  PHYSIOLOGY. 

fibers  of  nerves  (preganglionic  fibers)  coming  direct  from  the  medulla 
oblongata  and  spinal  cord. 

The  Parotid  Saliva. — The  parotid  saliva,  as  it  flows  from  the 
orifice  of  Stensen's  duct,  is  clear,  limpid,  free  from  viscidity,  dis- 
tinctly alkaline  in  reaction,  with  a  specific  gravity  of  1.003.  Chemic 
analysis  shows  that  it  consists  of  water,  a  small  quantity  of  proteid 
matter,  a  trace  of  a  sulphocyanogen  compound,  and  inorganic  salts. 
The  secretion  is  increased  during  mastication,  and  especially  on  the 
side  engaged  in  mastication.  Dry  food  causes  a  larger  flow  than 
moist  food.  The  situation  of  the  orifice  of  the  parotid  duct  is  such 
that  as  the  secretion  is  poured  into  the  mouth  it  is  at  once  incorporated 
with  the  food  by  the  movements  of  the  lower  jaw,  and  thus  fulfils  the 
physical  function  of  softening  and  moistening  it. 

The  Submaxillary  Saliva. — The  submaxillary  saliva  is  clear, 
slightly  viscid,  alkaline  in  reaction,  and  has  a  specific  gravity  of 
1.002.  It  consists  of  water,  proteid  matter  (mucin),  and  inorganic 
salts. 

The  Sublingual  Saliva.— The  subhngual  saliva  is  clear,  extremely 
viscid,  and  strongly  alkaline  in  reaction.  It  consists  of  water,  proteid 
matter  (chiefly  mucin),  and  inorganic  salts. 

The  small  racemose  glands  embedded  in  the  mucous  membrane 
on  the  inner  surface  of  the  cheeks  and  Hps,  on  the  hard  and  soft 
palate,  on  the  tongue  and  pharynx,  secrete  a  fluid  which  is  grayish 
in  color,  extremely  viscid  and  ropy.  It  contains  a  large  amount  of 
mucin. 

Mixed  Saliva. — The  saHva  of  the  mouth  is  a  complex  fluid  com- 
posed of  the  secretions  of  all  the  salivary  glands.  As  obtained  from 
the  mouth  it  is  frothy,  colorless,  shghtly  turbid,  and  somewhat  viscid. 
The  specific  gravity  is  low,  ranging  from  1.003  to  1.006.  The  re- 
action is  usually  distinctly  alkahne.  It  may,  however,  be  neutral 
or  even  acid  in  reaction  if  there  is  any  fermentation  of  food  particles 
in  the  mouth  or  as  a  result  of  disorders  of  the  alimentary  canal. 
When  examined  with  the  microscope,  the  saliva  is  seen  to  contain 
epithelial  cells,  salivary  corpuscles  resembling  leukocytes,  particles 
of  food,  various  microorganisms,  and  especially  Leptothrix  huccalis. 

The  chemic  composition  of  the  saliva  is  shown  in  the  following 
table : 

COMPOSITION  OF  HUMAN  SALIVA. 

Water, 995-i6  994.20 

Epithelium, ; 1.62  2.20 

Soluble  organic  matter, 1.34  1.40 

Potassium  sulphocyanid, 0.06  0.04 

Inorganic  salts, 1.82  2.20 

1000.00  1000.04 

(Jacubowitsch.)  (Hammerbacher.) 


DIGESTION.  165 

Water  constitutes  the  main  ingredient,  amounting  to  99.5  per  cent. 
The  soluble  organic  matter  is  proteid  in  character  and  is  a  mixture 
of  mucin,  globuhn,  and  serum-albumin.  The  potassium  sulpho- 
cyanid  is  mainly  derived  from  the  parotid  gland.  Its  presence  can  be 
demonstrated  by  the  addition  of  a  few  minims  of  a  dilute  solution  of 
slightly  acidulated  ferric  chlorid,  when  a  characteristic  red  color  is 
developed.  The  inorganic  constituents  comprise  the  sodium,  calcium 
and  magnesium  phosphates,  sodium  carbonate,  sodium  and  potas- 
sium chlorids. 

Quantity  of  Saliva.— The  estimation  of  the  total  quantity  of 
mixed  sahva  secreted  in  twenty-four  hours  is  exceedingly  difficult,  and 
the  results  obtained  must  be  only  approximative.  It  is,  of  course, 
subject  to  considerable  variation,  depending  upon  habit,  the  nature  of 
the  food,  etc.  The  experiments  of  Professor  Dalton  and  the  results 
obtained  by  him  are  eminently  trustworthy,  and  in  all  probabihty 
represent  as  nearly  as  possible  the  exact  amount  secreted.  He  found 
that  without  any  artificial  stimulus  he  was  enabled  to  collect  from 
the  mouth  about  36  grams  (540  grains)  of  saliva  per  hour,  but  that 
upon  the  introduction  of  any  stimulating  substance  into  the  mouth  the 
quantity  could  be  greatly  increased.  During  mastication  the  saUva 
was  poured  out  in  greater  abundance,  the  amount  depending  upon 
the  relative  dryness  of  the  food.  He  found  that  wheaten  bread  ab- 
sorbed 55  per  cent,  of  its  weight,  and  fresh  cooked  meat  48  per  cent. 
If,  therefore,  the  average  quantity  of  bread  and  meat  required  daily 
by  a  man  of  ordinary  physical  development  and  activity  be  assumed 
to  be  540  grams  (19  oz.)  of  the  former  and  450  grams  (16  oz.)  of  the 
latter,  these  two  substances  would  absorb  respectively  297  grams 
(4573.8  grains)  and  216  grams  (3326.4  grains),  making  a  total  of  513 
grams  (7900  grains).  If,  therefore,  the  amount  secreted  and  mixed 
with  the  food  during  an  estimated  two  hours  of  mastication  be 
added  to  the  amount  secreted  during  the  remaining  twenty-two 
hours,  supposing  that  it  continues  at  the  rate  of  36  grams  per  hour, 
we  have  a  total  amount  of  513  +  792  grams,  or  1305  grams  (19,780 
grains),  or  about  2.8  pounds. 

Histologic  Changes  in  the  Salivary  Glands  during  Secretion. 
— During  and  after  secretion  very  remarkable  changes  take  place  in 
the  cells  lining  the  acini,  which  are  in  some  way  connected  with  the 
production  of  the  essential  constituents  of  the  salivary  fluids.  In  the 
case  of  the  parotid  gland,  which  may  be  regarded  as  the  type  of  a 
serous  or  albuminous  gland,  the  following  changes  have  been  observed 
by  Langley  (Fig.  65).  During  the  period  of  rest  and  just  previous  to 
secretory  activity,  the  epithelial  cells  are  enlarged  and  swollen,  and 
encroach  on  the  lumen  of  the  acinus.  The  protoplasm  of  the  cells 
is  so  completely  filled  with  dark  fine  granules  as  not  only  to  obscure 
the  nucleus,  but  to  almost  obliterate  the  line  of  union  of  the  cells. 


1 66 


TEXT-BOOK  OF  PHYSIOLOGY. 


As  soon  as  secretion  becomes  active,  however,  the  granules  begin  to 
disappear  from  the  outer  region  of  the  cell  and  move  toward  the  inner 
border  and  into  the  lumen  of  the  acinus.  From  these  observations 
it  might  be  inferred  that  during  rest  the  protoplasm  of  the  cells  gives 
rise  to  granular  material,  and  that  during  and  after  secretory  activity 


Fig.  65. — Cells  of  the  Alveoli  of  a  Serous  or  Watery  Salivary  Gland.  A. 
After  rest.  B.  After  a  short  period  of  activity.  C.  After  a  prolonged  period  of 
activity. — {Yeo's  "  Text-Book  of  Physiology.") 


there  is  an  absorption  of  new  material  from  the  lymph  and  a  recon- 
struction of  the  granular  material. 

In  the  submaxillary  gland,  a  portion  of  which  may  be  taken 
as  a  type  of  a  mucous  gland,  similar  changes  have  been  observed 
(Fig.  66).  During  rest  the  epithehal  cells  are  large,  clear  in  appear- 
ance, highly  refractive,  and  loaded  with  small  globules  resembling 
mucin.  The  nucleus,  surrounded  by  a  small  quantity  of  proto- 
plasm, lies  near  the  margin  of 
the  cell.  That  the  granules 
are  not  protoplasmic  in  char- 
acter is  shown  by  the  fact 
that  they  do  not  stain  on  the 
addition  of  carmine.  When 
treated  with  water  or  dilute 
acids,  the  globules  swell  up, 
coalesce,  and  form  a  uniform 
mass.  The  chemic  relations 
of  this  substance  indicate  that 
it  is  the  precursor  of  mucin 
— namely,  mucigen.  During 
secretory  activity  the  cells  dis- 
charge these  mucigen  granules 
into  the  lumen  of  the  acinus,  where  they  are  transformed  into 
mucin.  Though  the  appearance  of  the  gland-cell  appears  to  in- 
dicate it,  there  is  no  evidence  for  the  view  that  the  cell  itself 
undergoes  disintegration  in  the  process. 

The  Physiologic  Actions  of  Saliva. — The  constant  presence  of 
sahvary  glands  in  the  different  classes  of  animals  and  the  large  amount 


m\ 


Fig.  66. — The  Appearance  Presented  by 
THE  Cells  of  the  Submaxillary 
Gland  of  the  Dog  after  Prolonged 
Secretion. — {Modified  from  Landois 
and  Stirling.) 


DIGESTION.  167 

of  secretion  they  pour  daily  into  the  ahmentary  canal  point  to  the 
conclusion  that  this  mixed  fluid  plays  an  important  role  in  the  general 
digestive  process.  Experiment  has  demonstrated  that  it  has  a  two- 
fold action,  physical  and  chemical. 

Physically,  saliva  softens  and  moistens  the  food,  unites  its  par- 
ticles into  a  consistent  mass  by  means  of  its  contained  mucin,  and 
thus  facihtates  swallowing. 

Chemically  it  converts  starch  into  sugar.  This  action  is  more 
marked  with  boiled  than  wdth  raw  starch,  a  fact  which  depends 
on  the  physical  structure  of  the  starch  grain.  In  the  natural  con- 
dition each  starch  grain  consists  of  a  cellulose  envelope  or  stroma 
in  the  meshes  of  which  is  contained  the  true  starch  material,  the 
granulose.  When  boiled  for  some  minutes,  the  starch  grain  absorbs 
water,  the  granulose  swells  and  ruptures  the  cellulose  envelope,  after 
which  it  passes  into  an  imperfect  opalescent  solution  more  or  less 
viscid,  depending  on  the  relative  amounts  of  water  and  starch.  This 
is  the  change  largely  brought  about  by  the  process  of  cooking.  If 
a  portion  of  this  hydrated  starch  be  kept  in  the  mouth  for  a  few 
minutes  it  will  be  converted  into  sugar,  a  fact  made  evident  by  the 
sense  of  taste. 

The  chemic  action  of  sahva  in  converting  starch  into  sugar,  as 
well  as  the  intermediate  stages,  can  be  experimentally  shown  in  the 
following  manner :  To  5  volumes  of  a  thin  starch  solution  in  a  test- 
tube  add  two  volumes  of  filtered  saliva.  Place  the  mixture  in  a 
water-bath  at  a  temperature  of  35°  C.  In  a  few  minutes  the  starch 
passes  into  a  soluble  condition  and  the  fluid  becomes  clear  and  trans- 
parent. On  testing  the  solution  from  time  to  time  with  iodin  the 
characteristic  blue  reaction  will  be  found  to  gradually  disappear,  the 
color  passing  from  blue  to  violet,  to  red,  to  yellow.  If  now  the  solu- 
tion be  boiled  with  a  solution  of  cupric  hydroxid  (Fehling's  solution) 
a  copious  red  or  yellow  precipitate  of  cuprous  oxid  is  formed,  which 
indicates  the  presence  of  sugar.  The  polariscope  shows  that  this 
sugar  is  maltose,  Cj2H220ir  During  the  conversion  of  the  starch 
intermediate  substances  are  formed  to  which  the  term  dextrin  is 
applied.  After  the  starch  has  been  rendered  soluble  it  undergoes 
a  cleavage  into  maltose  and  a  dextrin,  which,  as  it  gives  rise  to  a 
red  color  with  iodin,  is  termed  erythrodextrin.  At  a  later  stage  this 
erythrodextrin  also  undergoes  a  cleavage  into  maltose  and  a  second 
variety  of  dextrin,  which,  as  it  does  not  give  rise  to  any  color  with 
iodin,  is  termed  achroodextrin.  It  is  claimed  by  some  investigators 
that  this  form  can  also  in  time  be  transformed  into  sugar.  It  is 
possible  that  a  small  quantity  of  dextrose  is  also  formed. 

The  successive  stages  of  the  conversion  of  starch  into  sugar  may 
be  represented  by  the  following  schema : 


1 68  TEXT-BOOK  OF  PHYSIOLOGY. 

1^     .,      J     .  .  f  Achroodextrin. 

Starch    =    Soluble  Starch    =      f  Erythrodextnn     -=    |  Maltose. 

^  Maltose. 

This  change  consists  in  the  assumption  by  the  starch  of  a  molecule 
of  water,  and  for  this  reason  the  process  is  termed  hydrolysis.  The 
nature  of  the  chcmic  change  is  shown  in  the  following  formula: 

3(CeHio05)   +    HjO    =    C.jHj^Ou   +    CeH.oOs 

Starch         +    Water    ^        Maltrose         +      Dextrin. 

The  amylolytic  or  starch-changing  action  of  saliva  depends  on 
the  presence  of  an  unorganized  ferment  or  enzyme  known  as  ptyalin. 
This  enzyme  is  present  in  the  secretion  of  each  of  the  salivary 
glands.  The  chemic  character  of  ptyalin  is  unknown,  though  there 
are  reasons  for  believing  that  it  partakes  of  the  nature  of  a  proteid. 
It  is  a  product  in  all  probability  of  the  katabolic  activity  of  the  secre- 
tory cells.  According  to  Latimer  and  Warren,  ptyaHn  is  a  deriva- 
tive of  the  zymogen,  ptyalogen.  This  latter  compound  has  been 
shown  to  be  present  in  the  glands  of  the  dog,  cat,  and  sheep. 
Ptyalin  effects  the  transformation  of  starch  merely  by  its  presence, 
and  undergoes  no  perceptible  consumption  in  the  process.  The 
activity  of  this  enzyme  is  very  great,  and  unless  interfered  with  by  an 
excess  of  sugar  and  dextrin,  it  acts  practically  indefinitely. 

The  activity  of  ptyalin  is  modified  by  various  external  conditions, 
among  which  may  be  mentioned  the  chemic  reaction  of  the  medium 
in  which  it  is  placed.  It  is  most  active  when  the  medium  is  moder- 
ately alkaline.  Its  activity  is  arrested  either  by  strong  alkalies  or 
acids,  though  the  presence  of  a  small  percentage  of  an  acid  does  not 
appear  to  have  any  effect  in  either  hastening  or  retarding  the  process. 
This  fact  has  a  bearing  upon  the  question  as  to  whether  the  action  of 
the  saHva  is  interfered  with  in  the  stomach  by  the  presence  of  the 
gastric  juice.  At  present  it  is  a  disputed  matter,  but  the  weight  of 
authority  is  in  favor  of  the  view  that  the  transforming  action  may 
continue  for  almost  half  an  hour  during  the  early  stages  of  gastric 
digestion.  .The  temperature  also  influences  the  rapidity  with  which 
the  transformation  of  the  starch  is  effected.  At  a  temperature  of 
95°  to  io6°  F.  the  ptyalin  acts  most  energetically,  while  its  activity 
is  entirely  arrested  by  reducing  the  temperature  to  the  freezing-point 
or  raising  it  to  the  boihng-point. 

The  Nerve  Mechanism  of  the  Secretion  of  Saliva. — The 
secretion  of  the  saliva  is  a  complex  act  and  involves  the  cooperation 
of  gland-cells,  blood-vessels,  and  nerves.  During  the  intervals  of 
mastication  the  glands  are  practically  at  rest  as  far  as  the  discharge 
of  saliva  is  concerned.  The  cells,  however,  are  actively  engaged  in 
absorbing  from  the  surrounding  lymph-spaces  materials  derived 
from  the  blood  from  which  they  construct  their  characteristic  con- 


DIGESTION  169 

tents.  The  blood-vessels  possess  that  degree  of  dilatation  necessary 
for  nutritive  purposes. 

With  the  beginning  of  mastication  the  blood-vessels  suddenly 
dilate,  the  blood-supply  is  increased,  and  the  gland-cells  begin  to  dis- 
charge water,  inorganic  salts,  and  their  organic  constituents  into  the 
lumen  of  the  acinus.  This  continues  until  mastication  ceases,  when 
all  the  structures  return  to  their  former  condition  of  relative  inactivity. 
The  entire  process  is  reflex  in  character  and  takes  place  through  the 
medulla  oblongata.  It  requires  the  usual  mechanism  necessary  for 
all  reflex  acts — viz.,  a  sentient  surface,  afferent  nerves,  emissive 
cells,  efferent  nerves,  and  the  responsive  organs. 

With  the  introduction  of  food  into  the  mouth  impressions  are 
made  on  the  terminal  branches  of  the  afferent  nerves  distributed  in 
the  mucous  membrane.  Nerve  impulses  developed  by  the  mechanic 
and  chemic  action  of  the  food  are  then  transmitted  to  the  medulla 
oblongata  and  received  by  emissive  cells.  These  in  turn  discharge 
nerve  impulses  which  are  transmitted  through  efferent  ner\"es  to  the 
structures,  producing  the  vascular  and  secretory  effects  already  stated. 

The  nerves  and  nerve-centers  which  constitute  the  reflex  mechan- 
ism for  the  secretion  of  saliva  are  shown  in  the  following  table: 

Afferent  Nerves.  Nerve-centers.  Efferent  Nerves. 

1.  Lingual  branch  of  fifth      Medulla  oblongata.       Chorda  tympani  for  the  submax- 

nerve.  illary  and   subUngual   glands, 

auriculotemporal  branch  of  the 

2.  Taste     fibers     in     the  fifth     nerve    for    the    parotid 

chorda  tympani.  gland. 

3.  Glossopharyngeal.  Sympathetic  nerve. 

That  the  secretion  of  the  saliva  is  regulated  by  the  above  mechan- 
ism, and  that  the  lingual  branches  of  the  fifth  nerves  and  the 
glossopharyngeal  are  the  afjerent  nerves,  can  be  demonstrated  by 
exposing  the  glands  and  their  nerve  connections  and  subjecting  them 
to  experiment.  Under  such  circumstances,  if  a  cannula  be  placed 
in  the  duct  of  the  submaxillary  gland,  and  the  lingual  nerve  stimu- 
lated by  an  induced  electric  current  of  moderate  strength,  a  copious 
flow  of  saliva  at  once  takes  place.  If  now  the  glossopharyngeal  nerve 
be  stimulated  in  a  similar  manner,  the  effect  on  the  secretion  will  be  the 
same.  Division  of  these  two  nerves  in  an  animal,  in  such  a  way  as  to 
prevent  the  nerve  impulses  from  reaching  the  medulla  oblongata,  is 
followed  by  a  marked  diminution  in  the  amount  of  saliva  secreted. 
The  reflex  centers,  however,  may  receive  impulses  and  be  excited  to 
activity  by  impulses  coming  through  other  nerves — e.  g.,  the  pneumo- 
gastric,  when  the  mucous  membrane  of  the  stomach  is  stimulated; 
the  sciatic,  when  after  division  its  central  end  is  stimulated;  through 
nerve-fibers  that  originate  higher  up  in  the  brain  and  are  stimulated 
by  ideas  and  emotions. 


170 


TEXT-BOOK  OF  PHYSIOLOGY. 


Whenever  these  centers  are  stimulated,  either  by  nerve  impulses 
coming  through  afferent  nerves,  from  the  periphery  or  from  the 
brain,  impulses  are  generated  which  pass  outward  through  efferent 
nerves — the  chorda  tympani  nerve  to  the  submaxillary  and  sublingual 
glands,  and  the  auriculo-temporal  nerve  to  the  parotid  gland. 

The  chorda  tympani  nerve  is  a  branch  of  the  facial,  the  trunk  of 
which  it  leaves  in  the  aqueduct  of  Fallopius.  It  then  crosses  the 
tympanic  cavity,  emerges  through  the  Glaserian  fissure,  and  joins 
the   lingual   branch  of  the  inferior  maxillary  division  of  the  fifth 


Glosso-Pnaryntfe  al 


Otic  GanrjUon^ 
rarotid  via, 


Oiih Maxillary  Ulan 

uAarcla.  Tympani 


Oup.Ceroicul'  Uanollon. 
Sympathetic  Nerves 


Fig.  67. — Scheme  of  the  Nerves  Involved  in  the  Secretion  of  Saliva. 


nerve.  After  passing  forward  as  far  as  the  sublingual  gland,  nearly 
all  of  the  original  fibers  leave  the  lingual  nerve  by  four  or  five  strands 
to  become  connected  by  terminal  branches  with  nerve-ganglion  cells  in 
relation  with  the  submaxillary  and  sublingual  glands.  (See  Fig.  67.) 
The  nerve-fibers  which  conduct  nerve  impulses  outward  from 
the  medulla  to  the  parotid  gland  are  beheved  to  pass  through  the 
glossopharyngeal  nerve,  through  the  tympanic  branch  or  nerve  of 
Jacobson,  to  the  otic  ganglion,  with  which  they  become  connected. 
From  this  ganghon  new  nerve-fibers  arise,  the  terminal  branches  of 
which  become  connected  with  the  secretory  cells  of  the  gland. 


DIGESTION.  171 

The  effects  on  the  secretion  and  flow  of  sahva  from  the  submaxil- 
lary gland  which  follow  division  and  stimulation  of  the  chorda  tym- 
pani  nerve  are  shown  in  the  following  way:  a  cannula  is  inserted  into 
Wharton's  duct  and  the  rate  of  flow  estimated;  the  nerve  is  then 
divided,  after  which  the  flow  ceases.  The  peripheral  end  of  the 
nerve  is  then  stimulated  with  the  induced  electric  current,  when  a 
copious  secretion  of  a  thin  saliva  takes  place,  accompanied  by  a 
marked  dilatation  of  the  blood-vessels  of  the  gland.  The  quantity  of 
blood  passing  through  the  vessels  is  so  great  as  to  give  to  the  venous 
blood  an  arterial  hue  and  to  the  small  veins  a  distinct  pulsation.  It 
would  appear  from  these  effects  that  the  chorda  contains  two  sets  of 
fibers,  one  of  which  inhibits  the  action  of  a  local  vaso-motor  mechan- 
ism permitting  the  blood-vessels  to  dilate  (vaso-dilatator  fibers),  the 
other  of  which  stimulates  the  secretor  cells  to  activity,  either  directly 
or  through  the  intermediation  of  local  gangha.  That  local  ganglia 
are  involved  is  shown  by  the  effects  which  follow  the  injection 
of  nicotin  into  the  circulation.  After  a  sufficient  dose — 10  miUi- 
grams  for  the  cat — stimulation  of  the  chorda  has  no  effect.  Stimu- 
lation of  the  nerve-plexus  beyond  the  ganglion,  however,  is  at  once 
followed  by  vascular  dilatation  and  secretion. 

It  might  be  inferred  that  the  increase  in  the  flow  of  saliva  is  due 
to  filtration,  the  result  of  the  increased  blood-supply  to  the  gland,  and 
not  to  the  influence  of  any  true  secretor  fibers  stimulating  the 
activities  of  the  secretor  cells.  That  this  is  not  the  case,  however, 
can  be  demonstrated  in  several  ways:  First,  the  pressure  in  the  duct 
of  the  submaxillary  gland,  as  shown  by  the  mercurial  manometer, 
rises,  when  the  gland  is  secreting,  considerably  above  the  pressure  in 
the  carotid  artery,  which  could  not  be  the  case  if  it  were  due  to  a  mere 
filtration;  for  if  pressure  alone  were  the  cause,  the  flow  of  saliva  would 
cease  as  soon  as  the  pressure  in  the  tube  equaled  the  pressure  in  the 
blood-vessels.  Second,  even  in  the  absence  of  blood  the  gland  can 
be  made  to  yield  a  secretion,  as  shown  by  stimulating  the  nerve  in  a 
recently  killed  animal.  Third,  after  the  injection  of  atropin  into  the 
circulation  the  secretion  is  abolished,  but  the  local  vasomotor  mechan- 
ism is  unimpaired,  for  stimulation  of  the  nerve,  as  in  the  previous 
instance,  gives  rise  to  a  dilatation  of  the  vessels  and  an  increased 
blood-supply.  There  is  thus  abundant  proof  that  the  chorda  tym- 
pani  contains  two  sets  of  fibers — one  regulating  the  blood-supply  to 
the  gland,  the  other  stimulating  the  secretory  cells. 

The  influence  of  the  auriculo-temporal  branch  of  the  fifth  nerve 
upon  the  parotid  gland  is  similar  to  the  action  of  the  chorda  tympani 
on  the  submaxillary  gland.  The  active  fibers  of  this  nerve  are  prob- 
ably derived  from  the  ninth  nerve  or  glossopharyngeal.  If  the  nerve 
be  stimulated  by  the  induced  electric  current,  there  follows  a  dilata- 
tion of  the  blood-vessels  and  an  abundant  discharge  of  a  thin  saliva, 


172  TEXT-BOOK  OF  PHYSIOLOGY. 

rich  in  water  and  salts,  but  containing  a  small  amount  of  organic 
matter.  Division  of  the  nerve,  extirpation  of  the  otic  ganglion, 
division  of  Jacobson's  nerve,  is  followed  by  a  loss  of  reflex  secretion. 
Stimulation  of  Jacobson's  nerve,  as  shown  by  Heidenhain,  gives  rise 
to  the  secretion. 

The  sympathetic  fibers  which  influence  the  salivary  secretion 
emerge  from  the  spinal  cord  mainly  through  the  second,  third,  and 
fourth  thoracic  nerves.  After  passing  into  the  sympathetic  chain 
they  ascend  to  the  superior  cervical  ganglion,  with  the  cells  of  which 
they  become  connected  through  the  intermediation  of  fine  terminal 
branches.  From  this  point  non-medullated  nerve-fibers  follow  the 
branches  of  the  external  carotid  artery  to  the  different  glands.  There 
is  no  evidence  that  these  fibers  have  any  connection,  anatomic  or 
physiologic,  with  local  ganglia  at  or  near  the  submaxillary  or  sub- 
lingual glands.  If  the  sympathetic  nerve  in  the  neck,  especially  in 
the  dog,  be  divided  and  the  peripheral  end  stimulated  with  the  in- 
duced electric  current,  there  is  at  once  a  contraction  of  the  smaller 
blood-vessels  of  the  gland  and  a  diminution  of  the  blood-supply,  a 
result  showing  the  presence  of  vaso-constrictor  fibers.  Nevertheless 
both  the  submaxillary  and  sublingual  glands  pour  out  a  saliva  which 
is  different  from  that  poured  out  when  the  chorda  'tympani  is  stimu- 
lated. The  quantity  is  less,  it  is  more  viscid,  richer  in  organic  matter, 
of  a  higher  specific  gravity,  and  more  active  in  the  transformation  of 
starch  into  sugar.  Stimulation  of  the  fibers  passing  to  the  parotid 
gland  is  followed  by  contraction  of  the  vessels  and  an  abolition  of  the 
secretion;  but  at  the  same  time  there  is  an  increased  activity  of  the 
secretor  cells,  for  subsequent  stimulation  of  the  auriculo-temporal 
nerve  not  only  causes  an  increase  in  the  amount  of  water  and  in- 
organic salts,  but  an  increase  also  in  the  amount  of  organic  matter 
far  beyond  that  produced  when  the  auriculo-temporal  has  alone  been 
stimulated.  Histologic  examination  shows  that  the^small  ducts  of 
the  gland  are  filled  with  thick  organic  matter  after  stimulation  of 
the  cervical  sympathetic. 

DEGLUTITION. 

Deglutition  is  that  part  of  the  digestive  process  which  is  concerned 
in  the  transference  of  the  food  from  the  mouth  through  the  pharynx 
and  esophagus  into  the  stomach.  This  is  an  extremely  complex  act 
and  involves  the  action  of  a  large  number  of  structures,  all  of  which 
are  made  to  act  in  proper  sequence  under  the  coordinating  influence 
of  the  nervous  system.  The  deglutitory  canal  consists  of  the  mouth, 
pharynx,  and  esophagus,  each  of  which  presents  certain  anatomic 
features  on  which  its  physiologic  action  depends. 

The  cavity  of  the  mouth  communicates  posteriorly  with  the 
pharynx  by  a  narrow  orifice,  the  isthmus  of  the  fauces.     This  orifice 


DIGESTION.  173 

is  bounded  above  by  the  soft  palate,  laterally  by  the  anterior  and 
posterior  half  arches,  and  below  by  the  tongue. 

The  pharynx  is  an  oval-shaped  cavity  extending  from  the  base  of 
the  skull  to  the  lower  border  of  the  cricoid  cartilage,  a  distance  of 


/ML) 


MtT^s 


Z7 


Fig.  68. — Vertical  Section  of  the  Nasal  Fossa  and  Mouth,  i.  Left  nares 
2.  Lateral  cartilage  of  the  nose.  3.  Portion  of  the  internal  alar  cartilage  form- 
ing the  skeleton  of  the  lower  part.  4.  Superior  meatus.  5.  Middle  meatus. 
6.  Inferior  meatus.  7.  Sphenoidal  sinuses.  8.  External  boundary  of  the  pos- 
terior nares.  9.  Internal  elUptical  opening  of  the  Eustachian  tube.  10.  Soft 
palate.  11.  Vestibule  of  the  mouth.  12.  Vault  of  palate.  13.  Genioglossus 
muscle.  14.  Geniohyoid  muscle.  15.  Cut  margin  of  the  mylohyoid  muscle. 
16.  Anterior  pillar  of  the  palate  (anterior  half-arch),  presenting  a  triangular  figure 
with  the  base  inferiorly,  covering  partly  the  tonsil.  17.  Posterior  pillar  (poste- 
rior half-arch)  of  the  palate.  18.  Tonsil.  19.  FoUicular  (mucous)  glands  at  the 
base  of  the  tongue.  20.  Cavity  of  the  larynx.  21.  Ventricle  of  the  larynx. 
22.  Epiglottis.  23.  Cut  OS  hyoides.  24.  Cut  thyroid  cartilage.  25.  Thyro- 
hyoid membrane.  26.  Section  of  posterior  portion  of  the  cricoid  cartilage. 
27.  Section  of  the  anterior  portion  of  the  same  cartilage.  28.  Crico-thyroid 
membrane . — (5a  ppey.) 

about  12  centimeters.  (See  Fig.  68.)  Its  walls  are  formed  mainly  by 
three  pairs  of  muscles — the  superior,  middle,  and  inferior  constrictors 
— each  consisting  of  red,  striated  muscle- fibers,  and  hence  capable 
of  rapid  and  energetic  contractions.  Superiorly  the  pharynx  is 
attached  to  and  supported  by  the  basilar  process  of  the  occipital 
bone;  inferiorly  it  becomes  continuous  with  the  esophagus.     The 


174  TEXT-BOOK  OF  PHYSIOLOGY. 

anterior  wall  of  the  pharynx  is  imperfect  and  presents  openings  which 
communicate  with  the  nasal  chambers,  the  mouth,  and  the  larynx. 
The  lateral  wall  on  either  side  presents  the  opening  of  the  Eustachian 
tube  which  leads  directly  into  the  cavity  of  the  middle  ear.  The 
interior  of  the  pharynx  is  lined  by  mucous  membrane.  The  pharynx 
is  partially  separated  from  the  mouth  by  the  velum  pendulum  palati, 
a  muscular  structure  attached  above  to  the  hard  palate;  its  lower 
edge  or  border  is  directed  downward  and  backward  and  presents  in 
the  middle  hne  a  conical  process,  the  uvula.  On  either  side  the  palate 
presents  two  curved  arches,  the  anterior  and  posterior,  formed  re- 
spectively by  the  palato-glossei  and  palato-pharyngei  muscles.  The 
laryngeal  orifice  or  glottis  is  placed  just  beneath  the  base  of  the 
tongue.  It  is  triangular  in  shape,  wide  in  front,  narrow  behind,  and 
directed  downward  and  backward.  It  is  bounded  above  by  a  thin 
plate  of  cartilage,  the  epiglottis,  placed  just  behind  the  tongue  and  so 
arranged  that  it  can  easily  be  depressed  and  elevated. 

The  esophagus,  the  continuation  of  the  deglutitory  canal,  extends 
downward  from  the  lower  border  of  the  cricoid  cartilage  for  a  dis- 
tance of  from  2  2  to  25  centimeters,  to  a  point  opposite  the  ninth 
thoracic  vertebra,  where  it  expands  into  the  stomach.  Its  walls  are 
composed  of  an  internal  or  mucous  and  an  external  or  muscular  coat, 
united  by  areolar  tissue.  The  muscular  coat  consists  of  an  external 
layer  of  longitudinal  fibers  arranged  in  three  bands  and  of  an  internal 
layer  composed  of  fibers  arranged  circularly  in  the  upper  part  and 
obhquely  in  the  lower  part  of  the  esophagus.  In  the  upper  third 
the  fibers  are  striated;  in  the  middle  third  they  are  a  mixture  of  both 
striated  and  non-striated;  in  the  lower  third  they  are  entirely  non- 
striated. 

The  muscle  fibers  surrounding  the  esophago-gastric  orifice  are 
arranged  in  the  form  of  and  play  the  part  of  a  sphincter  muscle,  and 
for  this  reason  may  be  termed  the  sphincter  cardiae  muscle.  By 
its  action  it  prevents  a  return  under  normal  conditions  of  food  into 
the  esophagus. 

The  deglutitive  act  may  be  for  convenience  divided  into  three 
stages,  viz.: 

1.  The  passage  of  the  food  from  the  mouth  into  the  pharynx. 

2.  The  passage  of  the  food  through  the  pharynx  into  the  esophagus. 

3.  The  passage  of  the  food  through  the  esophagus  into  the  stomach. 

In  the  first  stage  the  bolus  of  food  is  placed  on  the  superior  surface 
of  the  tongue.  The  mouth  is  then  closed  and  respiration  is  momen- 
tarily suspended.  The  tip  of  the  tongue  is  placed  against  the  pos- 
terior surfaces  of  the  teeth.  The  tongue,  because  of  its  intrinsic 
musculature,  then  arches  from  before  backward  against  the  roof  of 
the  mouth  and  pushes  the  bolus  of  food  through  the  isthmus  of  the 
fauces  into  the  pharynx.     This  completes   the   first  stage.     It  is  a 


DIGESTION.  175 

voluntary  effort  and  accomplished  partly  by  the  tongue,   though, 
as  shown  by  Meltzer,  mainly  by  the  mylohyoid  muscles. 

The  second  and  third  stages,  or  the  passage  of  the  food  through 
the  pharynx  and  esophagus  into  the  stomach,  have  been  attributed 
until  quite  recently  entirely  to  peristaltic  movements  of  their  muscu- 
lature. It  has  been  stated  that  with  the  passage  of  the  food  through 
the  isthmus  of  the  fauces  the  posterior  wall  of  the  pharynx  advances 
and  seizes  the  food,  and  in  consequence  of  a  rapid  peristaltic  move- 
ment running  through  its  constrictor  muscles  from  above  downward 
is  transferred  to  the  esophagus;  that  with  the  entrance  of  the  food 
into  the  esophagus  a  similar  peristalsis,  varying  in  rapidity  in  different 
sections  in  consequence  of  a  change  in  the  character  of  its  muscula- 
ture, gradually  transfers  the  food  into  the  stomach.  There  can  be 
but  shght  doubt  that  by  this  method  the  bolus  of  food,  especially  if 
it  is  of  iirm  consistence  and  of  a  size  sufficient  to  distend  the  esoph- 
agus, is  transferred  into  the  stomach,  but  that  it  is  the  exceptional 
rather  than  the  usual  method  has  been  demonstrated  by  Kronecker, 
Falk,  and  Meltzer. 

In  1880  the  first  of  these  experimenters  made  the  observation  that 
the  sensation  in  the  stomach  following  the  swallowing  of  a  mouthful 
of  cold  water  occurred  too  quickly  to  be  explained  by  the  prevalent 
behef  that  its  transference  was  caused  by  ordinary  peristalsis,  the  rate 
of  progression  of  which  was  known  to  be  slow.  Falk  then  discovered 
the  fact,  by  introducing  through  the  mouth  into  the  pharynx  a  tube 
connected  externally  with  a  water  manometer,  that  during  the  act  of 
swallowing  there  is  a  sudden  rise  of  pressure  equal  to  about  twenty 
centimeters  of  water. 

These  experiments  demonstrated  that  at  the  beginning  of  degluti- 
tion there  is  a  sudden  rise  of  pressure,  the  result  of  a  quickly  acting 
force  resident  in  the  mouth  or  pharynx,  in  consequence  of  which  the 
food  is  rapidly  thrown  down  into  the  stomach,  peristalsis  playing  no 
part  in  the  process.  The  proof,  however,  of  these  statements  was 
furnished  by  Meltzer.  This  observer  introduced  into  the  pharynx 
and  esophagus  rubber  tubes,  the  ends  of  which  were  provided  with 
thin-walled  rubber  balloons  which  could  be  distended  with  air. 
The  outer  ends  of  the  tubes  were  connected  with  Marey's  recording 
tambours.  Any  compression  of  the  balloon  would  be  followed  by 
the  passage  of  the  air  into  the  tambour  and  an  elevation  of  the  lever. 
With  one  balloon  in  the  pharynx  and  the  other  in  the  esophagus  at 
varying  depths,  and  the  recording  levers  of  the  tambours  applied 
against  the  surface  of  a  revolving  cylinder,  it  became  possible,  with 
the  addition  of  a  chronogram,  to  obtain  a  graphic  representation  of 
the  time  relations  of  simultaneous  and  successive  compressions  of 
the  two  balloons. 


176 


TEXT-BOOK  OF  PHYSIOLOGY. 


It  was  found  as  the  result  of  many  experiments  that  no  matter 
how  deep  the  position  of  the  esophageal  balloon,  it  was  compressed 
simultaneously  with  the  pharyngeal  balloon,  as  shown  by  the  rise  of 
the  levers  on  swallowing  a  mouthful  of  water.  The  interval  of  time 
between  the  rise  of  the  two  levers  did  not  amount  to  more  than  the 
tenth  of  a  second.  The  inference  was  that  the  water  was  projected 
or  shot  down  the  pharynx  and  esophagus  in  this  period  of  time,  and 
in  its  passage  compressed  both  balloons  practically  at  the  same  in- 
stant. The  same  was  found  to  be  true  when  small  masses  of  more 
consistent  food  were  swallowed. 

The  curves  of  the  entire  deglutitive  act  recorded  by  the  two  levers 
are,  however,  different  in  form.  (See  Fig.  69.)  The  pharyngeal  curve, 
I,  presents  two  crests,  the  first,  A,  being  due  to  the  compression  caused 


Fig.  69. — Tracing  of  the  Act  of  Deglutition,  i.  A  indicates  the  compression  of 
the  elastic  bag  caused  by  the  bolus  projected  by  the  contraction  of  the  mylohyoid 
muscles.  B.  Contraction  of  the  pharynx.  2.  Line  marking  seconds.  3.  Trac- 
ing of  the  bag  in  the  esophagus  12  cm.  from  the  teeth.  C.  Compression  of  the 
bag  by  the  bolus  corresponding  to  A.  D.  Compression  by  the  residues  of  the 
bolus  carried  on  by  the  contraction  of  the  pharynx,  B.  E.  Contraction  of  the 
esophagus. — (Landois  and  Stirling.) 


by  the  passage  of  the  bolus,  the  second,  B,  due  to  the  compression 
exerted  by  the  contraction  of  the  pharyngeal  muscles.  The  interval 
of  time  between  these  two  crests  amounts  to  not  more  than  0.3 
second.  In  the  esophageal  curve,  3,  the  elevation,  C,  corresponds 
to  the  elevation.  A,  and  is  likewise  due  to  the  compression  exerted 
by  the  bolus.  The  interval  of  time  between  the  beginning  of  the 
first  and  second  curves  was  not  more  than  o.i  second,  regardless  of 
the  depth  to  which  the  esophageal  balloon  was  plunged.  At  a  later 
period  a  second  rise  of  the  lever  was  recorded ;  the  time  of  its  appear- 
ance, height,  duration,  etc.,  were  found  to  increase  with  the  depth  of 
the  balloon. 

These  facts  demonstrate  that  deglutition  consists  of  two  phases: 
(i)  a  rapid  rise  of  pressure  in  the  pharynx,  as  a  result  of  which  the 


DIGESTION.  177 

bolus  is  suddenly  shot  down  to  the  stomach;  (2)  a  peristaltic  con- 
traction of  the  musculature  of  the  canal,  which,  acting  as  a  supple- 
mentary force,  carries  onward  any  particles  of  food  in  the  canal  and 
forces  the  bolus  through  the  closed  sphincter  at  the  end  of  the  esoph- 
agus. 

The  immediate  cause  of  the  sudden  rise  of  pressure  was  shown  by 
Meltzer  to  be  the  contraction  of  the  mylohyoid  muscles.  When 
the  nerves  going  to  these  muscles  were  divided  in  a  dog,  deglutition 
was  practically  abohshed.  These  muscles  are  probably  assisted  in 
their  action  by  the  contraction  of  the  hyoglossus  muscles  as  well  as 
the  tongue  itself. 

It  was  also  demonstrated  in  these  experiments  that  the  contrac- 
tion of  the  esophagus  did  not  partake  of  the  character  of  ordinary 
peristalsis.  It  was  found  that  the  esophagus  contracted  in  three 
distinct  segments,  corresponding  in  all  probability  to  the  difference  in 
the  character  of  their  muscular  fibers.  The  first  segment,  about  six 
centimeters  in  length,  was  found  to  begin  to  contract  about  1.2 
seconds  after  the  beginning  of  the  first  curve  and  lasting  2  seconds; 
the  second  segment,  about  twelve  centimeters  in  length,  beginning 
to  contract  about  1.8  seconds  or  3  seconds  after  the  beginning  of 
the  first  section,  and  lasting  for  from  5  to  7  seconds;  the  third 
segment,  six  centimeters  in  length,  contracting  from  6  to  7  seconds. 
The  beginning  and  the  end  of  the  contraction  for  each  segment  oc- 
curred simultaneously  throughout  its  entire  extent.  If,  however,  a 
series  of  deglutitory  acts  follow  each  other  in  quick  succession, 
there  is  an  inhibition  of  the  peristaltic  contractions  until  after  the 
final  swallow. 

An  examination  of  the  action  of  the  esophagus  during  degluti- 
tion, made  by  Cannon  and  Moser  with  x-rays  and  the  fluoroscope, 
disclosed  the  fact  that  the  method  of  food  transmission  varied  in 
different  animals.  In  the  cat  and  dog  the  transmission  was  effected 
bv  peristalsis  alone.  The  time  required  for  the  food  to  reach  the 
stomach  varied  in  the  cat  from  nine  to  twelve  seconds  and  in  the 
dog  from  four  to  five  seconds.  The  descent  of  the  bolus  was  more 
rapid  in  the  upper  than  in  the  lower  part  of  the  esophagus.  In 
man,  hquids  descended  rapidly,  at  the  rate  of  several  feet  a  second, 
in  consequence  of  the  rapid  and  energetic  contraction  of  the  mylo- 
hyoid muscles.  A  peristaltic  contraction,  passing  over  the  entire 
esophagus,  was  necessar}-  to  the  passage  of  soHd  and  semisoHd  food 
through  it. 

Closure  of  the  Posterior  Nares  and  Larynx. — Notwithstand- 
ing the  rise  of  pressure  in  the  pharynx  during  the  act  of  swallowing, 
it  is  seldom  under  normal  circumstances  that  any  portion  of  the 
bolus  ever  finds  its  way  either  into  the  lar}mx  or  nasal  chambers, 


178 


TEXT-BOOK  OF  PHYSIOLOGY. 


for  the  reason  that  the  openings  of  these  cavities  are  fully  closed  by 
appropriate  means. 

At  the  moment  the  food  passes  into  the  pharynx  the  posterior 
nasal  openings  are  closed  against  the  entrance  of  the  food  by  a  septum 
formed  by  the  pendulous  veil  of  the  palate  and  the  posterior  half 
arches.  The  palate  is  drawn  upward  and  backward  until  it  meets 
the  posterior  wall  of  the  pharynx,  and  at  the  same  time  is  made  tense, 
by  the  action  of  the  levator  palati  and  tensor  palati  muscles  respec- 
tively (Fig.  70).  This  septum  is  completed  by  the  advance  toward 
the  middle  line  of  the  posterior  half  arches  caused  by  the  contrac- 
tion of  the  muscles  which  compose  them — the  palato-pharyngei. 
When   these   structures    are   impaired  in  their  functional  activity, 

as  in  diphtheritic  paralysis  and 
ulcerations,  there  is  not  infre- 
quently a  regurgitation  of  food, 
especially  liquids,  into  the  nose. 
The  larynx  is  equally  pro- 
tected against  the  entrance  of 
food  during  deglutition  under 
normal  circumstances.  That 
this  accident  occasionally  hap- 
pens, giving  rise  to  severe  spas- 
modic coughing,  and  even  in 
extreme  cases  to  suffocation, 
is  abundantly  shown  by  the 
records  of  chnical  medicine. 
Usually  it  does  not  occur,  for 
the  following  reasons:  Just 
preceding  and  during  the  act 
of  deglutition  there  is  a  com- 
plete suspension  of  the  act  of 
inspiration  by  which  particles 
of  food  might  otherwise  be 
drawn  into  the  larynx;  at  the 
same  time  the  larynx  is  always 
drawn  well  up  under  the  base  of  the  tongue  and  its  entrance 
closed  by  the  downward  and  backward  movement  of  the  epiglottis. 
The  glottis  itself  is  also  closed  by  the  constrictor  muscles  which 
surround  it. 

The  action  here  attributed  to  the  epiglottis  has  been  denied  by 
Stuart  and  McConnick.  These  observers  had  the  opportunity  of 
looking  into  a  naso-pharynx  which  had  been  laid  open  by  a  sur- 
gical operation  for  the  removal  of  a  morbid  growth.  In  this  patient, 
the  epiglottis,  at  the  time  of  deglutition,  was  always  more  or  less 
erect  and  closely  applied  to  the  base  of  the  tongue.     So  comj^lete 


Fig.  70. — Diagram  showing  the  Manner 
OF  Closure  of  the  Posterior  Nares 
AND  Larynx  during  Deglutition. — 
{Landois  and  Stirling.) 


DIGESTION.  179 

was  this  that  the  food  passed  over  its  posterior  or  inferior  surface 
for  a  certain  distance.  In  no  instance  was  it  ever  observed  to  fold 
backward  Hke  a  lid. 

Because  of  the  possibihty  that  this  position  of  the  epiglottis  was 
due  to  pathologic  causes,  Kanthack  and  Anderson  instituted  a  new 
series  of  experiments  with  a  view  of  determining  the  action  of  the 
epiglottis.  As  a  result  of  many  experiments  on  animals  and  of  ob- 
servations on  themselves,  these  observers  reathrm  the  generally 
accepted  view,  that  under  normal  conditions,  the  entrance  of  the 
larvnx  is  always  closed  by  the  epiglottis  after  the  manner  of  a  lid. 

The  Nerve  Mechanism  of  Deglutition. — Deglutition  is  almost 
exclusively  a  reflex  act  throughout  its  entire  extent,  and  requires  for 
its  inauguration  merely  a  stimulus  to  some  portion  of  the  mucous 
membrane  of  the  deglutitory  canal.  The  first  stage  is  primarily 
voluntary,  but  from  inattention  to  the  process  may  become  second- 
arily reflex.  The  origin  and  course  of  the  afferent  nerves,  stimu- 
lation of  which  excite  reflexly  the  movements  of  the  pharynx  and 
esophagus,  however,  are  practically  unknown.  In  the  rabbit  deg- 
lutition can  be  excited  by  stimulating  the  anterior  central  part  of 
the  soft  palate.  In  man  it  has  not  yet  been  possible  to  locate  an 
area  stimulation  of  which  will  give  rise  to  a  reflex  deglutitory  act. 
Though  electric  stimulation  of  the  superior  laryngeal  nerve  will  cause 
reflex  deglutitory  movements,  it  is  obvious  that  the  terminals  of  this 
nerve  can  not  be  the  source  of  the  natural  afferent  impulses.  Stimu- 
lation of  the  glossopharyngeal  nerve  causes  an  inhibition  of  the 
movements. 

The  center  from  which  emanate  nerve  impulses  which  excite  the 
various  muscles  to  action  has  been  located  experimentally  in  the 
medulla  oblongata  just  above  the  alas  cinereae.  The  efferent  nerves 
comprise  branches  of  the  facial,  hypoglossal,  motor  filaments  of  the 
third  division  of  the  fifth  ner\'e,  motor  filaments  of  the  glossopharyn- 
geal and  vagus  derived  in  all  probability  directly  from  the  medulla 
oblongata.  Inasmuch  as  the  different  mechanisms  act  not  only  in  a 
coordinate  but  sequential  manner,  it  is  possible  that  the  deglutition 
center  is  not  a  single  circumscribed  collection  of  cells,  but  a  series  of 
centers  corresponding  to  the  origin  of  the  efferent  nerves,  the  activi- 
ties of  which  are  coordinated  by  some  single  true  deglutition  center. 


GASTRIC  DIGESTION. 

After  the  food  has  passed  through  the  esophagus  it  is  received  by 
the  stomach,  where  it  is  retained  for  a  variable  length  of  time,  during 
which  important  changes  are  induced  in  its  physical  and  chemic  com- 
position. The  disintegration  of  the  food  inaugurated  by  mastication 
and  insalivation  is  still  further  carried  on  in  the  stomach  by  the  sol- 


i8o  TEXT-BOOK  OF  PHYSIOLOGY. 

vent  action  of  the  acid  fluid  there  present,  until  the  entire  mass  is 
reduced  to  a  hquid  or  semi-Hquid  condition. 

The  stomach  is  a  dilated  and  highly  specialized  portion  of  the 
alimentary  canal  intervening  between  the  esophagus  and  small  intes- 
tine. When  moderately  distended  with  food,  it  is  somewhat  conical 
or  pyriform  in  shape  and  shghtly  curved  on  itself.  It  is  situated 
obliquely  and  in  some  individuals  almost  vertically  in  the  upper  part 
of  the  abdominal  cavity,  extending  from  the  left  hypochondrium  to 
the  right  of  the  epigastrium.  The  dimensions  and  capacity  of  the 
stomach  undergo  considerable  periodic  variation  according  to  the 
extent  to  which  it  is  distended  by  food.  In  the  average  condition  it 
measures  in  its  long  diameter  from  25  to  35  centimeters,  in  its  vertical 
diameter  at  the  cardia  15  centimeters,  in  its  antero-posterior  diameter 
from  II  to  12  centimeters.  The  capacity  of  the  stomach  varies  from 
1500  to  1700  c.c.  In  the  empty  condition  its  walls  are  collapsed  and 
partly  in  contact,  and  the  entire  organ  is  drawn  up  into  the  upper 
part  of  the  abdominal  cavity.  The  opening  through  which  the 
food  passes  into  the  stomach  is  known  as  the  esophago-gastric  orifice 
or  the  cardia.  The  opening  through  which  it  passes  into  the  intes- 
tine is  known  as  the  pylorus,  the  pyloric  or  gastro-duodenal  orifice. 
Between  these  two  orifices  the  stomach  along  its  upper  border  pre- 
sents a  curve  and  along  its  lower  border  a  much  larger  curve,  known 
as  the  lesser  and  greater  curvatures  respectively.  The  left  end  of  the 
stomach  is  termed  the  fundus  or  cardiac  end;  the  right,  the  pyloric 
end.  Passing  from  the  fundus  toward  the  pylorus,  the  stomach 
gradually  narrows,  and  at  a  point  situated  about  5  cm.  from  the  pyloric 
opening  it  frequently  presents  a  constriction  which  divides  the  general 
cavity  into  two  portions:  viz.,  the  fundus  and  the  antrum  of  the 
pylorus. 

The  walls  of  the  stomach  are  formed  by  four  distinct  coats  united 
by  areolar  tissue  and  named,  from  without  inward,  as  the  serous, 
muscle-,  submucous,  and  mucous. 

The  external  or  serous  coat  is  thin  and  transparent  and  formed  by  a 
reduplication  of  the  general  peritoneal  membrane. 

The  middle  or  muscle-coat  consists  of  three  layers  of  non-striated 
muscle-fibers,  named  from  their  direction  the  longitudinal,  circular, 
and  oblique  (Fig.  71).  The  longitudinal  fibers  are  most  abundant 
along  the  lesser  curvature  and  are  a  continuation  of  those  of  the 
esophagus;  over  the  remainder  of  the  stomach  they  are  thinly  scat- 
tered, but  toward  the  pyloric  orifice  they  are  more  numerous  and 
form  a  tolerably  thick  layer  which  becomes  continuous  with  the 
fibers  of  the  small  intestine.  The  circular  fibers  form  a  complete 
ayer  encircling  the  entire  organ,  with  the  exception,  perhaps,  of  a 
portion  of  the  fundus.  The  fibers  of  this  coat  cross  the  longitudinal 
fibers  at  right  angles.     At  the  lower  end  of  the  esophagus  and  sur- 


DIGESTION.  i8i 

rounding  the  cardia  the  circular  muscle  fibers  form  a  true  sphincter 
which  is  known  as  sphincter  cardies.  At  the  junction  of  the  fundus 
with  the  pyloric  antrum  the  circular  fibers  are  arranged  in  a  well-de- 
fined bundle  termed  the  sphincter  antri  pylorici.  In  the  pyloric  region 
the  circular  fibers  are  more  closely  arranged,  forming  thick  well- 
defined  rings.  At  the  pyloric  opening  the  circular  fibers  are  again 
crowded  together  and  form  a  distinct  muscle  band, — the  sphincter 
pylori, — which  projects  for  some  distance  into  the  interior  of  the 
stomach.  It  has  been  stated  by  Riidinger  that  the  inner  fibers  of  the 
longitudinal  coat  become  connected  with  this  circular  band  and  con- 
stitute a  distinct  muscle,  the  dilatator  pylori.     The  oblique  fibers  are 


Fig.  71. — Fibers  Seen  WITH  THE  Stomach  Everted.  1,1.  Esophagus.  2.  Circular 
fibers  at  the  esophageal  opening.  3,  3.  Circular  fibers  at  the  lesser  curvature 
4,  4.  Circular  fibers  at  the  pylorus.  5,  5,  6,  7,  8.  Oblique  fibers.  9,  10.  Fibers 
of  this  layer  covering  the  greater  pouch.  11.  Portion  of  the  stomach  from  which 
these  fibers  have  been  removed  to  show  the  subjacent  circular  fibers. — (Sappey.) 


most  distinct  over  the  cardiac  portion  of  the  stomach,  but  extend 
from  left  to  right  as  far  as  the  junction  of  the  middle  and  last 
thirds  of  the  stomach.  They  are  continuations  of  the  circular  fibers 
of  the  esophagus. 

The  submucous  coat  consists  of  loose  areolar  tissue  carrying 
blood-vessels,  nerves,  and  lymphatics.  It  serves  to  unite  the  muscle 
to  the  mucous  coat.  Its  inner  surface  bears  a  thin  layer  of  muscular 
tissue,  the  muscularis  mucosa,  which  supports  the  mucous  membranes. 

The  internal  or  mucous  coat  is  loosely  attached  to  the  muscular 
coat.  In  the  empty  and  contracted  state  of  the  stomach  it  is  thrown 
into  longitudinal  folds  or  rugae,  which  are,  however,  obliterated  when 


I«2 


TEXT-BOOK  OF  PHYSIOLOGY. 


:^?f'*».. 


y'f"': 


Mucosa. 


■r-' 


the  organ  is  distended  with  food.  The  mucous  membrane  in  aduU 
life  is  smooth  and  velvety  in  appearance,  gray  in  color,  and  covered 
with  a  layer  of  mucus.  Its  average  thickness  is  about  one  millimeter. 
The  surface  of  the  membrane  is  covered  with  a  layer  of  columnar 
epithelial  cells,  each  of  which  possesses  a  nucleus  and  nucleolus.  At 
the  pylorus  there  is  a  circular  involution  of  the  mucous  membrane 
which  is  known  as  the  pyloric  valve.  This  is  strengthened  by  fibrous 
tissue  and  embraced  by  the  sphincter  muscle  previously  described. 

Gastric  Glands. — 
The  surface  of  the  mu- 
cous membrane  when 
examined  with  a  low 
magnifying  power  pre- 
sents throughout  in- 
numerable depressions 
polygonal  in  shape  and 
separated  by  slightly 
elevated  ridges.  At  the 
bottom  of  these  spaces 
are  to  be  seen  small 
orifices,  which  are  the 
mouths  of  the  glands 
embedded  in  the  mucous 
membrane.  A  vertical 
section  of  the  gastric 
walls  (Fig.  72)  shows  not 
only  the  position  and  ap- 
pearance of  the  glands, 
but  the  relation  of  the 
various  tissues  which  en- 
ter into  the  formation  of 
these  walls.  An  exam- 
ination of  the  mucous 
membrane  in  different 
regions  of    the   stomach 


Epilhelium. 


Tunic  J 
propria. 


Muscularis 
mucosae. 


SubniiKosa. 


Inner  cir- 
cular layer 
of  muscle. 


/I 


Muscu- 
laris. 


Outer  longi- 
tudinal layer 
of  muscle. 


Serosa. 

Fig.  72.^-Tr.'^nsverse  Section  or  the  Wall  of  a 
Human  Stomach.  X  i5-  The  tunica  propria 
contains  glands  standing  so  close  together  that 
its  tissue  is  visible  only  at  the  base  of  the  glands 
toward  the  muscularis  mucosas. — {Stolir.) 


reveals  two  distinct  types 
of  glands,  cardiac  or  fundic,  and  pyloric,  which  differ  not  only 
in  histologic  structure,  but  also  in  function.  Both  types  extend 
through  the  entire  thickness  of  the  mucosa. 

The  cardiac,  jundic,  or  peptic  glands,  are  formed  by  an  involution 
of  the  basement  membrane  of  the  mucosa  and  hned  by  epithehal 
cells.  Each  gland  may  be  said  to  consist  of  a  short  duct,  or  neck, 
and  a  body  or  fundus  (Fig.  73).  The  latter  portion  is  wavy  or 
tortuous  and  frequently  subdivided  into  as  many  as  8  to  10  distinct 
and  separate  tubules.     The  duct  is  lined  by  columnar  epithehal  cells 


DIGESTION. 


183 


similar  to  those  covering  the  surface  of  the  mucosa.  The  lumen  of 
the  fundus  is  bordered  by  epithelial  cells,  cuboid  in  shape,  and  con- 
sisting of  a  granular  protoplasm  containing  a  distinct  spherical  nucleus. 
These  cells  are  generally  spoken  of  as  the  chief  or  central  cells.  In 
addition  to  the  chief  cells,  the  fundus  contains  a  second  variety  of 

cell,  which  is  of  a  larger  size,  of 
a  triangular  or  oval  shape,  and 
consisting  of  a  finely  granular 
protoplasm.  From  this  situation 
just  beneath  the  gland  wall  they 
have  been  termed  parietal  or 
border  cells.  Each  parietal  cell 
appears  to  be  surrounded  and 
penetrated  by  a  system  of  pas- 
sages which  open  into  the  lumen 
of  the  gland  by  means  of  a  deli- 
cate cleft  or  canaliculus  (Fig. 
74).  Glands  with  these  histo- 
logic features  are  most  abundant 


-  Lumen. 


V^*^ 


Secretory 
capillaries 


Fig.  73. — Peptic  Gland  from 
Stomach  of  Dog.  a.  Wide 
mouth  and  duct  which  receive 
the  terminal  di\dsions  of  the 
gland,  b,  c.  Neck  and  fun- 
dus of  the  tubes,  e.  Central 
or  chief  cells,  d.  Parietal  or 
acid  cells. — (AJler  Piersol.) 


Fig.  74. — Section  of  Fundus  Gland 
OF  Mouse.  Left  upper  half  drawn 
after  an  alcohol  preparation,  right 
upper  half  after  a  Golgi  prepara- 
tion. The  entire  lower  portion  is  a 
diagrammatic  combination  of  both 
prepara  tions . — {Stoh  r . ) 


in  the  middle  zone  of  the  stomach.  Toward  the  extreme  left  end 
of  the  fundus  the  glands  are  largely,  if  not  entirely,  devoid  of  pari- 
etal cells. 

The  pyloric  glands  are  also    formed   by  an  involution  of  the 
mucous  membrane   and  lined   by  epithelial  cells    (Fig.    75).     The 


1 84 


TEXT-BOOK  OF  PHYSIOLOGY. 


ducts  are  much  longer  than  the  ducts  of  the  fundic  glands.  At 
its  extremity  each  duct  becomes  branched,  giving  rise  to  a  num- 
ber, from  2  to  1 6,  of  short  tubes,  each  of  which  has  a  large  lumen 
and  communicates  with  the  duct  by  a  narrow  short  neck.  The 
ducts  are  lined  throughout  by  columnar  epithelium.  Accord- 
ing to  Mall,  the  total  number  of  openings  on  the  surface  of  the 
raucous  membrane  of  the  dog's  stomach  is  somewhat  over  i,ooo,- 
ooo,  and  the  total  number  of  blind  tubes  opposite  the  muscularis 
mucosa  exceeds  16,500,000.  According  to  Sappe}*,  the  surface  of 
the  mucous  membrane  of  the  human  stomach  presents  over 
5,000,000  orifices  of  gastric  glands. 

Blood-vessels,  Nerves,   and  Lymphatics. — The  blood-vessels 

of  the  stomach  after  entering  the 
mucosa  break  up  into  a  number  of 
branches  which  are  distributed  to  the 
muscular  and  mucous  coats.  The 
branches  to  the  latter  soon  form  a 
capillary  network  with  oblong  meshes 
which  not  only  surround  the  tubules 
but  form  a  network  just  beneath  the 
surface  of  the  mucosa.  Veins  grad- 
ually arise  from  the  capillaries  which 
empty  into  the  larger  veins  of  the 
mucosa.  The  glands  are  also  sup- 
ported by  processes  of  smooth  mus- 
cle-fibers passing  up  from  the  muscul- 
aris mucosa. 

The  nerve-fibers  distributed  to  the 
stomach  are  derived  from  the  vagus 
and  the  sympathetic  branches  of  the 
solar  plexus.  After  piercing  the  ser- 
ous coat  the  fibers  form  or  unite  with 
a  plexus  of  fibers  situated  between 
the  circular  and  longitudinal  layers  of  the  muscle- coat.  At  the 
nodal  points  of  this  plexus  large  nerve-ganghon  cells  are  to  be 
found,  the  whole  forming  the  mechanism  known  as  Auerbach's 
plexus.  A  similar  plexus  of  cells  and  fibers  in  more  or  less  intimate 
anatomic  connection  with  the  foregoing  is  found  between  the  muscle 
and  submucous  coats,  and  is  known  as  Meissner's  plexus.  From 
this  plexus  fine  nerve  filaments  are  distributed  to  muscle-fibers, 
blood-vessels,  and  glands.  In  the  latter  structure  terminal  arbori- 
zations have  been  detected  in  close  contact  with  the  secreting  cells 
themselves. 

The  lymphatics,   which   are   quite  numerous,   originate   in   the 


Fig.  75. — Section  of  Pyloric 
Glands  from  Human  Stom- 
ach, a.  Mouth  of  gland 
leading  into  long,  wide  duct 
{b),  into  which  open  the  ter- 
minal divisions,  c.  Connec- 
tive tissue  of  the  mucosa. — 
(After  Piersol.) 


DIGESTION.  185 

meshes  of  the  mucosa.  The  larger  trunks  enter  lymph-glands  lying 
along  the  greater  and  lesser  curvatures  of  the  stomach. 

Gastric  Fistulae. — The  general  process  of  digestion,  as  it  takes 
place  in  the  stomach,  has  been  studied  in  human  beings  and  animals 
with  a  fistula  in  the  walls  of  the  stomach  and  abdomen,  the  result 
either  of  accident  or  of  necessary  surgical  or  experimental  procedures. 

The  earliest  observations  on  gastric  digestion  were  made  by  Dr. 
Beaumont  on  Alexis  St.  Martin,  who,  as  the  result  of  a  gunshot 
wound,  was  left  with  a  permanent  fistulous  opening  into  the  fundus 
of  the  stomach.  This  opening  two  years  after  the  accident  was  about 
two  and  a  half  inches  in  circumference  and  usually  closed  from  within 
by  a  fold  of  mucous  membrane  which  prevented  the  escape  of  the  food. 
This  valve  could  be  readily  displaced  by  the  finger  and  the  interior 
of  the  stomach  exposed  to  view.  After  the  complete  recovery  of  St. 
Martin,  Dr.  Beaumont  during  the  years  between  1825  and  1831  at 
intervals  made  numerous  experiments  on  the  nature  of  gastric  diges- 
tion. As  the  result  of  an  admirable  series  of  investigations  it  was 
estabhshed  that  the  digestion  of  the  food  is  largely  a  chemic  act,  due 
to  the  presence  of  an  acid  fluid  secreted  by  the  mucous  membrane; 
that  this  fluid  is  secreted  most  abundantly  after  the  introduction  of 
food  into  the  stomach;  that  different  articles  of  food  possess  varying 
degrees  of  digestibihty;  that  the  duration  of  digestion  varies  according 
to  the  nature  of  the  food,  exercise,  mental  states,  etc.,  and  that  the 
process  is  aided  by  continuous  movements  of  the  muscular  walls. 

Since  Dr.  Beaumont's  time  the  estabhshing  of  a  gastric  fistula  in 
human  beings  has  been  necessitated  by  pathologic  conditions  of  the 
esophagus.  After  recovery  these  cases  offered  fair  facihties  for  the 
study  of  the  process  when  the  food  was  introduced  through  the 
opening.  Similar  fistulce  have  been  established  in  both  carnivorous 
and  herbivorous  animals  with  a  view  of  studying  the  process  as  it 
takes  place  in  them.  The  results  obtained  in  these  instances  in  many 
respects  corroborate  those  obtained  by  Dr.  Beaumont,  though  many 
new  facts,  unobserved  by  him,  have  been  brought  to  light. 

Gastric  Juice. — The  gastric  juice  obtained  from  the  human 
stomach  free  from  mucus  and  other  impurities  is  a  clear,  colorless 
fluid  with  a  constant  acid  reaction,  a  slightly  saline  and  acid  taste, 
and  a  specific  gravity  varying  from  1.002  to  1.005.  The  juice  ob- 
tained from  the  dog's  stomach  possesses  essentially  the  same  char- 
acteristics, though  its  acidity  as  well  as  its  specific  gravity  are  shghtly 
greater.  When  kept  from  atmospheric  influences,  it  resists  putre- 
factive change  for  a  long  period  of  time,  undergoes  no  apparent 
change  in  composition,  and  loses  none  of  its  digestive  power.  It 
will  also  prevent  and  even  arrest  putrefactive  change  in  organic 
matter.  The  chemic  composition  of  the  gastric  juice  has  never 
been  satisfactorily  determined,  owung  to  the  fact  that  the  secretion 


i86  TEXT-BOOK  OF  PHYSIOLOGY. 

as  obtained  from  fistulous  openings  has  not  been  absolutely  normal. 
The  following  analyses  represent  the  composition  of  a  sample 
obtained  by  Schmidt  from  the  stomach  of  a  woman  who  had  a  fistula, 
but  who  was  nevertheless  in  good  health;  also  the  composition  of  the 
juice  from  a  dog: 

COMPOSITION  OF  GASTRIC  JUICE. 

Human.  Dog. 

Water, 994.40  973.06 

Organic  matter, 3.19  ^7-^3 

Hydrochloric  acid, 0.20?  3.34 

Calcium  chlorid, 0.06  0.26 

Sodium  chlorid, 1.46  2.50 

Potassium  chlorid, 0.55  1.12 

Calcium  phosphate  "|  1.73 

Magnesium     "           > 0.12  0.23 

Ferric               "           J  0.08 

Ammonium   chlorid, 0.47 

The  organic  matter  present  in  gastric  juice  is  a  mixture  of  mucin 
and  a  proteid,  products  of  the  metabolic  activity  of  the  epithehal 
cells  on  the  surface  of  the  mucous  membrane  and  of  the  chief  or 
central  cells  of  the  gastric  glands  respectively.  Associated  with  the 
proteid  material  are  two  ferment  or  enzyme  bodies,  termed  pepsin 
and  rennin.  As  is  the  case  with  other  enzymes,  their  true  chemic 
nature  is  practically  unknown. 

Pepsin,  though  present  in  gastric  juice,  is  not  present  as  such  in 
the  chief  cells  of  the  glands,  but  is  derived  from  a  zymogen,  pro- 
pepsin or  pepsinogen,  when  the  latter  is  treated  with  hydrochloric 
acid.  This  antecedent  compound  is  related  to  the  granules  ob- 
served in  and  produced  by  the  cell  protoplasm  during  the  period 
of  rest.  Though  pepsin  is  largely  produced  by  the  central  cells  of 
the  fundic  glands,  it  is  also  produced,  though  in  less  amount,  by 
the  cells  of  the  pyloric  glands.  Pepsin  is  the  chief  proteolytic  agent 
of  the  gastric  juice  and  exerts  its  influence  most  energetically  in  the 
presence  of  hydrochloric  acid  and  at  a  temperature  of  about  40°  C. 
Other  acids — e.  g.,  phosphoric,  nitric,  lactic,  etc.- — are  also  capable 
of  exciting  it  to  activity,  though  with  less  intensity. 

Rennin  or  pexin  is  present  in  the  gastric  juice  not  only  of  man  and 
all  the  mammaha,  but  also  of  birds  and  even  fish.  In  its  origin  from  a 
zymogen  substance,  in  its  relation  to  an  acid  medium  and  an  optimum 
temperature,  it  bears  a  close  resemblance  to  pepsin.  Its  specific 
action  is  the  curdling  of  milk,  a  condition  due  to  the  coagulation  of 
caseinogen. 

Hydrochloric  acid  is  the  agent  which  gives  to  the  gastric  juice  its 
normal  acidity.  Though  the  juice  frequently  contains  lactic,  acetic, 
and  even  phosphoric  acids,  it  is  generally  believed  that  they  are  the 
result  of  fermentation  changes  occurring  in  the  food,  the  result  of 


DIGESTION.  187 

bacterial  action.  The  percentage  of  hydrochloric  acid  has  been  the 
subject  of  much  discussion.  The  analysis  of  human  gastric  juice 
made  by  Schmidt  shows  a  percentage  of  0.02,  while  that  of  the  dog 
is  0.34.  It  is  probable,  however,  that  the  low  percentage  of  HCl  in 
human  gastric  juice  was  due  to  the  admixture  with  saliva.  At 
present  it  is  beheved  from  analyses  made  for  clinical  purposes  that 
the  acid  is  present  to  the  extent  of  at  least  0.2  per  cent.  This  degree 
of  acidity  is  not  constant  during  the  entire  process  of  digestion.  In 
the  earlier  as  well  as  in  the  later  stages  it  is  much  less. 

The  immediate  origin  of  the  hydrochloric  acid  is  difficult  of  ex- 
planation. That  it  is  derived,  however,  primarily  from  the  chlorids 
of  the  food  and  secondarily  from  the  blood-plasma  has  been  estab- 
lished by  direct  experiment.  If  all  the  chlorids  be  removed  from 
the  food  and  all  chlorids  be  withdrawn  from  the  animal  tissue  by  the 
administration  of  various  diuretics, — e.  g.,  potassium  nitrate, — there 
will  be  a  total  disappearance  of  hydrochloric  acid  from  the  stomach. 
On  the  addition  of  sodium  or  potassium  chlorids  to  the  food,  there  is 
at  once  a  reappearance  of  the  acid. 

As  to  the  nature  of  the  process  by  which  the  acid  is  formed,  noth- 
ing definite  is  known.  Various  theories  of  a  chemic  and  physical 
character  have  been  offered,  all  of  which  are  more  or  less  unsatis- 
factory. As  no  hydrochloric  acid  is  found  either  in  the  blood  or 
lymph,  the  most  plausible  view  as  to  its  origin  is  that  which  regards 
it  as  one  of  the  products  of  the  metaboHsm  of  the  gland-cells,  and 
more  particularly  of  the  parietal  or  border  cells,  and  which  for  this 
reason  have  been  termed  acid-producing  or  oxyntic  cells.  From  the 
chlorids  furnished  by  the  blood  the  chlorin  is  derived,  which,  uniting 
with  hydrogen,  forms  the  HCl.  The  base  set  free  returns  to  the  blood, 
which  in  part  accounts  for  its  increased  alkahnity  during  digestion 
as  well  as  the  diminished  acidity  of  the  urine.  The  acid  thus  formed 
passes  through  the  canalicuh,  which  penetrate  and  surround  the  cells, 
into  the  lumen  of  the  gland. 

Hydrochloric  acid  exerts  its  influence  in  a  variety  of  ways.  It  is  the 
main  agent  in  the  derivation  of  pepsin  and  rennin  or  pexin  from  their 
antecedent  zymogen  compounds,  pepsinogen  and  pexinogen  (Warren) ; 
it  imparts  activity  to  these  ferments ;  it  prevents  and  even  arrests  fer- 
mentative and  putrefactive  changes  in  the  food  by  destroying  micro- 
organisms; it  softens  connective  tissue,  it  dissolves  proteids  and  acid- 
ifies the  proteids,  thus  making  possible  the  subsequent  action  of  pepsin. 

The  inorganic  salts  of  the  gastric  juice  are  probably  only  inci- 
dental and  play  no  part  in  the  digestive  process. 

Mode  of  Secretion. — The  observations  of  Dr.  Beaumont  and 
the  experiments  of  many  physiologists  have  made  it  certain  that  the 
secretion  of  the  gastric  juice  is  intermittent  and  not  continuous,  that 
it  is  only  on  the  introduction  and  digestion  of  the  food  that  the  normal 


i88  TEXT-BOOK  OF  PHYSIOLOGY. 

amount  is  poured  out.  During  the  intervals  of  digestive  activity  the 
stomach  is  practically  free  from  all  traces  of  the  juice.  The  mucous 
membrane  is  pale  and  covered  with  a  layer  of  mucus  having  an  alka- 
line or  neutral  reaction.  The  introduction,  however,  of  small  por- 
tions of  food  or  irritation  with  a  glass  rod  causes  a  change  in  the 
appearance  of  the  mucous  membrane.  At  the  points  of  irritation 
the  membrane  becomes  red  and  vascular  and  in  a  few  minutes  small 
drops  of  the  secretion  make  their  appearance;  these  coalesce  and 
run  down  the  sides  of  the  stomach.  The  chemic  reaction  then 
changes  from  alkalinity  to  acidity. 

Though  the  secretion  of  the  gastric  juice  can  be  excited  by  these 
artificial  means,  the  amount  secreted,  owing  to  the  local  character  of 
the  stimulation,  is  but  slight  compared  with  the  quantity  secreted 
when  the  natural  stimulus — well-masticated  food  saturated  with 
alkaline  saliva — passes  into  the  stomach.  Under  such  circum- 
stances, the  stimulus  being  general,  the  blood-vessels  dilate,  the 
mucous  membrane  becomes  uniformly  red,  and  in  a  short  time  the 
secretion  makes  its  appearance. 

From  experimental  investigations  there  is  reason  to  believe  that 
the  physical  contact  of  the  food  with  the  mucous  membrane  is  not 
sufficient  to  maintain  a  continuous  secretion,  and  that  other  factors 
must  be  invoked.  For  it  is  not  until  digestion  is  well  under  way  that 
the  juice  is  secreted  in  normal  and  necessary  quantity.  Attempts 
have  been  made  to  determine  the  relative  degree  of  influence  of 
different  articles  of  food  on  the  rate  of  secretion.  Of  all  substances 
capable  of  increasing  the  flow  none  are  so  efficient  as  peptones,  their 
introduction  into  the  stomach  being  followed  by  a  copious  secretion. 
For  this  reason  it  has  been  asserted  that  after  the  primary  physical 
stimulation  of  the  food  there  is  a  secondary  chemic  stimulation  by 
peptones,  the  result  of  the  digestive  process.  As  to  whether  they  are 
absorbed  by  the  mucous  membrane  and  directly  stimulate  the  gland- 
cells,  or  whether  they  act  as  chemic  stimuli  to  afferent  nerves  in  the 
mucosa,  nothing  definite  can  be  stated. 

Histologic  Changes  in  the  Gastric  Cells  during  Secretion. — 
During  the  periods  of  rest  and  secretory  activity  the  cells  of  the 
gastric  glands  undergo  changes  in  histologic  structure  which  are 
believed  to  be  connected  with  the  production  of  the  ferments  and 
acid.  In  the  resting  period  the  protoplasm  of  the  chief  or  central 
cells  of  the  fundus  glands  becomes  crowded  with  large  and  well- 
defined  granules,  which  during  the  period  of  secretory  activity  largely 
disappear,  so  much  so,  that  only  the  luminal  border  of  the  cell  is 
occupied  by  them,  the  outer  border  being  clear  and  hyaline  in  appear- 
ance. The  parietal  cells  during  rest  are  large  and  finely  granular, 
but  after  secretion  they  are  smaller  in  size  though  still  granular. 
(See  Fig.  76,  A  and  B.) 


DIGESTION.  189 

The  cells  of  the  pyloric  glands,  though  containing  granules,  do  not 
show  any  marked  difference  between  the  resting  and  active  condition. 
According  to  some  observers,  they  contain  pepsinogen;  according  to 
others,  mucin.  The  epithelial  cells  lining  the  ducts  of  the  pylorus 
and  fundus  glands,  if  not  identical  with  the  epithehal  cells  on  the  sur- 
face of  the  mucous  membrane,  pass  by  transitional  forms  into  them. 
Among  these  cells  are  found  many  goblet  cells  which  secrete  a  portion 
of  the  mucus  found  in  the  stomach  and  gastric  juice.  In  the  period 
of  rest  the  protoplasm  of  the  epithelial  cells  absorbs  and  assimilates 
from  the  surrounding  lymph-spaces  material  which  eventually  makes 
its  reappearance  as  a  product  of  metabolism  in  the  form  of  granules 


br 


a, 9 


.„    ..C 


b 


nW. 


B 


Fig.  76. — Sections  of  Deep  Ends  of  Fundus  Glands  of  the  Cat  in  Different 
Secretfve  Phases.  X  1000. — (Bensley.)  A.  From  a  fasting  stomach.  The 
chief  cells  are  filled  with  large  zymogen  granules;  nuclei  near  the  outer  ends  of 
ceils.  Gentian-violet  preparation,  b  b  b.  Border  cells.  B.  Six  hours  after  an 
abundant  meal  of  raw  flesh.  The  chief  cells  exhibit  two  zones,  the  inner  occupied 
by  large  zymogen  granules,  the  outer  by  a  deeply  staining,  obscurely  fibrillar 
element,  prozymogen;  the  nuclei  lie  at  the  junction  of  the  two  zones,  b  bb.  Border 
cells,  pr.  Prozymogen.  c.  Mucin-secreting  cell,  similar  to  those  found  in  the 
neck  of  the  gland.     Gentian-violet  preparation. — {Hemmeter  after  Bensley.) 


and  hydrochloric  acid.  With  the  onset  of  digestive  activity  there  is  a 
dilatation  of  the  blood-vessels,  an  increase  in  the  blood-supply,  a 
stimulation  through  the  nerve-supply  of  the  cells,  and  an  output  of  a 
fluid  to  which  the  name  gastric  juice  is  given. 

Influence  of  the  Nerve  System. — The  secretion  of  gastric 
juice  is  largely  a  reflex  act  and  under  the  control  and  influence  of  the 
central  nerve  system.  Though  the  mechanism  involved  is  ob- 
scure, it  has  frequently  been  observed  that  the  sight  of  food  or  the 
chewing  of  food  without  its  passage  into  the  stomach  is  attended  by  a 
dilatation  of  the  blood-vessels  and  a  copious  flow  of  gastric  juice 


I90  TEXT-BOOK  OF  PHYSIOLOGY. 

within  a  few  minutes,  showing  the  cooperation  of  vaso-motor  and 
secretor  nerve  influences,  a  result  similar  to  that  which  occurs  when 
the  food  comes  into  contact  with  the  mucous  membrane  itself.  It 
was  also  observed  by  Dr.  Beaumont  that  mental  emotions,  such  as 
fear  and  anger,  will  arrest  the  normal  secretion. 

Many  attempts  have  been  made  with  var}dng  degrees  of  success 
to  determine  the  paths  of  the  efferent  impulses  to  the  glands.  As  the 
vagus  is  the  only  cranial  nerve  connecting  the  stomach  with  the 
central  nerve  system,  it  has  been  the  subject  of  much  experimenta- 
tion. The  results  obtained,  however,  have  not  been  uniform.  The 
recent  investigations  of  Pawlow  are  most  reliable.  He  found  that 
division  of  both  vagi  was  followed  by  a  loss  of  reflex  action.  Stimu- 
lation of  the  peripheral  ends  with  induction  shocks,  one  per  second, 
after  a  latent  period  of  about  seven  minutes,  caused  a  flow  of  gastric 
juice. 

The  Physiologic  Action  of  Gastric  Juice. — In  the  study  of  the 
physiology  of  gastric  digestion  as  it  takes  place  under  normal  con- 
ditions it  is  important  to  bear  in  mind  that  the  foods  introduced  into 
the  stomach  are  heterogeneous  compounds  consisting  of  both  nutritive 
and  non  nutritive  materials,  and  that  before  the  former  can  be  digested 
and  utilized  for  nutritive  purposes  they  must  be  freed  from  their 
combinations  with  the  latter.  This  is  accomphshed  by  the  solvent 
action  of  the  gastric  juice,  which  in  virtue  of  the  chemic  activity  of  its 
constituents  on  proteids,  gradually  disintegrates  the  food  and  reduces 
it  to  the  liquid  or  semi-liquid  condition. 

The  nature  of  this  change  and  the  respective  influence  which  the 
acid  and  pepsin  exert  can  be  studied  w'ith  almost  any  form  of  proteid. 
The  most  suitable  form,  however,  is  coagulated  fibrin  obtained  from 
blood  by  wdiipping  and  thoroughly  freed  from  blood  by  washing 
under  a  stream  of  water.  The  chemic  features  of  proteids,  as  well  as 
the  typical  forms  contained  in  the  different  articles  of  food,  have 
been  considered  in  connection  with  the  chemic  composition  of  the  body 
and  the  composition  of  foods  (see  pages  31  and  136).  For  purposes  of 
experimentation  artificial  gastric  juice  may  be  employed.  This  is  as 
effective  as  the  normal  secretion  and  in  no  essential  respect  differs 
from  it.  A  glycerin  extract  of  the  mucous  membrane  acidulated 
with  0.2  per  cent,  hydrochloric  acid  is  probably  the  best. 

If  small  pieces  of  fibrin  be  suspended  in  clear  gastric  juice  and 
kept  at  a  temperature  of  104°  F.  (40°  C.)  for  an  hour  or  two,  they  will 
be  dissolved  and  will  entirely  disappear,  giving  rise  to  a  slightly 
opalescent  mixture.  In  the  early  stages  of  the  process  the  fibrin  be- 
comes swollen  and  transparent  and  partly  dissolved.  If  at  this  time 
the  solution  be  carefully  neutralized,  the  dissolved  portion  can  be 
regained  in  the  form  of  acid-albumin  or  syntonin — a  fact  which  in- 
dicates that  the  first  effect  of  the  gastric  juice  is  the  acidification  of  the 


DIGESTION.  191 

proteids.  This  having  been  accompUshed,  the  pepsin  becomes  opera- 
tive, and  in  a  varying  length  of  time  transforms  the  acid-albumin  into 
a  new  form  of  proteid,  termed  peptone.  This  form  of  proteid  differs 
from  all  other  forms  of  proteid  in  being  soluble  in  both  acids  and 
alkalies  and  non-coagulable  by  heat.  In  the  transformation  of  acid- 
albumin  into  peptone  it  is  possible  to  isolate  by  the  addition  of 
magnesium  sulphate  and  ammonium  sulphate  intermediate  bodies 
to  which  the  term  alhumoses  or  proteoses  has  been  given,  and  which 
differ  somewhat  in  their  solubility.  The  proteoses  are  termed,  from 
the  order  in  which  they  make  their  appearance,  primary  and  second- 
ary. The  primar}^  proteoses  are  precipitated  by  magnesium  sulphate, 
the  secondar}^  by  ammonium  sulphate.  As  some  of  the  primar}- 
proteoses  are  soluble  in  water  while  others  require  in  addition  so- 
dium chlorid  for  their  solution,  they  have  been  divided  into  two  groups 
— -viz.:  proto-  and  hetero-albumoses.  The  secondary  proteoses  or 
deutero-albumoses  are  soluble  in  water.  Though  in  the  subjoined 
scheme  two  forms  of  dcutero-albumose  are  represented  and  two  forms 
of  peptone  developed  out  of  them,  the  results  of  chemic  investiga- 
tion would  indicate  that  there  is  but  one  form  of  deutero-albumose 
and  hence  but  one  form  of  peptone.  This  supposed  change  pro- 
duced by  gastric  juice  is  represented  by  the  following  scheme: 

Albumin 
Acid-albumin 

Proto-albumose  =   ( -n     .       '        =  Hetero-albumose 
■  Proteoses ' 

Deutero-albumose    (  'ti     <.         '  I  =  Deutero-albumose 
Proteoses  ' 

Peptone         (Ampho-peptones)  Peptone. 

From  the  fact  that  when  peptones  are  subjected  to  the  prolonged 
action  of  pancreatic  juice  there  arise  compounds  such  as  leucin, 
tyrosin,  aspartic  acid,  arginin,  etc.,  it  was  believed  that  two  kind 
of  peptones  were  formed  out  of  a  simple  proteid  one  of  which  suc- 
cumbed to  the  destructive  action  of  pancreatic  juice,  while  the  other 
resisted  it;  for  this  reason  the  latter  was  termed  anti-  and  the  former 
hemi-peptone.  The  two  were  included  under  the  term  ampho-pep- 
tone.  It  is  generally  admitted  now,  however,  that  the  body  termed 
anti-peptone  is  not  a  peptone  at  all,  but  a  compound  termed  carnic 
acid  and  which  is  also  separable  into  leucin,  tyrosin,  etc.  Hemi- 
peptone  has  never  been  isolated.  The  probabilities  are,  therefore, 
that  but  one  form  of  peptone  is  developed  from  any  given  simple  pro- 
teid. 


192  TEXT-BOOK  OF  PHYSIOLOGY. 

Nearly  all  forms  of  proteid  are  in  a  similar  manner  transformed 
into  peptones  by  gastric  juice.  Beyond  this  stage,  however,  there 
does  not  seem  to  be  any  further  change,  peptones  apparently  being 
the  final  products  of  gastric  digestion.  The  intimate  nature  of  this 
change  is  practically  unknown,  but  there  are  reasons  for  thinking 
that  it  is  a  process  of  hydration,  attended  by  cleavage,  with  increasing 
solubility  of  the  resulting  products. 

Characters  0}  Peptones. — The  peptones  resulting  from  the  diges- 
tion of  different  proteids,  though  resembling  each  other  in  many  re- 
spects, yet  possess  different  chemic  characteristics,  as  shown  by  their 
reaction  to  various  chemic  reagents.  Though  having  some  resem- 
blance to  the  proteids  from  which  they  are  derived,  they  are  to  be 
distinguished  from  them  by  the  following  general  characteristics: 

1.  They  are  not  coagulable  either  by  heat  or  by  nitric  acid. 

2.  They  are  soluble  in  water,  either  hot  or  cold,  and  in  acid  and 

alkahne  solutions. 

3.  They  are  diffusible,   passing  through  animal  membranes  with 

great  rapidity.  It  has  been  demonstrated  that  peptones  diffuse 
about  twelve  times  as  rapidly  as  the  proteids  from  which  they 
are  derived. 

Neither  on  fat  nor  starch  has  gastric  juice  any  appreciable  effect, 
and  when  these  substances  are  introduced  into  the  stomach  they 
pass  into  the  intestine  unchanged.  It  has  been  stated,  however,  that 
the  gastric  mucosa  produces  a  fat-splitting  enzyme,  but  that  its 
action  is  prevented  by  the  presence  of  the  hydrochloric  acid.  Not- 
withstanding the  fact  that  dilute  solutions  of  hydrochloric  acid  (0.3 
per  cent.)  will  promptly  invert  cane-sugar  into  dextrose  and  lewilose, 
and  that  gastric  juice  will  accomphsh  the  same  result  in  test-tubes, 
there  is  no  strong  evidence  for  the  behef  that  the  inversion  of  cane- 
sugar  takes  place  to  any  marked  extent  in  the  stomach  under 
normal  conditions.  The  starch,  however,  which  has  been  subjected 
to  the  action  of  the  saliva  still  continues  to  be  converted  into  maltose 
during  the  first  fifteen  to  thirty  minutes  or  e\'en  longer.  Even  though 
gastric  juice  is  being  secreted  and  though  hydrochloric  acid  solutions 
with  a  strength  of  0.3  per  cent,  will  arrest  the  action  of  ptyalin, 
starch  digestion  continues  for  the  reason  that  the  acid  combines  with 
the  proteids  and  is  thus  rendered  inoperative  and  for  the  further 
reason  that  the  food  is  largely  retained  in  the  extreme  cardiac  end 
of  the  stomach  where  the  gastric  juice  is  not  abundant.  After  the 
above-mentioned  period,  free  acid  makes  its  appearance  when 
saUvary  digestion  ceases. 

Action  of  Gastric  Juice  on  Foods. — The  action  of  gastric 
juice  on  proteids  affords  a  key  to  its  influence  in  the  reduction  of 
foods  to  the  liquid  or  semi-Hquid  condition.  It  is  evident  that  it  will 
be  most  active  in  the  digestion  of  food  consisting  largely  of  proteid 


DIGESTION.  193 

materials,  such  as  meat,  eggs,  milk,  etc.  Meat  is  disintegrated  first 
by  the  conversion  of  the  proteids  of  the  connective  tissue,  which  have 
been  more  or  less  gelatinized  by  cooking,  into  peptones.  The 
sarcolemma  of  the  muscle-fibers  which  have  been  thus  separated 
is  in  a  similar  manner  attacked  and  converted  into  peptones.  The 
true  muscle  or  sarcous  substance,  consisting  largely  of  myosin,  un- 
dergoes a  corresponding  change.  If  the  quantity  of  meat  be  not  too 
large  and  the  gastric  juice  be  secreted  in  proper  amount,  it  is  possible 
that  all  the  meat  will  be  digested  in  the  stomach.  It  is  quite  probable, 
however,  that  this  is  not  the  case  and  that  a  portion  of  the  semi- 
digested  meat  passes  into  the  intestine,  where  its  final  solution  is 
effected. 

The  white  of  egg,  especially  when  slightly  boiled,  is  much  more 
readily  digested  than  when  raw  or  firmly  coagulated  by  prolonged 
boihng.  In  either  condition,  however,  the  connective  tissue  is  dis- 
solved and  peptonized,  after  which  the  native  albumin  undergoes  the 
same  change.  The  yolk  of  the  egg  consists  largely  of  fat  held  in  sus- 
pension by  a  proteid  substance,  vitellin,  which  is  also  capable  of 
transformation  into  peptone. 

Adipose  tissue  is  similarly  reduced.  The  proteids  of  the  con- 
nective tissue  and  of  the  fat  vesicles  are  dissolved  and  peptonized 
and  the  fat-drops  set  free. 

Milk  undergoes  a  peculiar  change  in  composition  before  its 
proteid  constituents  can  be  transformed  into  peptones.  The  case- 
inogen  in  the  presence  of  calcium  salts  is  always  in  the  soluble 
state.  When  acted  on  by  the  gastric  juice,  the  caseinogen  under- 
goes coagulation  which  consists  in  the  formation  of  a  solid  com- 
pound, casein,  and  a  soluble  albumin.  This  change  is  due  to  the 
presence  and  activity  of  the  ferment,  rennin.  The  necessity  for  this 
change,  how^ever,  is  not  apparent.  The  coagulated  casein  presents 
itself  in  the  form  of  a  flocculent  curd,  which  is  finer  in  human  than 
in  cow's  milk,  and  hence  more  easily  digestible.  The  casein  is 
acidified  by  the  hydrochloric  acid  and  then  converted  by  the  pepsin 
into  peptone. 

Vegetables,  though  consisting  of  a  woody  or  cellulose  framework, 
undergo  a  partial  disintegration  in  the  stomach.  When  boiled  and 
physically  disintegrated  by  the  teeth,  the  gastric  juice  is  enabled  to 
penetrate  the  framework  and  dissolve  and  peptonize  the  various 
proteid  constituents.  As  a  general  rule,  the  vegetable  proteids  are 
more  difficult  of  digestion  than  the  animal  proteids. 

Duration  of  Gastric  Digestion. — The  length  of  time  the  food 
remains  in  the  stomach  and  the  relative  digestibihty  of  different 
articles  of  food  were  carefully  studied  by  Dr.  Beaumont  on  St.  Martin, 
and  though  the  results  obtained  by  him  may  not  be  absolutely  correct, 
viewed  in  the  light  of  recent  knowledge  of  the  digestive  process,  yet 


194  TEXT-BOOK  OF  PHYSIOLOGY. 

in  the  main  they  have  been  corroborated  in  various  ways.  As  a 
result  of  many  observations  Dr.  Beaumont  came  to  the  conclusion 
that  the  average  length  of  time  an  ordinary  meal  consisting  of  meat, 
bread,  potatoes,  etc.,  remained  in  the  stomach  undergoing  digestion 
was  about  three  and  a  half  hours,  the  duration  of  the  process,  how- 
ever, being  increased  when  an  excessive  quantity  of  food  was  taken  or 
the  quantity  and  quaUty  of  the  gastric  juice  impaired  by  abnormal 
conditions  of  the  system.  As  soon  as  the  food  is  liquefied  by  the 
gastric  juice  that  portion  not  absorbed  by  the  gastric  vessels  passes 
into  the  intestines,  this  continuing  for  two  to  three  hours  until  the 
stomach  is  completely  emptied.  The  relative  digestibihty  of  the  dif- 
ferent foods  was  also  made  the  subject  of  many  experiments  by  Dr. 
Beaumont.  After  repeating  and  verifying  his  observations  made 
under  varying  conditions,  he  summed  up  his  results  in  a  table,  of 
which  the  following  is  an  abstract,  in  which  the  mode  of  preparation 
and  the  time  required  for  the  digestion  of  different  foods  are  exhibited : 

TABLE  SHOWING  DIGESTIBILITY  OF  VARIOUS  ARTICLES  OF  FOOD 

Hours.  Minutes. 

Eggs,  whipped, i  20 

"     soft  boiled, 3 

"      hard  boiled, 3  30 

Oysters,  raw,    2  55 

"        stewed, 3  30 

Lamb,  broiled, 2  30 

Veal,  broiled, 4 

Pork,  roasted, 5  15 

Beefsteak,  broiled, 3 

Turkey,  roasted, 2  25 

Chicken,  boiled, 4  __ 

"         fricasseed, 2  45 

Duck,  roasted, 4 

Soup,  barley,  boiled, 1  30 

"      bean,         "      3 

"      chicken,    "      3 

"      mutton,     "      3  30 

Liver,  beef,  broiled, 2 

Sausage,           "          3  20 

Green  corn,  boiled,   3  45 

Beans,             "          2  30 

Potatoes,  roasted, 2  30 

"        boiled, 3  30 

Cabbage,      "      4  30 

Turnips,       "      3  30 

Beets,            "      3  45 

Parsnips,       "       2  30 

Movements  of  the  Stomach. — During  the  period  of  gastric 
digestion  the  muscle  walls  of  the  stomach  become  the  seat  of  a 
series  of  movements,  peristaltic  in  character,  which  not  only  incor- 
porate the  gastric  juice  with  the  food,  but  also  serve  to  eject  the 
liquefied  portions  of  the  food  into  the  small  intestine. 


DIGESTION.  195 

The  movements  of  the  human  stomach  as  described  by  Beau- 
mont, as  well  as  the  movements  of  the  dog's  stomach  as  stated  by 
different  observers,  are  not  in  agreement  in  all  respects,  and  are, 
moreover,  open  to  question  for  the  reason  that  they  were  not  ob- 
served under  strictly  physiologic  conditions.  The  more  recent 
investigations  of  Cannon  have  thrown  new  light  on  this  subject. 
By  means  of  the  Rontgen  rays  he  has  been  enabled  to  study  the 
movements  in  the  living  animal  and  under  normal  conditions. 
The  animal  (the  cat)  was  fed  with  bread  and  milk,  to  which  was 
added  subnitrate  of  bismuth.  This  substance,  being  opaque, 
rendered  the  movements  of  the  stomach  walls  visible  on  the 
fluorescent  screen.  With  paper  placed  over  the  screen  it  was  pos- 
sible to  sketch  the  changes  in  shape  that  the  stomach  undergoes 
at  different  periods  of  the  digestive  act.  Some  of  these  changes  are 
represented  in  Fig.  77.  The  anatomic  features  of  the  cat  stomach 
of  interest  in  this  connection  are  represented  in  Fig.  78. 

These  investigations  show  that  different  portions  of  the  stomach 
walls  exhibit  different  forms  of  activity,  which  for  convenience  of 
description  are  separately  described  by  Cannon  as  follows: 

I.  The  Movements  0}  the  Pyloric  Part. — Within  five  minutes  after 
a  cat  has  finished  a  meal  of  bread  there  is  visible  near  the  du- 
odenal end  of  the  antrum  a  slight  annular  contraction  which  moves 
peristaltically  to  the  pylorus;  this  is  followed  by  several  waves  re- 
curring at  regular  intervals.  Two  or  three  minutes  after  the  first 
movement  is  seen,  very  slight  constrictions  appear  near  the  middle  of 
the  stomach,  and,  pressing  deeper  into  the  greater  curvature,  course 
slowly  tovv'ard  the  pyloric  end.  As  new  regions  enter  into  constric- 
tion, the  fibers  just  previously  contracted  become  relaxed,  so  that 
there  is  a  true  moving  wave,  with  a  trough  between  two  crests. 
When  a  wave  swings  round  the  bend  in  the  pyloric  part,  the  indenta- 
tion made  by  it  deepens;  and  as  digestion  goes  on  the  antrum 
elongates  and  the  constrictions  running  over  it  grow  stronger,  but,  until 
the  stomach  is  nearly  empty,  they  do  not  entirely  divide  the  cavity. 
After  the  antrum  has  lengthened,  a  wave  takes  about  thirty-six  sec- 
onds to  move  from  the  middle  of  the  stomach  to  the  pylorus.  At  all 
periods  of  digestion  the  waves  recur  at  intervals  of  almost  exactly  ten 
seconds.  It  results  from  this  rhythm  that  when  one  wave  is  just 
beginning  several  others  are  already  running  in  order  before  it. 
Between  the  rings  of  constriction  the  stomach  is  bulged  out,  as  shown 
in  the  various  outlines  in  Fig.  77. 

Movements  of  the  Pyloric  Sphincter, — During  the  first  ten  or 
fifteen  minutes  after  the  first  constriction  of  the  antrum  the  pylorus 
is  tightly  closed.  After  this  period  it  opens  at  irregular  intervals  to 
permit  the  passage  of  liquefied  food  which  is  ejected  by  peristaltic 
waves  for  a  distance  of  two  or  three  centimeters  into  the  duodenum. 


196 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  frequency  with  which  the  pylorus  opens  depends  apparently  on 
the  degree  to  which  the  food  is  softened.  When  the  food  is  hard, 
the  pylorus  closes  more  tightly  and  remains  closed  a  longer  period 

than  when  it  is  soft. 

The  Activity  0}  the  Cardiac 
Portion. — As  digestion  proceeds, 
the  pre-antral  part  of  the  stomach 
elongates  and  assumes  the  shape 
of  a  tube,  which  becomes  the  seat 
also  of  peristaltic  constriction  waves. 
As  a  result,  some  of  the  food  is 
gradually  forced  into  the  antrum  to 
succeed  that  which  has  been  prepar- 
ed and  ejected  into  the  duodenum. 
As  the  pre-antral  tube  is  emptied  of 
its  contents  the  longitudinal  and 
circular  fibers  of  the  fundus  stead- 
ily contract  and  gradually  force  its 
contents  into  the  tubular  portion. 
This  continues  until  the  fundus  is 
completely  emptied.     The  changes 


Left 


Fig.  77. — Shadow  Sketches 
OF  THE  Outlines  of 
THE  Stomach  of  a  Cat 
Immediately  after  a 
Meal  (ii.o),  and  at 
Various  Intervals 
Afterward  (at  12.0,  at 
2.0,  3.30,  4.30).— (PF.  B. 
Cannon.) 


Post 


Fig.  78. — The  cardiac  portion  is  all  that 
part  to  the  left,  as  the  stomach  Ues  in  the 
body,  of  WX.  The  cardia  is  at  C  The 
pylorus  is  at  P,  and  the  pyloric  portion 
is  the  part  between  P  and  WX.  This 
has  two  divisions:  the  antrum,  between 
P  and  YZ,  and  the  pre-antral  part,  be- 
tween WX  and  YZ.  The  lesser  curva- 
ture is  on  the  top  of  the  outline  between 
C  and  P,  and  the  greater  curvature  be- 
tween the  same  points  along  the  lower 
border. — (Amer.  Jour,  of  Physiology, 
Cannon.) 


in  shape  which  the  cardiac  portion  undergoes  during  digestion  are 
represented  in  Fig.  77.  The  fundus  acts  as  a  reservoir  for  the  food 
and  forces  out  its  contents  a  httle  at  a  time  as  the  antral  mechan- 
ism   is   ready  to   receive   them.      Since  peristaltic  movements  are 


DIGESTION.  197 

absent  from  the  cardiac  portion  the  food  is  not  mixed  with  gastric 
juice,  and  therefore  sahvary  digestion  can  continue  for  a  considerable 
period.  There  is  no  evidence  of  a  circulation  of  food  in  the  stomach 
as  usually  described.  On  the  contrary,  the  movement  through  the 
pre-antral  tube  and  antrum  is  in  general  a  progressive  though  an 
oscillating  one.  As  the  constriction  waves  rapidly  pass  over  the 
food  it  is  advanced  toward  the  pyloric  opening,  but  as  this  is  closed 
the  food  is  forced  backward  through  the  advancing  constricted  ring 
for  a  variable  distance. 

The  effect  of  the  constriction  waves  is  to  mix  the  food  with  the 
gastric  juice,  triturate  and  soften  it.  So  soon  as  this  is  eft'ected,  the 
pylorus  relaxes,  when  the  advancing  constriction  \vave  expels  it  into 
the  intestine.  With  its  expulsion  room  is  afforded  for  an  additional 
quantity  of  food,  and  hence  there  is  a  general  advance  of  the  food 
mass  toward  the  pylorus. 

Though  these  observations  were  made  on  the  cat,  evidence  is 
accumulating  which  goes  to  show  that  in  human  beings  the  walls 
of  the  stomach  exhibit  constriction  waves  which  are  similar  in  all 
respects  to  those  above  described. 

The  Nerve  Mechanism  of  the  Stomach. — In  preceding  para- 
graphs it  was  stated  that  during  the  period  of  gastric  digestion  the 
food  is  retained  in  the  stomach  because  of  the  closure  of  the  cardia 
(the  esophago-gastric  orifice)  and  of  the  pylorus  (the  gastro-duode- 
nal  orifice)  both  orifices  being  tightly  closed  by  the  tonic  contraction 
of  sphincter  muscles;  that  both  sphincters  relax  from  time  to  time, 
the  one  to  permit  the  entrance  of  food  into  the  stomach  for  further 
digestion,  the  other  to  permit  the  exit  of  food  into  the  intestine  after 
its  more  or  less  complete  digestion,  after  w^hich  in  both  instances  the 
sphincters  again  contract  and  close  the  orifices;  that  the  pyloric  or 
antral  muscles  are  vigorously  active  throughout  the  digestive  period, 
triturating  the  food,  mixing  it  with  gastric  juice,  and  finalh'  driving 
it  through  the  temporarily  open  pylorus  into  the  intestine. 

These  separate  but  related  groups  of  muscle-fibers,  by  reason  of 
their  endowments,  and  possibly  by  virtue  of  the  presence  of  local 
nerve  mechanisms,  exhibit  activities  wiiich  are  independent  of  the 
central  nerve  svstem.  Thus  the  isolated  stomach  of  the  dog  and  of 
other  animals  as  well,  if  kept  warm  and  moist,  will  exhibit  rhythmic 
movements  for  a  period  of  time  varying  from  an  hour  to  an  hour  and 
a  half.  Though  nerve-cells  and  nerve-fibers  (Auerbach's  plexus) 
are  present  in  the  vvalls  of  the  stomach  between  the  layers  of  muscle- 
fibers,  it  is  not  believed  that  they  are  the  immediate  sources  of  the 
stimulus  to  the  contraction,  though  they  may  act  as  a  coordinating 
mechanism.  The  stimulus  in  all  probability  develops  in  the  muscle- 
fiber  itself  and  is  therefore  myogenic  in  origin. 

The  degree  of  acti\dty  of  both  the  sphincter  and  antral  muscles 


198  TEXT-BOOK  OF  PHYSIOLOGY. 

is  modified  by  the  central  nerve  system  cither  in  the  way  of  inhibition 
or  augmentation  and  in  response  to  gastric  stimulation.  The  nerves 
more  especially  concerned  in  the  maintenance  and  regulation  of  the 
gastric  contractions,  are  the  vagi  and  the  splanchnics.  The  afferent 
fibers  through  which  nerve  impulses  pass  to  the  nerve  centers  are  in  all 
probability  contained  in  the  trunk  of  the  vagus  nerve;  the  efjerent 
fibers  through  which  nerve  impulses  from  the  centers  reach  the 
stomach,  are  contained  partly  in  the  trunk  of  the  vagus  and  partly  in 
the  trunk  of  the  splanchnic  nerve. 

If  the  vagus  nerves  are  divided  in  the  neck,  there  is  a  loss  of 
muscle  tonus  though  the  contractions  do  not  wholly  disappear. 
Stimulation  of  the  peripheral  end  of  one  divided  vagus  is  followed  by 
an  augmentation  in  the  vigor  of  the  contraction  of  the  antral  muscles 
an  increase  in  the  tone  of  the  fundus  muscles  as  well  as  an  increase 
in  the  contraction  of  the  sphincter  pylori  and  sphincter  cardiac. 
Though  this  is  the  usual  result  there  may  be  a  primary  relaxation  or 
inhibition  of  short  duration  of  one  or  all  of  these  structures  before  the 
augmicntation  occurs.  May  states  that  this  was  alwa}"S  the  case  in 
his  experiments.  A  similar  inhibition  may  be  brought  about  reflexly 
by  stimulation  of  the  central  end  of  a  divided  vagus.  This  result 
will  not  be  produced  if  the  opposite  vagus  has  previously  been  divided. 
The  vagi,  therefore,  contain  both  inhibitor  and  augmentor  nerve- 
fibers  lor  the  gastric  musculature. 

Stimulation  of  the  peripheral  end  of  a  divided  splanchnic  is 
followed  by  an  inhibition  of  the  peristalsis  and  a  loss  of  tone.  Morat, 
however,  has  observed  a  primary  opposite  effect.  From  these  facts 
it  would  appear  that  the  gastric  muscles  receive  both  inhibitor  and 
augmentor  fibers  from  two  different  sources. 

The  excitatory  cause  for  the  activity  of  this  mechanism  is  doubtless 
connected  with  the  chemic  and  mechanic  stimulation  by  the  gastric 
contents.  The  relaxation  or  inhibition  of  the  sphincter  p}lori  ap- 
pears to  be  caused  by  the  presence  of  free  acid  at  the  pylorus:  its 
contraction,  by  the  presence  of  acids  in  the  duodenum.  Similar 
conditions  throughout  the  interior  of  the  stomach  may  be  the  cause 
of  the  cooperative  antagonism  of  these  specialized  muscle  structures. 

INTESTINAL  DIGESTION. 

The  physical  and  chemic  changes  which  the  ahmentary  principles 
undergo  in  the  small  intestine,  and  which  collectively  constitute  in- 
testinal digestion,  are  probably  more  important  and  complex  than 
those  taking  place  in  the  stomach,  for  the  food  is,  in  this  situation, 
subjected  to  the  solvent  action  of  the  pancreatic  and  intestinal  juices, 
as  well  as  to  the  action  of  the  bile,  each  of  which  exerts  a  transforming 
influence  on  one  or  more  substances  and  still  further  prepares  them 
for  absorption  into  the  blood. 


DIGESTION.  199 

To  i-ightly  appreciate  the  physiologic  actions  of  the  digestive 
juices  poured  into  the  intestine,  the  nature  of  the  partially  digested 
food  as  it  comes  from  the  stomach  must  be  kept  in  mind.  This 
consists  of  water,  inorganic  salts,  acidified  proteids,  albumoses,  pep- 
tones, starch,  maltose,  liquefied  fat,  saccharose,  lactose,  dextrose, 
cellulose,  and  the  indigestible  portion  of  meats,  cereals,  and  fruits. 
Collectively  they  are  known  as  chyme.  As  this  acidified  mass  passes 
through  the  duodenum  its  contained  acids  excite  a  reflex  secretion 
and  discharge  of  the  intestinal  fluids:  e.  g.,  pancreatic  juice,  bile, 
and  intestinal  juice.  Inasmuch  as  these  fluids  are  alkahne  in  re- 
action they  exert  a  neutrahzing  and  precipitating  influence  on  vari- 
ous constituents  of  the  chyme.  As  soon  as  this  has  taken  place, 
gastric  digestion  ceases  and  those  chemic  changes  are  inaugurated 
which  eventuate  in  the  transformation  of  all  the  remaining  undigested 
nutritive  materials  into  absorbable  and  assimilable  compounds  which 
collectivelv  constitute  intestinal  digestion. 


THE  SMALL  INTESTINE. 

The  small  intestine,  in  which  this  stage  of  digestion  takes 
place,  is  a  convoluted  tube,  measuring  about  seven  meters  in  length 
and  3.5  cm.  in  diameter,  and  extending  from  the  pyloric  orifice  of 
the  stomach  to  the  beginning  of  the  large  intestine. 

The  intestine  consists  of  four  coats:  viz.,  serous,  muscular,  sub- 
mucous, and  mucous. 

The  serous  coal  is  the  most  external  and  is  formed  by  a  reflection 
of  the  general  peritoneal  membrane.  It  is,  however,  wanting  in  the 
duodenal  portion. 

The  muscle  coat,  situated  just  beneath  the  former,  surrounds 
the  entire  intestine.  It  is  composed  of  non-striated  fibers  which  are 
more  abundant  and  better  developed  in  the  upper  than  in  the  lower 
portions  of  the  intestine.  The  muscle  coat  consists  of  two  layers 
of  fibers:  (i)  an  external  or  longitudinal,  and  (2)  an  internal  or 
circular  layer.  The  longitudinal  fibers  are  most  marked  at  that 
border  of  the  intestine  free  from  peritoneal  attachment,  though  they 
form  a  thin  layer  all  over  the  intestine.  The  circular  fibers  are  much 
more  numerous,  and  completely  encircle  the  intestine  throughout 
its  entire  extent.  It  has  been  demonstrated  that  at  the  junction  of 
the  ileum  and  colon,  and  surrounding  the  orifice,  the  ileo-colic,  common 
to  both,  the  muscle-fibers  are  arranged  in  the  form  of,  and  play  the 
part  of,  a  sphincter  muscle,  which  has  been  turned  the  ileo-colic 
sphincter.  It  is  usuallv  in  a  state  of  tonic  contraction  and  regulates 
the  passage  of  materials  from  the  small  into  the  large  intestine,  and 
possibly  also  in  the  reverse  direction  under  special  circumstances. 

The  submucous  coat  consists  of  areolar  tissue  and  serves  to  unite 


200  TEXT-BOOK  OF  PHYSIOLOGY. 

the  muscular  with  the  mucous  coat.  A  thin  layer  of  muscle-fibers, 
the  muscularis  mucosa,  is  placed  on  its  inner  surface. 

The  mucous  coat  is  soft  and  velvety  in  appearance  and  covered 
by  a  single  layer  of  columnar  epithelium.  Its  entire  surface  is  covered 
with  small  conical  projections  termed  villi.  Throughout  its  entire 
extent,  with  the  exception  of  the  lower  portion  of  the  ileum  and 
the  duodenum,  the  mucous  membrane  presents  a  series  of  transverse 
folds — the  valvulae  conniventes,  or  valves  of  Kirkring.  These  folds 
vary  from  one-fourth  to  half  an  inch  in  width  and  extend  one-half 
to  two-thirds  of  the  distance  around  the  interior  of  the  bowel. 
Each  valve  consists  of  two  layers  of  the  mucous  membrane  perman- 
ently united  by  fibrous  tissue.  It  is  beheved  that  the  valves  retard 
to  some  extent  the  passage  of  the  food  through  the  intestine  and 
present  a  greater  surface  for  absorption. 

Blood-vessels,  Nerves,  and  Lymphatics. — The  blood-vessels, 
of  the  small  intestine,  which  are  very  numerous,  are  derived  mainly 
from  the  superior  mesenteric  artery.  After  penetrating  the  intestinal 
walls  the  smaller  vessels  ramify  in  the  submucous  coat  and  send 
branches  to  the  muscle  and  mucous  coats,  supplying  all  their  struc- 
tures with  blood.  After  circulating  through  the  capillary  vessels  the 
blood  is  returned  by  small  veins,  which  subsequently  unite  to  form 
the  superior  mesenteric  vein,  which,  uniting  with  the  splenic  and  gas- 
tric veins,  forms  the  portal  vein.  The  nerves  are  derived  from  the 
lower  part  of  the  solar  plexus.  The  branches  follow  the  blood-vessels 
and  become  associated  with  two  plexuses,  one  (Auerbach's)  lying 
between  the  muscle  coats,  the  other  (Meissner's)  lying  in  the  sub- 
mucous coat.  The  lymphatics,  which  originate  in  the  mucous  and 
muscle  coats,  are  very  abundant.  They  unite  to  form  those  vessels 
seen  in  the  mesentery  and  empty  into  the  thoracic  duct. 

Intestinal  Glands. — The  gland  apparatus  of  the  intestine  by 
which  the  intestinal  juice  is  secreted  consists  of  the  duodenal  (Brun- 
ner's)  and  the  intestinal  (Lieberkiihn's)  glands. 

The  duodenal  glands  are  situated  beneath  the  mucous  membrane 
and  open  by  a  short  wide  duct  on  its  free  surface.  They  are  racemose 
glands  lined  by  nucleated  epithelium.  The  secretion  of  these  glands 
is  clear,  slightly  viscid,  and  alkaline.  Its  chemic  composition  and 
function  are  unknown. 

The  intestinal  glands  or  foUicles  are  distributed  throughout  the 
entire  mucous  membrane  in  enormous  numbers.  They  are  formed 
mainly  by  an  inversion  of  the  mucous  membrane  and  hence  open  on 
its  free  surface.  Each  tubule  consists  of  a  thin  basement  membrane 
lined  by  a  layer  of  spheric  epithelial  cells,  some  of  which  undergo 
distention  by  mucin  and  become  converted  into  mucous  or  goblet 
cells.  The  epithelial  secreting  cells  consist  of  granular  protoplasm 
containing  a  well-defined  nucleus.     The  intestinal  foUicles  constitute 


DIGESTION.  20I 

the  apparatus  which  secretes  the  chief  portion  of  the  intestinal 
juice. 

Intestinal  Juice. — Owing  to  its  admixture  with  other  secretions 
and  to  the  profound  disturbance  of  the  digestive  function,  caused  by 
the  estabhshment  of  intestinal  fistulas,  this  fluid  has  rarely  been  ob- 
tained in  a  state  of  purity  or  in  quantities  sufficient  for  accurate 
analyses  or  for  experimental  purposes.  Its  physiologic  properties 
and  functions  are  therefore  imperfectly  known.  Various  attempts 
have  been  made  by  physiologists,  by  the  employment  of  different 
methods,  to  obtain  this  secretion.  The  method  usually  employed 
is  that  of  Thiry  and  Vella.  This  consists  in  dividing  the  intestine  at 
two  places,  about  eight  or  ten  inches  apart,  restoring  the  continuity 
of  the  intestine,  and  then  uniting  the  two  ends  of  the  resected  portion 
to  the  edges  of  two  openings  in  the  abdominal  walls.  The  resected 
portion,  being  supplied  with  blood-vessels  and  nerves,  maintains  its 
nutrition  and  secretes  a  more  or  less  normal  juice. 

When  obtained  from  a  dog  under  these  circumstances  the  intes- 
tinal juice  is  watery  in  consistence,  slightly  opalescent,  light  yellow  in 
color,  alkahne  in  reaction,  with  a  specific  gravity  of  i.oio.  Chemic 
analysis  reveals  the  presence  of  proteids,  mucin,  and  sodium  car- 
bonate. 

The  intestinal  juice  obtained  by  Tubbey  and  Manning  from  a 
small  portion  of  the  human  intestine  (ileum)  was  opalescent,  occa- 
sionally brownish  in  color,  alkaline,  and  had  a  specific  gravity  of  1.006. 
On  the  addition  of  hydrochloric  acid,  carbonic  acid  was  given  off, 
showing  the  presence  of  carbonates.  It  contained  proteids  and 
mucins. 

PANCREAS. 

The  pancreas  is  a  long  flattened  gland,  situated  deep  in  the 
abdominal  cavity,  lying  just  behind  the  stomach.  It  measures  from 
six  to  eight  inches  in  length,  two  and  a  half  in  breadth,  and  one  in 
thickness.  It  is  usually  divided  into  a  head,  body,  and  tail.  The 
head  is  directed  to  the  right  side  and  is  embraced  by  the  curved 
portion  of  the  duodenum;  the  tail  is  directed  to  the  left  side  and 
extends  as  far  as  the  spleen  (Fig.  79).  The  pancreas  communicates 
with  the  intestine  by  means  of  a  duct.  This  duct  commences  at  the 
tail  and  runs  transversely  through  the  body  of  the  gland.  As  it 
approaches  the  head  of  the  gland  it  gradually  increases  in  size  until 
it  measures  about  one-tenth  of  an  inch  in  diameter.  It  then  curves 
downward  and  forward  and  opens  into  the  duodenum.  In  its  course 
through  the  gland  it  receives  branches  which  enter  it  nearly  at  right 
angles.  The  pancreas  is  richly  supplied  with  blood-vessels  and 
nerves,  the  latter  coming  from  the  solar  plexus. 


20:; 


TEXT-BOOK  OF  PHYSIOLOGY. 


Histologic  Structure. — In  its  structure  the  pancreas  resembles 
the  salivary  glands.  It  consists  oi  a  connective-tissue  framev^ork 
which  divides  the  gland  tissue  into  lobules.  Each  lobule  is  com- 
posed of  a  number  of  acini  or  alveoH,  more  or  less  elongated  or 


Tail. 


Pancreatic  ducts  Common  bile  duct 


Primary  pancreatic  duct 


Fig.  79. — Pancreas  and  Duodenum  Removed  from  the  Body  and  Seen  from 
Behind.     The  Gland  is  Cut  to  Show  the  Ducts. — {Landois  and  Stirling.) 

tubular  in  shape.  Each  acinus  gives  origin  to  a  small  duct  which, 
uniting  with  adjoining  ducts,  forms  the  lobular  duct,  which  becomes 
tributary  to  the  main  duct.  The  acinus  is  lined  by  a  layer  of  cylin- 
dric   epithehal   cells   characterized  by  a  difference  in  structure  be- 


FiG.  80. — Section  of  Hum.an  Pan- 
creas, INCLUDING  Several  Acini 
AND  Two  Ducts.  The  Cells 
Present  a  Central  Granular 
AND  A  Peripheral  Clear  Zone. 
— (Piersol.) 


Fig.  81. — Section    of    Human    Pan- 
creas   SHOWING,    a,   a.    Islands 

OF       LANGERHANS,       AND       b      THE 

Usual  Acini. — {Piersol). 


tween  the  central  and  peripheral  ends  (Fig.  80).  The  central  end, 
that  bordering  the  lumen  of  the  acinus,  is  dark  in  appearance  and 
filled  with  dark  granules,  while  the  peripheral  end  is  clear  ahd  homo- 
geneous.    The  relative  depth  of  these  two  zones  varies  according  to 


DIGESTION.  203 

the  functional  activity  of  the  gland.  During  the  intervals  of  digestion 
the  granular  layer  is  very  deep  and  occupies  almost  the  entire  cell; 
after  active  digestion  the  granular  layer  is  very  narrow,  while  the 
clear  zone  is  largely  increased  in  depth.  The  blood-vessels  of  the 
pancreas  are  arranged  around  the  acini  in  a  manner  similar  to  that 
observed  in  the  salivary  glands.  The  ultimate  terminations  of  the 
nerves  in  the  epithelium  are  probably  by  means  of  the  usual  end-tufts. 

The  Islands  of  Langerhans . — Throughout  the  body  of  the  pan- 
creas and  especially  in  the  outer  extremity  there  are  found  between 
and  among  the  acini  collections  of  globular  cells  arranged  in  the 
form  of  rods  or  columns,  separated  from  the  acini  and  from  one 
another  by  layers  of  connective  tissue  in  which  ramify  large  tortuous 
capillary  blood-vessels.  These  columnar  bodies,  seen  in  cross- 
section  in  Fig.  81,  have  been  named,  after  their  discoverer,  the 
islands  of  Langerhans. 

Embryologic  investigations  have  shown  that  these  cells  are 
outgrowths  from  the  primitive  acini,  to  which  they  remain  attached 
for  some  time  by  means  of  a  foot-stalk.  This  subsequently  becomes 
constricted  by  the  connective  tissue  and  the  cells  become  completely 
detached.  The  cells  then  assume  the  columnar  arrangement,  after 
which  vascularization  takes  place. 

From  the  fact  that  complete  extirpation  of  the  pancreas  as  well  as 
its  various  diseases  is  followed  by  serious  disturbances  of  the  carbohy- 
drate metabolism  it  has  been  suggested  that  the  islands  of  Langerhans 
have  a  function  separate  and  distinct  from  that  of  the  glandular  portion 
of  the  pancreas;  that  they  secrete  a  specific  material  which  partakes  of 
the  nature  of  an  internal  secretion  which  is  absorbed  by  the  blood 
circulating  around  them  and  carried  to  different  tissues.  The  effect 
on  the  metabolism  of  the  body  which  follows  extirpation  of  the  pan- 
creas will  be  referred  to  in  a  subsequent  chapter. 

Pancreatic  Juice. — The  pancreatic  juice  may  be  obtained  by 
introducing  a  silver  cannula,  through  an  opening  in  the  abdominal 
wall,  into  the  duct,  and  securing  it  by  a  ligature.  In  a  short  time 
the  juice  flows  from  the  distal  end  of  the  cannula,  when  it  can 
be  collected.  According  to  Bernard,  normal  juice  can  only  be  ob- 
tained during  the  first  twenty-four  hours.  The  juice  obtained  from 
a  temporary  fistula  is  clear,  slightly  opalescent,  viscid,  of  a  decidedly 
alkaline  reaction,  and  has  a  specific  gravity  in  the  dog  of  1.040. 
When  cooled  to  0°  C,  it  assumes  a  gelatinous  consistence.  At  100°  C. 
it  completely  coagulates.  When  obtained  from  a  permanent  fistula, 
the  juice  is  watery  and  the  solid  constituents  are  very  much  diminished 
in  amount. 

The  chemic  composition  of  the  pancreatic  juice  of  the  dog  as  deter- 
mined by  Schmidt  is  as  follows:  water,  goo. 76;  organic  matter,  90.44; 
inorganic  salts,  8.80.     Of  the  inorganic  salts,  sodium  carbonate  is 


204  TEXT-BOOK  OF  PHYSIOLOGY. 

probably  the  most  essential,  as  it  is  this  salt  which  gives  to  the  juice 
its  alkaline  reaction. 

Mode  of  Secretion. — The  secretion  of  the  juice  is,  in  the  rabbit 
and  dog  at  least,  almost  continuous  during  a  period  of  twenty-four 
hours  after  a  single  average  meal,  though  the  rate  of  flow  varies  con- 
siderably during  this  period.  As  soon  as  food  enters  the  stomach 
the  flow  of  the  pancreatic  juice  begins  and  steadily  increases  in 
amount  until  about  the  third  hour,  when  it  reaches  its  maximum; 
after  this  period  the  flow  diminishes  until  the  sixth  hour,  when  it 
again  increases  for  about  an  hour.  It  then  gradually  diminishes 
until  it  ceases  entirely.  During  the  period  of  secretory  activity  the 
gland  becomes  red  and  vascular  from  a  dilatation  of  the  blood-vessels. 

The  discharge  of  the  juice  associated  with  the  introduction  of 
food  into  the  stomach  is  brought  about  in  all  probability  through 
the  agency  of  the  nerve  system,  though  the  exact  mechanism  is 
imperfectly  understood.  It  is  probable  that  impressions  made  on 
the  terminal  filaments  of  the  pneumogastric  nerve  ascend  to  the 
medulla,  whence  impulses  pass  outward  through  vaso-motor  and 
secretor  nerves  to  the  blood-vessels  and  secreting  cells  of  the  glands. 
Stimulation  of  the  peripheral  end  of  the  divided  vagus  gives  rise  to 
increased  secretion.  Inasmuch  as  various  agents,  such  as  mineral 
and  organic  acids,  placed  on  the  duodenal  mucous  membrane  excite 
the  flow,  it  is  quite  probable  that  the  passage  of  the  acid  contents  of 
the  stomach  through  the  duodenum  also  acts  as  a  powerful  stimulus 
to  the  discharge  of  the  juice.  But  as  the  secretion  and  discharge 
of  the  juice  is  excited  by  the  same  conditions  after  the  division  of  all 
related  nerves,  other  explanations  were  sought  for.  Bayliss  and 
Starhng  made  the  discovery  that  the  secretory  activity  of  the  pancreas 
is  initiated  and  maintained  by  the  action  of  a  specific  substance  to 
which  they  have  given  the  term  secretin.  This  substance  is  developed 
in  the  duodenal  glands  out  of  a  precursor,  prosecretin,  in  consequence 
of  the  action  of  the  acids  in  the  chyme,  after  which  it  is  carried  by  the 
blood-stream  to  the  pancreas.  An  extract  of  the  duodenal  mucous 
membrane  made  with  hydrochloric  acid  0.4  per  cent,  and  presumably 
containing  secretin,  when  injected  into  the  blood  will  evoke  a  pro- 
fuse discharge  of  pancreatic  juice.  Hydrochloric  acid  alone  will  not 
have  this  effect.  The  total  amount  of  pancreatic  juice  secreted  in 
twenty-four  hours  has  been  only  approximately  determined;  the 
estimates  based  upon  the  amount  obtained  from  dogs  vary  from 
175  to  800  grams. 

Histologic  Changes  in  the  Cells  during  Secretory  Activity.— 
Reference  has  already  been  made  to  the  fact  that  the  cells  lining  the 
acini  consist  of  two  zones:  an  outer  one,  clear  and  homogene- 
ous; and  an  inner  one,  dark  and  granular.  The  position  of  the 
nucleus  of  the  cell  varies,  being  at    one  time  in  the  outer,  at  an- 


DIGESTION. 


205 


\  -'v 


other  time  in  the  inner,  zone.  If  the  pancreas  be  examined 
microscopically  during  the  intervals  of  digestion,  it  will  be  ob- 
served that  the  inner  zone  is  broad,  highly  granular,  occupying 
nearly  the  entire  cell,  while  the  outer  zone  is  narrow  and  clear. 
If,  however,  the  gland  be  examined  shortly  after  a  period  of  active 
secretion,  the  reverse  conditions  will  be  observed;  that  is,  the  inner 
zone  will  be  narrow,  containing  relatively  few  granules,  while  the 
outer  zone  will  be  clear  and  wide.  This  change  in  the  cell  has 
been  witnessed  in  the  pancreas  of  the  Hving  animal — rabbit — by 
Kiihne  and  Lea.  They  observed  that  as  soon  as  digestion  set  in, 
the  granules  of  the  broad  inner  zone  began  to  pass  toward  the 
lumen  of  the  acinus  and  to  gradually  disappear  as  the  secretion 
was  poured  out,  while 
the  outer  zone  in- 
creased in  width  un- 
til almost  the  entire 
cell  became  clear  and 
homogeneous.  (See 
Fig.  82.)  After  secre- 
tion ceased  the  gran- 
ules again  made  their 
appearance,  the  result, 
in  all  probability,  of 
metabolic  activity. 

Physiologic  Ac- 
tion of  Pancreatic 
Juice . — Exper  i  m  e  n  - 
tal  investigations  have 
demonstrated  the  fact 
that  pancreatic  juice 
is  the  most  complex  in 
its  physiologic  action 
of  all  the  digestive 
fluids.     In  virtue  of  its  contained  enzymes,  pancreatic  juice  acts: 

1.  On  starch.  When  normal  pancreatic  juice  or  a  glycerin 
extract  of  the  gland  is  added  to  a  solution  of  hydrated  starch,  the 
latter  is  speedily  transformed  into  maltose,  passing  through  the  inter- 
mediate stage  of  dextrin.  The  process  is  in  all  respects  similar  to 
that  observed  in  the  digestion  of  starch  by  saliva.  Pancreatic  juice, 
however,  is  more  energetic  in  this  respect  than  sahva.  The  enzyme 
which  effects  this  change  is  termed  amylopsin.  When  the  starch 
which  escapes  salivary  digestion  passes  into  the  small  intestine  and 
mingles  with  pancreatic  juice,  it  is  very  promptly  converted  into 
maltose  by  the  action  or  in  the  presence  of  this  enzyme. 

2.  On  proleid.     When  proteid  bodies  are  subjected  to  the  action 


Fig.  82. 


Fig.  83. 


One  Saccule  of  the  Pancreas  of  the  Rabbit  in 
Different  States  of  Activity.  Fig.  82. — After 
a  period  of  rest,  in  which  case  the  outlines  of  the 
cells  are  indistinct  and  the  inner  zone — i.  e.,  the 
part  of  the  cells  (a)  next  the  lumen  (c) — is  broad 
and  filled  with  fine  granules.  Fig.  83. — After 
the  gland  has  poured  out  its  secretion,  when  the 
cell  outUnes  {d)  are  clearer,  the  granular  zone  (a) 
is  smaller,  and  the  clear  outer  zone  is  wider. — 
{Yeo's  "Texl-hook  of  Physiology,"  ajter  Kiihne 
and  Lea.) 


2o6  TEXT-BOOK  OF  PHYSIOLOGY. 

of  pancreatic  juice,  they  are  transformed  into  peptones  which  do  not 
differ  in  essential  respects  from  those  formed  by  gastric  juice.  The 
intermediate  stages,  however,  are  beheved  to  be  somewhat  different. 
The  enzyme  which  effects  this  change  is  termed  trypsin. 

When  fibrin,  for  example,  is  added  to  trypsin  in  a  solution  rendered 
alkaline  by  sodium  carbonate,  it  does  not  swell  up  and  become  trans- 
lucent, as  it  does  when  treated  with  hydrochloric  acid  and  pepsin. 
On  the  contrary,  it  becomes  corroded  on  the  surface,  fragile,  and  in  a 
short  time  undergoes  solution.  The  first  product  is  a  compound 
termed  alkali-albumin.  After  solution  has  taken  place,  various  chemic 
changes  are  initiated  which  eventuate  in  the  production  of  peptone  and 
certain  nitrogenized  bodies,  leucin,  tyrosin,  aspartic  acid,  etc.  The 
intermediate  stages  in  this  process  have  not  been  satisfactorily  deter- 
mined. At  no  time  during  artificial  pancreatic  digestion  is  there  any 
evidence  of  the  presence  of  the  primar}^  proteoses  (proto-albumose 
and  hetero-albumose).  The  secondary  proteoses  (deutero-albumose) 
are  usually  present.  It  will  be  recalled  that  when  the  peptone  of 
peptic  digestion  is  subjected  to  the  action  of  trypsin  a  portion  of  it 
is  decomposed  into  leucin  and  tyrosin,  while  another  portion  pre- 
sumably is  not  so  decomposed,  for  which  reason  the  latter  was  called 
anti-  and  the  former,  Aewi-peptone.  It  is  now  believed  that  anti- 
peptone  is  not  a  peptone  at  all,  but  a  compound  termed  carnic  acid, 
which  can  be  decomposed  into  simpler  nitrogen-holding  bodies  such 
as  leucin,  tyrosin,  arginin,  etc. 

The  action  of  tr\psin  on  proteids  in  an  alkaline  medium  may  be 
illustrated  by  the  following  scheme: 

»  Proteid 

Alkali-albumin 

Deutero-proteose  or  deutero-albumose 

Peptone 


Leucin         Tyrosin      Aspartic  acid      Arginin         Ammonia 

When  the  proteids  which  have  escaped  digestion  in  the  stomach 
pass  into  the  small  intestine  and  mingle  with  the  pancreatic  juice, 
they  are  doubtless  digested  in  the  course  of  the  intestinal  canal, 
passing  through  the  stages  just  described.  As  leucin  and  tyrosin  are 
found  in  the  intestine  during  digestion,  it  is  probable  that  a  portion 
of  the  peptone  undergoes  decomposition  into  these  bodies;  but  as 
to  the  extent  to  which  this  takes  place  or  in  how  far  it  is  a  necessary 
process  under  normal  conditions,  nothing  definite  can  be  said.  It  is 
probable  that  it  takes  place  when  there  is  an  excess  of  proteid  food 
or  when  for  any  reason  digestion  is  prolonged  or  absorption  is  delayed. 

While  the  view  that  the  final  stage  in  the  digestion  of  proteids  is 


DIGESTION.  207 

the  formation  of  peptones,  which  in  due  time  are  absorbed  and  syn- 
thetized  into  blood  albumin,  is  generally  accepted,  there  is  some 
evidence  that  it  is  not  wholly  true,  and  that  the  final  stage  may  be 
the  formation  of  the  nitrogen-holding  compounds  above  mentioned; 
in  other  words,  that  the  cleavage  of  the  proteids  is  far  more  com- 
plete than  has  heretofore  been  assumed.  Ever  since  the  discovery 
by  Cohnheim  of  the  existence  in  the  intestinal  juice  of  a  substance 
termed  by  him  erepsin,  which  is  capable  of  splitting  proteoses  and 
peptones  into  simple  nitrogen-holding  compounds,  there  has  been 
slowly  developing  the  idea  that  normally  during  intestinal  digestion 
the  proteoses  and  peptones  are  reduced  by  this  agent  to  leucin,  tyrosin, 
histidin,  arginin,  aspartic  acid,  etc.,  which  in  turn  are  absorbed  and 
synthetized  to  blood  or  tissue  albumin.  The  discovery  by  Vernon  of 
erepsin  in  pancreatic  juice  lends  further  support  to  this  view.  Until 
more  convincing  evidence  is  furnished,  however,  it  may  be  assumed 
that  peptone  represents  the  final  stage  in  the  digestion  of  proteids. 
3.  On  jat.  If  pancreatic  juice  be  added  to  a  perfectly  neutral 
fat — olein,  palmitin,  or  stearin — and  kept  at  a  temperature  of  about 
100°  F.  (38°  C),  it  will  at  the  end  of  an  hour  or  two  be  partially  de- 
composed into  glycerin  and  the  particular  fatty  acid  indicated  by 
the  name  of  the  fat  used — e.  g.,  oleic,  palmitic,  stearic.  The  oil  will 
then  exhibit  an  acid  reaction.  The  reaction  is  represented  in  the 
following  formula: 

CsHjCCsHjjO^),     +     3H2O     =     (CisHj.Oj),     -f     C3H5(HO)3 
Triolein.  Water.  Oleic  Acid.  Glycerin. 

If  to  this  acidified  oil  there  be  added  an  alkah,  e.  g.,  potassium  or 
sodium  carbonate,  the  latter  will  at  once  combine  with  the  fatty  acid 
to  form  a  salt  known  as  a  soap.  The  reaction  is  expressed  in  the 
following  equation: 

Sodium  Carbonate.  Oleic  Acid.  Sodium  Oleate.  Carbonic  Acid. 

NajCOa     +      CjaHj^Oj     =     NajOCigHjjOj     +      H2CO3 

Coincident  with  the  formation  of  the  soap  the  remaining  neutral  oil 
undergoes  division  into  drops  of  microscopic  size,  which  float  in  the 
soap  solution,  forming  what  has  been  termed  an  emulsion,  which  is 
white  and  creamy  in  appearance.  The  action  of  the  pancreatic  juice 
may  then  be  said  to  consist  in  the  cleavage  of  the  neutral  fats  into 
fatty  acids  and  glycerin,  after  which  the  formation  of  the  soap  and 
the  division  of  the  fat  takes  place  spontaneously.  The  enzyme 
which  produces  the  cleavage  of  the  neutral  fats  has  been  termed 
stcapsin.  The  extent  to  which  the  cleavage  of  the  fat  takes  place  in 
the  intestine  has  not  been  definitely  determined.  There  are  some 
who  think  the  amount  is  relatively  small,  while  others  consider  that 
it  is  large,  practically  all  of  the  fat  undergoing  this  decomposition, 
with  the  formation  of  soap  and  glycerin  prior  to  their  absorption. 


2o8  TEXT-BOOK  OF  PHYSIOLOGY. 

According  to  Pawlow,  the  relative  amounts  of  the  pancreatic 
enzymes  produced  are  conditioned  by  the  character  and  amounts 
of  the  food  principles  consumed.  Thus,  if  chyme  contains  an  ex- 
cess of  either  starch,  proteid,  or  fat,  there  is  a  corresponding  increase 
in  the  amount  of  either  amylopsin,  trypsin,  or  steapsin  produced. 
The  pancreas  apparently  adapts  its  activities  to  the  character  of 
the  food.  Though  it  is  probable  that  each  enzyme  is  a  derivative 
of  a  special  zymogen,  it  is  only  positively  known  that  this  is  the  case 
with  trypsin.  This  enzyme  is  a  derivative  of  the  zymogen,  trypsin- 
ogen,  the  production  of  which  is  thought  to  be  the  special  function 
of  secretin.  The  pancreatic  juice  at  the  moment  of  its  discharge 
into  the  intestine  does  not  contain  trypsin  but  trypsinogen.  The 
transformation  of  the  latter  into  the  former  is  accomplished,  ac- 
cording to  Pawlow,  by  a  special  ferment  secreted  by  the  epithelium 
of  the  small  intestine  and  termed  enter okinase. 

The  rapidity  with  which  pancreatic  juice  in  the  presence  of  bile 
and  hydrochloric  acid  (under  conditions  such  as  are  present  in  the 
duodenum)  can  develop  sufhcient  fatty  acid  to  form  an  emulsion  was 
determined  by  Rachford  to  be  two  minutes.  The  activity  of  steapsin 
is  thus  shown  to  be  very  great. 

Physiologic  Action  of  the  Intestinal  Juice. — The  part  played 
by  the  intestinal  juice  in  the  digestive  process  is  yet  a  subject  of  dis- 
cussion, as  the  results  obtained  by  different  observers  are  in  some 
respects  contradictory,  due  to  the  fact  that  animals,  including  human 
beings,  have  been  the  subjects  of  experimentation.  Notwithstanding 
the  actions  of  saliva,  gastric  and  pancreatic  juice,  there  yet  remain 
in  the  food  saccharose,  maltose,  and  lactose,  three  forms  of  sugar 
which  are  believed  by  most  observers  to  be  non-assimilable  and 
therefore  require  some  change  before  they  can  be  absorbed  and 
assimilated.  An  extract  of  the  intestinal  mucous  membrane  or  the 
intestinal  juice  of  a  dog,  added  to  a  solution  of  saccharose,  will  in  a 
very  short  time  convert  it  into  dextrose  and  levulose,  which  together 
constitute  invert  sugar.  The  enzyme  by  which  this  inversion  is 
produced,  though  nothing  definite  is  known  as  to  its  nature,  has  been 
termed  invertin.  Tubbey  and  Manning  state  that  the  human  intes- 
tinal juice  as  obtained  by  them  has  the  same  action.  In  the  case 
of  intestinal  fistulae  reported  by  Busch,  which  were  supposed  to  be 
located  in  the  upper  third  of  the  intestine,  it  was  found  that  when 
saccharose  was  introduced  into  the  lower  opening,  it  was  not  inverted 
but  appeared  in  the  feces  unchanged. 

Maltose  is  also  rapidly  transformed  into  dextrose.  Lactose 
appears  to  be  unaffected  by  the  pure  juice.  As  it  is  non-assimilable 
it  has  been  supposed  to  undergo  conversion  into  dextrose  and  galac- 
tose while  passing  through  the  epithelial  cells  of  the  intestinal  mu- 


DIGESTION.  209 

cosa.     In  either  case  the  transformation  is  brought  about  by  two 
ferments  known  respectively  as  maltase  and  lactase. 

Intestinal  juice  also  has  a  sHght  diastatic  action  on  starch. 


THE  LIVER. 

The  liver  is  a  highly  vascular  conglomerate  gland  situated  in 
the  right  hypochondriac  region  and  connected  with  the  intestine  by 
a  duct. 

Inasmuch  as  the  liver  performs  several  functions  related  to  both 
secretion  and  excretion,  a  consideration  of  its  structure  and  its  vari- 
ous functions  will  be  deferred  to  a  subsequent  chapter.  In  this 
connection  the  bile,  its  physical  properties  and  chemic  composition 
in  relation  to  the  digestive  process,  will  only  be  considered. 

The  bile  is  a  product  of  the  secretory  activity  of  the  liver  cells. 
As  it  is  poured  into  the  intestine  in  man  and  most  mammals  at 
a  point  corresponding  to  the  orifice  of  the  pancreatic  duct, 'and  most 
abundantly  at  the  time  the  food  is  passing  through  the  duodenum, 
it  is  usually  regarded  as  a  digestive  fluid  possessing  an  influence 
favorable  if  not  necessary  to  the  completion  of  the  general  digestive 
process. 

Anatomic  Relations  of  the  Biliary  Passages. — After  its  forma- 
tion by  the  liver  cells  the  bile  is  conveyed  from  the  hver  by  the  bile 
capillaries,  which  uniting  finally  form  the  main  hepatic  duct.  This 
duct  emerges  from  the  liver  at  the  transverse  fissure.  At  a  distance 
of  about  5  centimeters  it  is  joined  by  the  cystic  duct,  the  distal  ex- 
tremity of  which  expands  into  a  pear-shaped  reservoir,  the  gall- 
bladder, in  which  the  bile  is  temporarily  stored  (Fig.  84).  The  duct 
formed  by  the  union  of  the  hepatic  and  cystic  ducts,  the  common 
bile-duct,  passes  downward  and  forward  for  a  distance  of  about  7 
centimeters,  pierces  the  walls  of  the  intestine  and  passes  obhquely 
through  its  coats  for  about  a  centimeter  and  opens  on  the  surface  of 
a  papilla  in  conjunction  with  the  pancreatic  duct.  The  walls  of  the 
bihary  passages  are  composed  of  a  mucous  membrane  internally, 
a  fibrous  and  muscular  coat  externally.  The  termination  of  the 
common  bile-duct  is 'provided  with  a  distinct  band  of  circularly 
disposed  muscle-fibers,  which  when  in  action  completely  close  the 
orifice  and  prevent  the  discharge  of  bile.  It  may  therefore  be  re- 
garded as  a  true  sphincter  muscle.  Small  racemose  glands  are 
embedded  in  the  mucous  membrance  of  the  main  ducts. 

Physical  Properties  and  Chemic  Composition  of  Bile. — The 
bile  obtained  directly  from  the  hver  through  a  fistulous  opening  in 
the  hepatic  duct  is  always  thin  and  watery,  while  that  obtained  from 
the  gall-bladder  is  more  or  less  viscid  from  admixture  with  mucin, 
the  degree  of  this  viscidity  depending  on  the  length  of  time  it  remains 
14 


2IO  TEXT-BOOK  OF  PHYSIOLOGY. 

in  this  reservoir.  The  specific  gravity  of  human  bile  varies  within 
normal  limits  from  i.oio  to  1.020.  The  reaction  is  invariably  alka- 
line in  the  human  subject  when  first  discharged  from  the  liver,  but 
may  become  neutral  in  the  gall-bladder.  The  alkahnity  depends 
on  the  presence  of  sodium  carbonate  and  sodium  phosphate.  When 
fresh,  it  is  inodorous;  but  it  readily  undergoes  putrefactive  changes, 
and  soon  becomes  offensive.  Its  taste  is  decidedly  bitter.  When 
shaken  with  water,  it  becomes  frothy — a  condition  which  lasts  for 


Fig.  84. — Gall-bladder,  Hepatic,  Cystic,  and  Common  Ducts,  i,  2,  3.  Duode- 
num. 4,  4,  5,  6,  7,  7.  8.  Pancreas  and  pancreatic  ducts.  9,  10,  11,  12,  13.  Liver. 
14.  Gall-bladder.  15.  Hepatic  duct.  16.  Cystic  duct.  17.  Common  duct.  18. 
Portal  vein.  19.  Branch  from  the  ceHac  axis.  20.  Hepatic  artery.  21.  Coro- 
nary artery  of  the  stomach.  22.  Cardiac  portion  of  the  stomach.  23.  Splenic 
artery.  24.  Spleen.  25.  Left  kidney.  26.  Right  kidney.  27.  Superior  mesen- 
teric artery  and  vein.     28.  Inferior  vena  cava. — (Sappey.) 


some  time  and  which  is  due  to  the  presence  of  mucin.     In  ox  bile  the 
mucin  is  replaced  by  a  nucleo-proteid. 

The  color  of  bile  obtained  from  the  hepatic  duct  is  variable, 
usually  a  shade  between  a  greenish-yellow  and  a  brownish-red.  In 
different  animals  the  color  varies.  In  the  herbivorous  animals  it  is 
usually  green;  in  the  carnivorous  animals  it  is  orange  or  brown.  In 
man  it  is  green  or  a  golden  yellow.  The  colors  are  due  to  the  pres- 
ence of  pigments.  Microscopic  examination  does  not  show  the 
presence  of  structural  elements. 


DIGESTION.  211 

Human  bile  obtained  from  an  accidental  biliary  fistula  was  shown 
by  Jacobson  to  contain  the  following  ingredients,  viz. : 

COMPOSITION  OF  HUMAN  BILE. 

Water, 1 9774o 

Sodium    glycocholate, 9.94 

Sodium  taurocholate, a  trace 

Cholesterin, c.54 

Free  fat, ^..  o.io 

Sodium  palmitate  and  stearate, 1.36 

Lecithin, 0.04 

Other  organic  matters, 2.26 

Sodium  chlorid, 5.45 

Potassium  chlorid, 0.28 

Sodium   phosphate, 1.33 

Calcium   phosphate, 0.37 

Sodium  carbonate, l 0.93 

1000.00 

In  this  analysis  the  soHd  ingredients  constitute  22.5  parts  per  1000, 
of  which  two-thirds  are  organic  and  one-third  inorganic.  The 
amount  of  sohds  varies  according  to  the  animal  from  which  the  bile 
is  obtained. 

Sodium  Glycocholate  and  Taurocholate. — Of  the  various  in- 
gredients of  the  bile  none  are  more  important  than  these  two  salts, 
usually  known  as  the  bile  salts.  The  sodium  glycocholate  is  found 
most  abundantly  in  the  bile  of  herbivora,  the  sodium  taurocholate 
in  the  bile  of  the  carnivora.  These  salts  are  compounds  of  sodium 
and  glycocholic  and  taurocholic  acids.  When  separated  from  the 
sodium,  the  acids  will  crystallize  in  the  form  of  fine  acicular  needles. 
Under  the  influence  of  hydrating  agents,  such  as  dilute  acids  and 
alkalies,  both  acids  will  undergo  cleavage  into  their  respective  com- 
ponents— e.  g.,  glycocoll  and  cholalic  acid,  taurine  and  cholahc  acid. 
Glycocoll  and  taurine  are  crystallizable  nitrogenized  compounds 
known  chemically  as  amido-acetic  and  amido-isothionic  acids  re- 
spectively. The  bile  salts  are  produced  in  the  liver  by  a  true  act 
of  secretion,  as  they  are  not  found  in  any  of  the  tissues  and  fluids  of 
the  body.  After  being  discharged  into  the  intestine  they  undergo 
chemic  changes,  after  which  they  can  no  longer  be  recognized.  In 
all  probability  they  are  reabsorbed  into  the  blood  and  play  some 
ulterior  part  in  the  nutrition  of  the  body. 

Cholesterin. — Cholesterin  is  a  constant  ingredient  of  bile,  though 
it  is  not  confined  to  this  fluid,  as  its  presence  has  been  determined  in 
the  crystaUine  lens,  blood-corpuscles,  nerve-tissue,  and  various  patho- 
logic fluids.  It  is  an  organic  non-nitrogenized  substance  resembling 
the  fats  in  some  particulars,  but  differing  from  them  in  not  being 
capable  of  saponification  with  alkahes.  It  presents  itself  in  the  form 
of  thin  transparent  rectangular  crystals,  insoluble  in  water  but  soluble 


212  TEXT-BOOK  OF  PHYSIOLOGY. 

in  ether  and  boiling  alcohol  (Fig.  85).  It  is  held  in  solution  in  bile  by 
the  bile  salts.  If  they  are  deficient  in  amount,  the  cholesterin  may 
pass  out  of  solution,  collect  around  some  foreign  matter,  and  form 
a  gall-stone.  Cholesterin  is  a  product  of  the  metabolism  largely  of 
nerve-tissue,  from  which  it  is  absorbed  by 
the  blood,  carried  to  the  liver,  and  excreted. 
In  the  intestine  it  is  converted  into  stercorin 
and  discharged  from  the  body  in  the  feces. 

Bilirubin,  Biliverdin. — These  two  pig- 
ments impart  to  the  bile  its  red  and  green 
colors  respectively.     Bilirubin  is  present  in 
^  the  bile  of  human  beings  and  the  carnivora, 
^'^'c^ivTx^A'^s.-CWoil'^biHverdin  in  the  bile  of  the  herbivora.     As 
and  Stirling.)  the  former  pigment  readily  undergoes  oxi- 

dation in  the  gall-bladder,  giving  rise  to  the 
latter  pigment,  almost  any  specimen  of  bile  may  present  any  shade  of 
color  between  red  and  green.  Bihrubin  is  regarded  as  a  derivative 
of  hematin,  one  of  the  cleavage  products  of  hemoglobin,  the  coloring- 
matter  of  the  blood.  In.  the  liver  the  hematin  combines  with  water, 
loses  its  iron,  and  is  changed  to  bilirubin.  By  continuous  oxidation 
there  are  formed  bihverdin,  bihcyanin,  and  choletelin.  After  their 
discharge  into  the  intestine  the  bile  pigments  are  finally  reduced  to 
hydrobilirubin,  which  becomes  one  of  the  constituents  of  the  feces. 
An  oxidation  of  the  bilirubin  can  be  produced  by  nitroso-nitric  acid. 
If  this  agent  is  added  to  a  thin  layer  of  bile  on  a  porcelain  surface, 
a  series  of  colors  will  rapidly  succeed  one  another,  commencing  with 
green  and  passing  to  blue,  orange,  purple,  and  yellow.  This  is  the 
basis  of  the  well-known  test  for  bile  pigments  suggested  by  Gmelin. 

Lecithin  is  one  of  the  products  of  the  metabolism  of  nerve-tissue. 
It  is  removed  from  the  blood  by  the  liver  cells  and  thus  becomes  one 
of  the  constituents  of  the  bile,  in  which  it  is  held  in  solution  by  the 
bile  salts. 

The  Mode  of  Secretion  and  Discharge  of  Bile.^ — The  manner 
in  which  the  bile  flows  from  the  liver  into  the  main  hepatic  ducts, 
the  variations  in  the  rate  of  its  discharge  into  the  intestine,  as  well 
as  the  total  quantity  secreted  daily,  have  been  approximately  de- 
termined by  fistulous  openings  either  in  the  hepatic  ducts  or  in  the 
gall-bladder.  Although  the  liver  presents  some  physiologic  peculi- 
arities, there  is  no  reason  to  believe  that  the  conditions  of  secretion 
therein  are  different  from  those  in  any  other  secretory  organ,  or  that 
any  other  structure  than  the  cell  is  engaged  in  this  process.  As 
shown  by  chemic  analysis,  the  bile  consists  of  compounds,  some  of 
which,  like  the  bile  salts,  are  formed  in  the  fiver  cells  out  of  ma- 
terial furnished  by  the  blood,  by  a  true  act  of  secretion,  while  others, 
such  as  cholesterin  and  lecithin,  principles  of  waste,  are  merely  ex- 


DIGESTION.  213 

creted  from  the  blood  to  be  finally  eliminated  from  the  body.     The 
bile  is  thus  a  compound  of  both  secretory  and  excretory  principles. 

The  flow  of  bile  from  the  liver  is  continuous  but  subject  to  con- 
siderable variation  during  the  twenty-four  hours.  The  introduction 
of  food  into  the  stomach  at  once  causes  a  shght  increase  in  the  flow, 
but  it  is  not  until  about  two  hours  later  that  the  amount  secreted 
reaches  its  maximum;  after  this  period  it  gradually  decreases  up  to 
the  eighth  hour,  but  never  entirely  ceases.  During  the  intervals  of 
digestion  though  a  small  quantity  passes  into  the  intestine,  the  main 
portion  is  diverted  into  the  gall-bladder,  because  of  the  closure  of 
the  common  bile-duct  by  the  sphincter  muscle  near  its  termination, 
where  it  is  retained  until  required  lor  digestive  purposes.  When 
acidulated  food  passes  over  the  surface  of  the  duodenum  there  is 
excited,  through  reflex  action,  a  contraction  of  the  muscle  walls  of 
the  gall-bladder  and  ducts,  a  relaxation  of  the  sphincter,  and  a  gush 
of  bile  into  the  intestine,  the  discharge  continuing  intermittently 
until  digestion  ceases  and  the  intestine  is  emptied  of  its  contents. 

The  storage  and  the  discharge  of  bile,  brought  about  by  the 
alternate  contraction  and  relaxation  of  the  muscle  walls  of  the  gall- 
bladder and  of  the  sphincter  is  regulated  by  the  nerve  system.  As 
the  result  of  his  experiments  Doyon  concludes,  that  during  the  inter- 
vals oi  intestinal  digestion  the  vagus  nerve  is  carr}  ing  nerve  impulses 
which  on  the  one  hand  augment  the  contraction  of  the  sphincter 
and  inhibit  the  contraction  of  the  walls  of  the  gall-bladder,  thus 
estabhshing  the  concitions  lor  the  storage  of  bile;  but  when  intesti- 
nal digestion  is  inaugurated  the  splanchnic  nerve  carries  nerve  im- 
pulses which  inhibit  the  sphincter  and  augment  the  contraction  of 
the  walls  of  the  gall-bladder,  thus  establishing  the  condition  for  the 
discharge  ol  the  bile. 

The  total  quantity  of  bile  secreted  daily  has  been  estimated  to  be 
from  500  to  800  grams. 

Physiologic  Action  of  Bile.— Notwithstanding  our  knowledge 
of  the  complex  composition  of  bile,  the  quantity  discharged  daily,  and 
the  time  and  place  of  its  discharge,  its  exact  relation  to  the  digestive 
process  has  not  been  fully  determined.  No  specific  action  can  be 
attributed  to  it.  It  has  but  a  slight,  if  any,  diastatic  action  on  starch. 
It  is  without  influence  on  proteids.  By  virtue  of  the  bile  salts  it  con- 
tains, it  hastens  the  action  of  pancreatic  juice  in  sphtting  neutral  oils 
into  fatty  acids  and  glycerin,  and  in  this  way  aids  in  their  digestion. 
The  bile  salts  also  dissolve  insoluble  soaps,  which  may  be  formed 
during  digestion. 

Bile  favors  the  digestion  of  fat.  If  it  be  excluded  from  the  intes- 
tine there  is  found  in  the  feces  from  22  to  58  per  cent,  of  the  ingested 
fats.  At  the  same  time  the  chyle,  instead  of  presenting  the  usual 
white  creamy  appearance,  is  thin  and  shghtly  yellow.     The  manner 


214  TEXT-BOOK  OF  PHYSIOLOGY. 

in  which  the  bile  promotes  fat  digestion  is  yet  a  subject  of  investiga- 
tion. If  all  the  fat  is  converted  into  fatty  acid  and  glycerin,  with  the 
formation  of  soaps,  as  seems  probable,  the  action  of  the  bile  becomes 
more  apparent  from  the  fact,  already  stated,  that  it  dissolves  and 
holds  in  solution  the  soaps  so  formed  which  would  be  necessary  to 
their  absorption  by  the  epithelial  cells.  As  an  aid  to  digestion  the 
bile  has  been  regarded  as  important,  for  the  reason  that  its  entrance 
into  the  intestine  is  attended  by  a  neutralization  and  precipitation  of 
the  proteids  which  have  not  been  fully  digested  and  are  yet  in  the 
stage  of  acid-albumin.  In  this  way  gastric  digestion  is  arrested  and 
the  foods  are  prepared  for  intestinal  digestion. 

Though  bile  possesses  no  antiseptic  properties  outside  the  body, 
itself  undergoing  putrefactive  changes  very  rapidly,  it  has  been 
believed  that  in  the  intestine  it  in  some  way  prevents  or  retards  putre- 
factive changes  in  the  food.  There  can  be  no  doubt  that  if  the  bile 
be  prevented  from  entering  the  intestine  there  is  an  increase  in  the 
formation  of  gases  and  other  products  which  impart  to  the  feces 
certain  characteristics  which  are  indicative  of  putrefaction.  As  to 
the  manner  in  which  bile  retards  this  process  nothing  definite  can  be 
stated. 

Bile  has  been  supposed  to  be  a  stimulant  to  the  peristaltic  move- 
ments of  the  intestine,  inasmuch  as  these  movements  diminish  when 
bile  is  diverted  from  the  intestine. 

Though  no  definite  nor  specific  action  on  any  of  the  different 
classes  of  food  principles  can  be  attributed  to  the  bile,  there  is  abun- 
dant evidence  to  show  that  its  presence  in  the  alimentary  canal  during 
digestion  is  essential  to  the  maintenance  of  the  nutrition  of  the  body. 
That  the  bile  as  a  whole,  or  at  least  part  of  its  constituents,  favorably 
influences  digestion  and  general  nutrition  is  evident  from  the 
phenomena  which  follow  its  total  exclusion  from  the  intestine,  as 
when  the  common  bile-duct  is  ligated  and  a  fistula  of  the  gall-bladder 
is  established.  The  following  phenomena  were  observed  in  a  young 
dog  so  prepared  by  Professor  Flint.  During  the  first  five  days 
succeeding  the  operation  the  abdomen  was  tumid  and  there  was 
some  rumbling  in  the  bowels.  Though  the  animal  ate  every  day,  the 
discharge  of  fecal  matter  became  infrequent,  the  matter  passed  being 
grayish  in  color  and  highly  offensive.  After  two  weeks  the  alvine 
discharges  took  place  three  and  four  times  daily.  For  four  days  the 
weight  remained  normal;  afterward  it  began  to  diminish,  and  from 
this  time  the  animal  continued  to  lose  strength  and  weight  until  its 
death,  thirty-eight  days  after  the  operation.  Ten  days  after  the  oper- 
ation the  appetite,  which  had  been  very  good,  increased,  but  did 
not  become  ravenous  until  a  few  days  before  death.  The  animal 
usually  ate  from  a  pound  to  a  pound  and  a  half  of  beef-heart  daily, 
always  refusing  fat.     There  was  an  absence  at  all  times  of  jaundice, 


DIGESTION.  215 

fetor  of  the  breath,  and  falhng  of  the  hair.  Postmortem  examination 
showed  that  the  bile-duct  was  obhterated,  and  there  was  no  evidence 
that  any  bile  could  have  passed  into  the  intestine.  The  results  of  this 
and  similar  cases  go  to  show  that  that  portion  of  the  bile  which  is 
secretory  in  character  is  essential  to  digestion  and  the  nutrition  of 
the  body — that,  though  large  quantities  of  food  are  consumed,  pro- 
gressive diminution  of  weight  takes  place  until  nearly  40  per  cent,  of 
the  body  is  consumed.  In  some  instances  the  breath  becomes  fetid 
and  there  is  a  falling  of  the  hair,  showing  some  profound  disturbance 
of  the  general  nutritive  process. 

The  Movements  of  the  Intestine. — The  movements  of  the 
intestine  have  been  studied  by  means  of  the  Roentgen  rays  by  Cannon. 
The  method  adopted  was  to  mix  with  the  food,  subnitrate  of  bismuth, 
which  being  opaque  rendered  the  movements  of  the  intestinal  con- 
tents and  thereby  the  movements  of  the  intestinal  walls  visible,  on  the 
fluorescent  screen.  These  investigations  revealed  the  presence  of 
two  forms  of  activity,  one  of  which  is  more  or  less  stationar}^  and  due 
to  rhythmic  contraction  of  circular  muscle-fibers,  the  other  progres- 
sive, passing  from  abo^•e  downward  and  due  to  the  contraction  of 
circular  and  longitudinal  muscle-fibers.  The  former  activity,  which 
is  by  far  the  more  common,  results  in  a  division  of  the  intestinal 
contents  into  small  segments  and  for  this  reason  was  termed  by  Can- 
non, rhvtlimic  segmentation;  the  latter  activity  is  the  well-known 
peristaltic  wave. 

Alter  bands  of  circular  muscle-fibers,  situated  at  variable  dis- 
tances one  from  another,  contract  and  divide  a  mass  of  food  into  seg- 
ments, they  at  once  relax  and  are  followed  by  contraction  of  other 
bands  in  the  segments  of  the  intestines  overlying  the  segments  of 
food.  The  result  is  again  a  division  of  the  food  into  two  new  seg- 
ments. The  lower  hah  of  each  segment  then  unites  with  the  upper 
half  of  the  segment  below  to  commingle  with  it  and  expose  new  sur- 
faces of  the  lood  mass  to  contact  with  the  actively  absorbing  mu- 
cosa. The  continual  repetition  of  this  process  results  in  a  thorough 
mixing  of  the  food  with  the  digestive  juices.  From  the  manner  in 
which  these  contractions  make  their  appearance  it  would  seem  that 
the  mere  presence  of  a  segment  in  the  lumen  of  the  intestine  is  suffi- 
cient to  excite  the  overhing  fibers  to  activity. 

In  certain  regions  of  the  intestine  rhythmic  segmentation  may 
continue  for  half  to  three-quarters  of  an  hour  without  moving  the 
food  forward  to  any  marked  extent.  In  the  cat  the  segmentation 
may  proceed  at  the  rate  of  thirty  divisions  a  minute. 

Ba}hss  and  Starling  state,  from  observations  made  on  the  ex- 
posed intestine  of  a  dog,  that  in  addition  to  the  usual  peristaltic 
movement  the  intestinal  coils  exhibit  a  swaying  or  pendulum  move- 
ment accompanied  by  shght  waves  of  contraction  which  may  arise 


2i6  TEXT-BOOK  OF  PHYSIOLOGY. 

apparently  at  any  point  and  pass  down  the  intestine.  These  con- 
tractions may  occur  from  ten  to  twelve  times  a  minute  and  travel 
at  a  rate  varying  from  two  to  five  centimeters  a  second.  In  how  far 
this  movement  represents  the  normal  movement  as  it  takes  place 
under  ph}siologic  conditions  and  as  observed  by  Cannon,  remains 
for  further  investigators  to  decide. 

Alter  the  food  has  been  prepared  by  the  process  above  described, 
it  is  then  slowly  carried  downward  by  what  is  known  as  the  vermicular 
or  peristaltic  wave.  This  wave  is  characterized  by  a  contraction  of 
the  circular  fibers  behind  a  bolus  and  a  relaxation  of  the  fibers  in 
advance  of  it.  The  result  is  a  movement  forward  of  the  bolus,  and 
as  it  moves  it  is  followed  by  a  ring  of  constriction  and  preceded  by  a 
ring  of  relaxation  or  inhibition.  The  rate  of  movement  of  the  peri- 
staltic wave  is  extremely  slow. 

The  Nerve  Mechanism  of  the  Intestine. — The  causes  of  these 
two  forms  of  intestinal  activity,  rhythmic  segmentation  or  pendulum 
movement  and  peristalsis,  have  been  the  subject  of  much  investiga- 
tion. Because  of  the  presence  of  a  network  or  plexus  (Auerbach's 
and  Meissner's)  of  nerve  cells  and  nerve-fibers  in  the  walls  of  the 
intestines  and  in  close  relation  to  the  muscle  cells,  and  because  of 
the  fact  that  the  intestines  will  contract  for  some  time  alter  removal 
from  the  body  of  the  animal,  it  has  been  difficult  to  decide  whether 
the  contractions  are  of  myogenic  or  neurogenic  origin. 

As  the  rhythmic  contractions  continue,  though  the  peristaltic 
are  abolished  by  the  introduction  of  nicotin  into  the  blood,  an  agent 
which  temporarily  paral}ses  peripheral  nerve  cells,  it  was  concluded 
by  Bayhss  and  Starling  that  the  rhythmic  contractions  are  of  myo- 
genic origin  and  propagated  from  fiber  to  fiber  and  that  the  peri- 
staltic contractions  are  reflex  in  character,  the  coordination  being 
carried  out  by  the  local  nerve  mechanisms  and  initiated  by  stimu- 
lation of  the  intestine.  Whether  this  is  the  case  or  not,  the  general 
contractions  of  the  intestine  are  augmented  and  inhibited  by  the 
central  nerve  system  through  the  vagus  and  splanchnic  nerves. 

Stimulation  of  the  vagus  is  followed  by  an  augmentation  of  the 
contraction,  though  not  inlrequently  there  is  a  primary  inhibition  of 
short  duration.  Stimulation  of  the  splanchnic  is  followed  by  a 
relaxation  or  inhibition  of  the  contraction,  though  according  to  some 
observers  there  is  at  times  an  opposite  effect. 

The  Large  Intestine. — The  large  intestine  is  that  portion  of  the 
alimentary  canal  situated  between  the  termination  of  the  ileum  and 
the  anus.  It  varies  in  length  from  four  and  a  half  to  five  feet,  in 
diameter  from  one  and  a  half  to  two  and  a  half  inches.  It  is  divided 
into  the  cecum,  the  colon  (subdivided  into  an  ascending,  transverse, 
and  descending  portion,  including  the  sigmoid  flexure),  and  the 
rectum. 


DIGESTION.  217 

The  cecum  is  situated  in  the  right  iliac  fossa.  It  is  that  dilated 
portion  of  the  large  intestine  below  the  orifice  of  the  small  intestine. 
The  posterior  and  inner  wall  presents  a  small  opening  which  leads 
into  a  narrow  round  process  about  four  inches  in  length — the  vermi- 
form appendix.  The  opening  of  the  small  intestine  into  the  cecum  is 
narrow  and  elongated  and  bordered  by  two  folds  of  mucous  mem- 
brane strengthened  by  fibrous  and  muscle-tissue.  These  folds 
constitute  the  so-called  ileo-cecal  valve.  When  the  cecum  is  dis- 
tended the  margins  of  these  folds  are  approximated  and  effectually 
prevent  the  return  of  material  into  the  small  intestine. 

The  closure  of  this  opening  is  now  attributed  to  a  sphincter  mus- 
cle— the  ileo- colic — the  action  of  which  is  regulated  by  the  nerve 
system. 

The  colon  ascends  to  the  under  surface  of  the  hver,  where  it  bends 
at  a  right  angle,  crosses  the  abdominal  ca-\dty  to  the  spleen,  bends 
again,  and  descends  to  the  left  iliac  fossa.  At  this  point  it  turns  upon 
itself  to  form  the  sigmoid  flexure.  The  rectum  is  a  dilated  pouch, 
situated  within  the  true  pehis.  It  measures  from  15  to  18  centi- 
meters in  length.  Within  an  inch  of  its  termination  at  the  anus  it 
presents  a  constriction  formed  by  a  circular  band  of  muscle-fibers 
known  as  the  internal  sphincter.  The  margin  of  the  anus  is  also 
surrounded  by  bands  of  muscle-fibers  known  collectively  as  the  ex- 
ternal sphincter. 

The  walls  of  the  large  intestine  consist  of  three  coats:  viz.,  serous, 
muscular,  and  mucous. 

The  serous  is  a  reflection  of  the  general  peritoneal  membrane. 

The  muscle  is  composed  of  both  longitudinal  and  circular 
fibers.  The  longitudinal  fibers  are  collected  into  three  narrow 
bands  which  are  situated  at  points  equidistant  from  one  another. 
At  the  rectum  they  spread  out  so  as  to  completely  surround  it.  As 
the  longitudinal  bands  are  shorter  than  the  intestine  itself,  its  surface 
becomes  sacculated,  each  sac  being  partially  separated  from  adjoining 
sacs  by  narrow  constrictions.  The  circular  fibers  are  arranged  in 
the  form  of  a  thin  layer  over  the  entire  intestine.  Between  the  sac- 
cuH,  however,  they  are  more  closely  arranged.  In  the  rectum  they 
are  well  developed,  and  at  a  point  an  inch  above  the  anus  they 
form,  as  stated  above,  the  internal  sphincter. 

The  mucous  membrane  of  the  large  intestine  possesses  neither 
vilH  nor  valvulce  conniventes.  It  contains  a  large  number  of  tubules 
consisting  of  a  basement  membrane  lined  by  columnar  epithehum. 
They  resemble  the  follicles  of  Lieberkiihn.  The  secretion  of  these 
glands  is  thick  and  \'iscid  and  contains  a  large  quantity  of  mucin. 

Contents  of  the  Large  Intestine. — ^As  a  result  of  the  actions 
of  saliva,  of  gastric,  intestinal,  and  pancreatic  juice,  and  of  the  bile, 
the  food  is  disintegrated  and  hquefied.  The  nutritive  principles, 
proteid,  starches,  sugars,  and  fats,  undergo  chemic  changes  and  are 


2i8  TEXT-BOOK  OF  PHYSIOLOGY. 

transformed  into  peptones,  dextrose,  soap  and  glycerin,  fatty  acids, 
under  which  forms  they  are  absorbed.  After  the  more  or  less  com- 
plete digestion  and  absorption  of  these  nutritive  substances  the 
residue  of  the  food,  comprising  the  indigestible  and  undigested 
matter,  passes  out  of  the  small  intestine  into  the  large  intestine 
and  forms  a  portion  of  its  contents.  This  residue  consists  of 
the  hard  parts  of  the  cereals,  vegetable  seeds,  cellulose,  etc.,  the 
quantity  and  variety  of  which  depend  on  the  nature  of  the  food. 
These  substances,  passing  into  the  large  intestine  along  with  the 
excrementitious  matters  of  the  bile,  become  incorporated  with  the 
mucous  secretions  and  assist  in  the  formation  of  the  feces.  Under 
the  influence  of  a  peristaltic  movement  similar  to  that  witnessed  in 
the  small  intestine,  all  this  excrementitious  matter,  deprived  by 
absorption  of  the  excess  of  its  contained  water  and  nutritive  material 
is  gradually  carried  downward  to  the  sigmoid  flexure,  where  it  accu- 
mulates prior  to  its  extrusion  from  the  body. 

The  efi'ects  of  the  peristaltic  waves  are  to  some  extent  interfered 
with  by  anti-peristaltic  waves  which  beginning  in  the  transverse 
colon  nm  toward  and  to  the  cecum.  An  antiperistaltic  wave  occurs 
in  the  cat  about  every  fifteen  minutes  and  lasts  for  about  five  min- 
utes. The  intestinal  contents  are  thereby  driven  back  toward  the 
cecum.  The  effect  is  a  still  further  admixture  with  the  secretions 
and  exposure  to  the  absorbing  mucosa.  There  is  some  evidence  also 
that  the  antiperistaltic  wave  may  force  some  of  the  liquefied  con- 
tents through  the  ileo-colic  opening  into  the  small  intestine  because 
of  the  relaxation  of  the  sphincter  muscle. 

Intestinal  Fermentation. — Owing  to  the  favorable  conditions 
in  the  intestine  for  fermentative  and  putrefactive  processes — e.  g., 
heat,  moisture,  oxygen,  and  the  presence  of  various  microorganisms 
— the  food,  when  consumed  in  excessive  quantity  or  when  acted  on 
by  defective  secretions,  undergoes  a  series  of  decomposition  changes 
which  are  attended  by  the  production  of  gases  and  various  chemic 
compounds.  Dextrose  and  maltose  are  partially  reduced  to  lactic 
acid;  this  to  butyric  acid,  carbon  dioxid,  and  hydrogen.  Fats  are 
reduced  to  glycerol  and  fatty  acids;  the  glycerol,  according  to  the 
organisms  present,  yields  succinic  acid,  carbon  dioxid,  and  hydrogen. 
The  proteids  under  the  prolonged  action  of  the  pancreatic  juice  are 
decomposed,  with  the  production  of  leucin  and  tyrosin.  These 
crystalline  compounds  are  in  turn  reduced  to  simpler  forms.  The 
former  yields  valerianic  acid,  ammonia,  and  carbon  dioxid;  the 
latter  gives  rise  to  indol,  which  is  the  antecedent  of  indican,  found  in 
the  urine.  Skatol,  another  derivative  of  the  proteid  molecule,  due 
to  bacterial  action,  gives  the  characteristic  odor  to  the  feces. 

Feces. — The  feces  consist  of  water,  mucin,  the  indigestible  resi- 
due of  the  food,  decomposition  products,  and  inorganic  salts.     The 


DIGESTION.  219 

consistency  of  the  fecal  matter  varies  from  fluid  to  semi-fluid,  depend- 
ing largely  on  the  length  of  time  it  remains  in  the  intestine  and  the 
extent  to  which  absorption  of  its  watery  portion  has  taken  place.  The 
odor  is  due  to  the  presence  of  sulphuretted  hydrogen  and  skatol. 
The  color  is  due  partly  to  the  altered  coloring-matter  of  the  bile,  hydro- 
bilinibin  or  stercohilin,  and  partly  to  the  character  of  the  food.  The 
total  quantity  discharged  daily  varies  from  four  to  six  ounces. 

Defecation. — Defecation  is  the  final  act  of  the  digestive  process 
and  consists  in  the  expulsion  of  the  indigestible  residue  of  the  food 
from  the  intestine.  This  act  usually  takes  place  in  the  human  being 
but  once  in  twenty-four  hours,  as  the  diet  contains  but  a  minimum 
quantity  of  indigestible  matter.  Previous  to  their  expulsion  the  feces 
which  have  accumulated  in  the  sigmoid  flexure  must  pass  downward 
into  the  rectum.  In  so  doing  they  develop  the  sensation  which  leads 
to  the  act  of  defecation.  The  descent  of  the  feces  is  accompUshed  by 
the  peristaltic  contraction  of  the  intestinal  wall.  Coincident  with  the 
passage  of  the  feces  into  the  rectum  there  is  a  relaxation  of  the  sphinc- 
ter muscles  and  a  contraction  of  the  longitudinal  and  circular  mus- 
cular fibers,  in  consequence  of  which  the  feces  are  expelled.  These 
complex  muscular  actions  are  also  aided  by  the  voluntary  contrac- 
tions of  the  diaphragm  and  abdominal  muscles. 

Nerve  Mechanism  of  Defecation. — The  act  of  defecation  is 
primarily  reflex,  though  somewhat  influenced  by  voluntary  efforts. 
Under  normal  conditions  the  sphincter  muscles  governing  the  anal 
orifice  are  firmly  contracted,  thus  preventing  the  escape  of  gases  or 
semi-sohd  matter.  This  tonic  contraction  is  maintained  by  a  nerve- 
center  located  in  the  lumbar  region  of  the  spinal  cord.  The  circu- 
lar and  longitudinal  muscle-fibers  of  the  rectum  are  at  the  same 
time  in  a  relaxed  condition.  When  the  desire  to  evacuate  the  bowels 
is  experienced,  the  impressions  made  by  the  feces  on  the  afferent 
nerves  of  the  rectal  mucous  membrane  develop  nerve  impulses, 
which  transmitted  to  the  rectal  center  and  to  the  brain,  influence  in 
one  direction  or  another,  their  activities.  If  the  act  of  defecation  is 
to  take  place  there  is  an  inhibition  of  the  contraction  of  the  sphincter 
ani  muscles  and  an  augmentation  of  the  contraction  of  the  rectal 
muscles.  In  their  expulsive  efforts,  these  latter  muscles  are  assisted 
by  the  contraction  of  the  diaphragm,  abdominal  and  other  muscles. 
After  the  expulsion  of  the  feces  there  is  a  return  to  the  former  con- 
dition, viz.,  a  relaxation  of  the  rectal  muscles  and  a  contraction  of 
the  sphincters. 

If  the  act  is  to  be  suppressed,  the  controlling  influence  of  the 
rectal  or  sphincter  center  is  strengthened  and  the  reflex  mechanism 
for  a  while  held  in  abeyance. 

The  exact  course  of  the  afferent  and  efferent  nerves  concerned 
in  this  reflex  is  yet  a  subject  of  investigation. 


220  TEXT-BOOK  OF  PHYSIOLOGY. 

The  nerve  supply  for  the  circular  and  longitudinal  muscles  of 
the  lower  part  of  the  colon  and  rectum  varies  somewhat  in  different 
animals,  though  it  is  usually  derived  from  the  second,  third,  and  fourth 
lumbar  and  the  second  and  third  sacral  or  pelvic  nerves. 

The  lumbar  nerves  pass  into  and  through  the  sympathetic  chain 
and  thence  to  the  inferior  mesenteric  ganglion,  around  the  cells  of 
which  most  of  the  nerves  arborize.  From  this  ganghon  nerve- 
fibers  pass  to  both  the  circular  and  longitudinal  fibers. 

The  sacral  or  pelvic  nerves  pass  to  the  hypogastric  plexus  and 
are  ultimately  connected  with  ganghon  cells,  which  in  turn  send  fibers 
to  both  the  longitudinal  and  circular  fibers.  The  explanation  of 
the  action  of  this  complex  mechanism  is  a  subject  of  discussion 
because  of  the  want  of  agreement  in  the  results  that  follow  stimula- 
tion of  these  nerves. 


CHAPTER  X. 
ABSORPTION. 

Absorption  is  the  process  by  which  nutritive  material  is  trans- 
ferred from  the  tissues,  from  the  serous  cavities,  and  from  the  mucous 
surfaces  of  the  body,  into  the  blood.  The  most  important  of  these 
surfaces,  especially  in  its  relation  to  the  formation  of  blood,  is  the 
mucous  surface  of  the  alimentary  canal,  for  it  is  from  this  organ  that 
the  new  materials  are  derived  which  maintain  the  quantity  and 
quahty  of  the  blood.  The  absorption  of  material  from  the  inter- 
stices of  the  tissues  and  from  the  serous  cavities  may  be  regarded  as 
an  act  of  resorption,  or  a  return  to  the  blood  of  liquid  nutritive  material 
which  has  escaped  through  the  walls  of  the  capillary  blood-vessels 
for  purposes  of  nutrition,  and  which,  if  not  returned,  would  lead  to 
an  accumulation  and  the  development  of  edematous  conditions. 

The  anatomic  mechanisms  involved  in  the  absorptive  process 
are,  primarily,  the  tissue  or  lymph-spaces,  the  lymph-  and  blood- 
capillaries;  secondarily,  the  lymph-vessels  and  the  veins.  • 

Tissue  or  Lymph-spaces;  Lymph-capillaries. — Everywhere 
throughout  the  body,  in  the  connective-tissue  system  and  in  the  inter- 
stices of  the  several  structures  of  which  an  organ  is  composed,  are 
found  spaces  or  clefts  of  irregular  shape  and  size,  determined  largely 
by  the  structure  of  the  organ  in  which  they  are  found,  which  have 
been  termed  tissue  or  lymph-spaces,  from  the  fact  that  they  contain 
a  clear  fluid,  the  lymph.  These  spaces  are  devoid  for  the  most  part  of 
any  endothehal  lining,  but  as  they  communicate  more  or  less  freely 
one  with  another,  there  is  a  circulation  of  lymph  through  them  and 
around  the  islets  of  tissue  (Fig.  86).  In  addition  to  the  connective- 
tissue  lymph-spaces,  different  observers  have  described  special  spaces 
or  clefts  in  organs  such  as  the  kidney,  liver,  spleen,  testicle,  and  in 
all  secreting  glands  between  their  basement  membrane  and  the  sur- 
rounding blood-vessels,  all  of  which  contain  a  greater  or  less  quan- 
tity of  lymph.  Within  the  brain,  spinal  cord,  bone,  and  other  tissues 
it  has  been  shown  that  the  smallest  blood-vessels  and  capillaries  are 
bounded  and  limited  by  a  cylindrical  sheath  containing  lymph,  which 
is  known  as  a  perivascular  lymph-space.  A  similar  sheath  surrounds 
the  smallest  nerve-bundles  and  fibers,  enclosing  a  perineural  lymph- 
space.  The  large  serous  cavities  of  the  body,  pleural,  peritoneal, 
pericardial,  etc.,  are  also  to  be  regarded  as  lymph-spaces.  The  sur- 
faces of  these  cavities,  however,  are  covered  with  a  layer  of   endo- 


TEXT-BOOK  OF  PHYSIOLOGY. 


thclial  cells  with  sinuous  margins.  At  intervals  between  these  cells 
are  to  be  found  small  free  openings  which  have  received  the  name 
of  stomata. 

The  lymph-capillaries  in  which  the  lymph-vessels  proper 
take  their  origin  are  arranged  in  the  form  of  plexuses  of  quite  irreg- 
ular shape.  In  most  situations  they  are  intimately  interwoven 
with  the  blood-vessels,  from  which  they  can  be  readily  distinguished 
by  their  larger  caliber  and  irregular  expansions.  The  wall  of  the 
lymph-capillary  is  formed  by  a  single  layer  of  endothelial  cells 
with  characteristic  sinuous  outlines.     These  capillaries  anastomose 

very  freely  one  with  an- 
other and  communicate, 
on  the  one  hand,  with 
the  lymph-spaces  and 
on  the  other  with  the 
lymph  -  vessels  proper. 
As  the  shape,  size,  etc., 
of  both  lymph -spaces 
and  capillaries  are  de- 
termined largely  by  the 
nature  of  the  tissue  in 
which  they  are  found,  it 
is  not  always  possible  to 
separate  one  from  the 
other.  Their  function, 
however,  may  be  re- 
garded as  similar:  viz., 
the  reception  and  collec- 
tion of  the  lymph  which 
has  transuded  through 
the  walls  of  the  blood- 
vessels and  its  transmis- 
sion onward  into  the 
regular  lymph- vessels. 
The  blood-capilla- 
ries not  only  permit  of  a  transudation  of  the  Hquid  nutritive  material 
from  the  blood  through  their  delicate  walls,  but  are  also  engaged,  if 
not  in  the  reabsorption  of  a  portion  of  this  transudate,  at  least  in  the 
absorption  of  waste  products  resulting  from  tissue  metabolism. 

Lymph-vessels. — The  lymph-vessels  constitute  a  system  of 
minute,  delicate,  transparent  vessels  found  in  nearly  all  the  organs 
and  tissues  of  the  body,  and  take  their  origin  from  the  lymph- 
capillaries  and  spaces  above  described.  From  their  origin  they 
gradually  converge  toward  the  trunk  of  the  body,  and  finally  empty 
into   the   thoracic    duct.      In   their   course    they    anastomose    very 


Fig.  86. — Origin  of  Lymphatics  from  the  Cen- 
tral Tendon  of  the  Diaphragm  Stained 
with  Nitrate  of  Silver,  s.  The  lymph- 
spaces  and  lymph-canals,  communicating  at  x 
with  the  lymphatics,  a.  Origin  of  the  lym- 
phatics by  the  confluence  of  several  juice 
canals.  B.  Capillary  blood-vessel. — {Landois 
and  Stirling.) 


ABSORPTION. 


223 


freely  with  adjoining  vessels.  The  diameter  of  a  lymph-vessel 
varies  from  i  to  2  mm.  After  the  lymphatics  have  emerged  from 
the  lymph-capillaries  they  acquire  three  distinct  coats,  each  of  which 
possesses  definite  histologic  features. 

The  internal  coat  is  composed  of  a  delicate  lamina  of  longitudinally 
disposed  elastic  fibers  covered  with  a  layer  of  flattened  nucleated 
endothelial  cells  with  wavy  outlines. 

The  middle  coat  consists  of  white  fibrous  tissue  arranged  longi- 


FiG.  87. — Lymph-vessels  and  Lymph-glands  of  the  Head  and  Neck. — (From 
Gould's  Dictionary.) 

tudinally  and  of  non-striated  muscle  and  elastic  fibers  arranged 
transversely. 

The  external  coat  consists  of  practically  the  same  structures,  though 
the  muscle-fibers  are  longitudinally  disposed. 

The  lymphatics  are  provided  with  valves  which  are  so  numerous 
and  located  at  such  short  intervals  as  to  give  the  vessels  a  beaded 
appearance.  These  valves  are  arranged  in  pairs  and  consist  of  two 
semilunar  folds  with  their  concavities  directed  toward  the  larger 
vessels.     They  are  formed  by  a  reduplication  of  the  Hning  membrane, 


224 


TEXT-BOOK  OF  PHYSIOLOGY. 


which  is  strengthened  by  fibrous  tissue  derived  from  the  middle 

coat. 

Lymph-glands. — In  their  course  toward  the  thoracic  duct  the 

lymph-vessels  pass  through  a  num- 
ber of  small  lenticular  bodies 
termed  lymph-glands.  These  are 
exceedingly  abundant  in  some  situ- 
ations, as  the  cervical,  axillary,  and 
inguinal  regions,  and  the  abdominal 
cavity.  As  the  lymph-vessels  ap- 
proach a  gland  they  divide  into  a 
number  of  branches  before  entering 
it,  known  as  the  afferent  vessels. 
From  the  opposite  side  of  the  gland 
the  lymphatics  again  emerge  as 
efferent  vessels  to  unite  to  form 
larger  trunks.  A  section  of  a  gland 
shows  that  it  consists  of  an  outer 
dense  cortical  and  an  inner  soft 
pulpy  medullary  portion.  Each 
gland  is  covered  externally  by  a 
dense  membrane  of  fibrous  tissue 
containing  in  its  meshes  non-stri- 
ated muscle-fibers.  From  the  inner 
surface  of  this  membrane  there  pass 
inward  septa  of  connective  tissue 
which,  as  they  converge  toward  the 
center  of  the  gland,  divide  the  outer 
zone  of  the  gland  into  small  conical 
compartments  or  alveoli.  When  the 
septa  reach  the  medullary  portion, 
they  subdivide  and  form  bands  or 
cords  which  interlace  in  every  direc- 
tion and  constitute  a  loose  mesh- 
work  the  spaces  of  which  communi- 
cate with  one  another  and  with  the 
alveoli  (Fig.  90).  Within  the  meshes 
of  this  framework  the  proper  gland 
substance  is  contained.  In  the  cor- 
tical compartments  it  is  moulded 
into  pear-shaped  masses;  in  the 
medullary  mesh  work  it  assumes  the 

form   of    rounded   cords   which  are  connected  with   one   another. 

In  both  regions,  however,  it  is  separated  from  the  septa  by  a  space 

termed  a  lymph  sinus,  through  which  the  lymph  flows  as  it  passes 


—Lymph-vessels 
Arm. — {Deaver. ) 


ABSORPTION. 


225 


through  the  gland.  The  lymph  sinus  is  crossed  by  a  network  of 
retiform  connective  tissue  which  offers  considerable  resistance  to 
the  passage  of  the  lymph.  The  gland  substance  consists  also  of  a 
framework  of  retiform  connective  tissue  in  the  meshes  of  which  large 
numbers  of  lymph-corpuscles  are  contained.  The  gland  substance 
is  separated  from  the  lymph  sinus  by  a  dense  layer  of  a  reticulum, 
which,  however,  does  not  prevent  lymph  and  even  corpuscles  from 
passing  through  it  into  the  lymph  sinus. 

The  lymph-glands  are  abundantly  supphed  with  blood-vessels. 
The  arteries  enter  the  gland  at  the  hilum,  penetrate  into  the  medullary 
substance,  and  terminate  in  a  line  capillary  plexus  which  is  supported 
by  the  connective  tissue.  The  veins  arising  from  this  plexus  leave 
the  gland  also  at  the  hilum. 


Fig.  89. — Diagrammatic  Section  of  a  Lymph-gland,  a.  I.,  Afferent,  e.  I.,  Efferent 
lymphatics.  C.  Cortical  substance.  M.  Reticular  cords  of  medulla.  /.  5.  Lymph 
sinus,     c.  Capsule,  with  trabeculas,  tr.- — (Landois  and  Stirling.) 


The  lymph-vessels  which  enter  a  gland  first  ramify  in  the  in- 
vesting membrane  and  then  open  directly  into  the  lymph  sinus. 
The  vessels  which  leave  the  gland  are  also  in  communication  with 
the  sinus.  After  the  lymphatics  enter  the  gland  they  lose  their 
external  and  middle  coats,  retaining  only  the  internal  or  endothehal 
coat,  which  lines  the  inner  surface  of  the  lymph  sinus.  The  current 
of  lymph,  therefore,  is  from  the  afferent  vessels  through  the  lymph 
sinus  into  the  efferent  vessels.  In  addition  to  this  primary  current, 
there  is  a  secondary  current  flowing  from  the  capillary  blood-vessels 
outward  and  into  the  sinus,  which  carries  with  it  large  numbers  of 
lymph-corpuscles.  It  is  quite  probable  that  the  movement  of  the 
15 


226 


TEXT-BOOK  OF  PHYSIOLOGY. 


lymph  through  this  compHcated  system  of  passages  is  aided  by  the 
contraction  of  the  muscle-fibers  in  the  capsule  of  the  gland. 

The  lymph-corpuscles  or  lymphocytes  originate  for  the  most  part 
in  the  gland  substance  of  the  cortical  alveoli.     In  this  situation  there 


Fig.  90. — Diagram  showing  the  Course  of  the  Main  Trunks  of  the  Absorbent 
System.  The  lymphatics  of  lower  extremities  (D)  meet  the  lacteals  of  intestines 
(LAC)  at  the  receptaculum  chyli  (RC),  where  the  thoracic  duct  begins.  The 
superficial  vessels  are  shown  in  the  diagram  on  the  right  arm  and  leg  (S),  and 
the  deeper  ones  on  the  arm  to  the  left  (D).  The  glands  are  here  and  there  shown 
in  groups.  The  small  right  duct  opens  into  the  veins  on  the  right  side.  The 
thoracic  duct  opens  into  the  union  of  the  great  veins  of  the  left  side  of  the  neck 
(T). — {Yeo's  "  Text-book  of  Physiology.") 


are  groups  of  cells,  so-called  germ  centers,  which  divide  very  rapidly 
by  mitosis  and  give  rise  constantly  to  groups  of  young  cells  which 
soon  find  their  way  into  the  lymph  stream. 


ABSORPTION.  227 

The  Thoracic  Duct. — The  thoracic  duct  is  the  general  trunk  of 
the  lymph  system,  into  which  the  vessels  of  the  lower  extremities, 
of  the  abdominal  organs,  of  the  trunk,  of  the  left  arm,  and  of  the  left 
side  of  the  head  empty  their  contents.  It  is  about  fifty  centimeters 
in  length  and  four  millimeters  in  diameter.  It  extends  upward 
from  the  third  lumbar  vertebra  along  the  vertebral  column  to  the 
seventh  cervical  vertebra,  where  it  empties  into  the  venous  system 
at  the  junction  of  the  internal  jugular  and  subclavian  veins  on  the  left 
side.  The  thoracic  duct  wall  has  the  same  general  structure  as  the 
wall  of  the  lymph-vessel:  viz.,  an  internal  or  endothelial;  a  middle 
elastic  and  muscular;  an  external  or  fibrous.  It  is  also  provided 
with  numerous  valves. 

The  lymph-vessels  of  the  right  side  of  the  head,  of  the  right  arm, 
and  a  portion  of  the  right  side  of  the  trunk  terminate  in  the  right 
thoracic  duct,  which  is  about  25  to  30  mm.  in  length  and  which  emp- 
ties into  the  venous  system  at  the  junction  of  the  internal  jugular  and 
subclavian  veins  on  the  right  side.  The  general  arrangement  of  the 
lymphatic  system  is  diagrammatically  shown  in  Fig.  90. 


LYMPH. 

Lymph  is  the  clear  fluid  found  within  the  tissue  spaces  and  with- 
in the  lymph-vessels.  Inasmuch  as  there  are  reasons  for  the  view 
that  lymph  varies  in  composition,  as  well  as  in  function,  in  these 
different  regions  it  will  be  found  conducive  to  clearness  to  designate 
the  lymph  found  in  the  tissue  spaces  as  intercellular  lymph,  and  that 
found  in  the  lymph-vessels  as  intravascular  lymph. 

The  Physical  Properties  of  Lymph. — Whether  obtained  from 
tissue  spaces  or  from  lymph-vessels,  the  lymph  presents  practically 
the  same  physical  properties.  The  lymph  obtained  from  the  tho- 
racic duct  during  the  intervals  of  digestion  or  from  one  of  the  large 
trunks  of  the  leg  is  a  clear,  colorless  or  slightly  opalescent  fluid  hav- 
ing an  alkaline  reaction  and  a  specific  gravity  of  1.020  to  1.040. 
Examined  microscopically  it  is  seen  to  hold  in  suspension  a  large 
number  of  corpuscles  similar  to  those  seen  in  the  lymph  glands  and 
to  the  white  corpuscles  of  the  blood.  Their  number  has  been  esti- 
mated at  about  8200  per  cubic  millimeter,  though  this  count  will 
vary  within  wide  Hmits  according  as  the  lymph  examined  has  passed 
through  a  large  or  smaller  number  of  glands.  The  lymph  corpuscle 
consists  of  a  small  quantity  of  protoplasm  in  which  is  embedded  a 
distinct  nucleus.  Some  of  these  lymphocytes  contain  distinct  gran- 
ules, more  or  less  refractive,  which  impart  to  the  corpuscle  a  dis- 
tinctly granular  appearance.  When  withdrawn  from  the  vessels 
lymph  undergoes  a  spontaneous  coagulation,  though  the  coagulum 
is  never  as  firm  as  that  observed  in  the  coagulation  of  the  blood. 


228  TEXT-BOOK  OF  PHYSIOLOGY. 

The  cause  of  the  coagulation  is  the  appearance  of  librin.  After  a 
variable  length  of  time  the  coagulum  separates  into  a  liquid  and  a 
solid  portion,  the  serum  and  the  clot. 

The  Chemic  Composition  of  Lymph. — Although  the  lymph 
obtained  from  the  tissue  spaces,  from  the  lymph-vessels,  as  well  as 
from  the  so-called  serous  cavities  has  the  same  general  chemic  char- 
acteristics, there  is  reason  for  the  view  that  it  varies  in  its  ultimate 
composition  according  as  it  is  derived  from  one  region  of  the  body 
or  from  another.  The  needs  of  any  individual  tissue  as  well  as  the 
character  of  its  metabolic  products  will  in  all  probability  not  only 
change  its  normal  composition,  but  also  the  relative  amounts  of  its 
normal  constituents. 

Chemic  analysis  has  shown  that  the  lymph  from  the  thoracic  duct 
contains  from  3.4  to  4.1  per  cent,  of  proteids  (serum-albumin,  fibrin- 
ogen), 0.046  to  0.13  per  cent,  of  substances  soluble  in  ether  (probably 
fat),  0.1  per  cent,  of  sugar,  and  from  0.8  to  0.9  per  cent,  of  inorganic 
salts,  of  which  sodium  chlorid  (0.55  per  cent.)  and  sodium  carbonate 
(0.24  per  cent.)  are  the  most  abundant  (Munk).  There  are  usually 
in  most  specimens  small  quantities  of  potassium,  calcium,  and  mag- 
nesium salts.  Fibrinogen  is  seldom  present  beyond  o.i  per  cent., 
which  will  account  for  the  feeble  and  slow  coagulation.  Lymph 
contains  both  free  oxygen  and  carbon  dioxid.  Of  the  former,  how^- 
ever,  there  is  but  a  small  percentage;  of  the  latter,  about  45  vols,  per 
cent.,  partially  in  the  free  state  and  partially  combined  with  sodium. 
Urea  is  also  present  in  very  small  amounts.  This  analysis  indicates 
that  lymph  resembles  blood-plasma  in  the  character  of  its  constitu- 
ents, though  their  relative  quantities  vary  considerably.  With  the 
exception  that  it  contains  no  red  corpuscles,  lymph  may  be  regarded 
as  a  diluted  blood. 

The  Production  of  Lymph. — Though  blood  is  the  common  res- 
ervoir of  nutritive  material,  the  latter  is  not  available  for  nutritive 
purposes  as  long  as  it  is  confined  within  the  blood-vessels.  The 
capillary  wall,  thin  as  it  is,  and  composed  of  but  a  single  layer  of 
endothehal  cells,  would  be  sufficient  to  prevent  its  utilization  by  the 
tissues,  if  it  were  not  permeable  to  the  liquid  portion  of  the  blood. 
As  this  is  the  case,  however,  it  is  found  that  as  the  blood  flows 
through  the  capillary  vessels  a  portion  of  the  blood-plasma  passes 
through  the  capillary  wall  and  is  received  into  the  tissue  spaces, 
where  it  comes  into  intimate  contact  with  the  tissue-cells. 

The  forces  concerned  in  the  passage  of  the  constituents  of  the 
blood-plasma  across  the  capillary  wall  have  been  the  subject  of  much 
investigation.  According  to  some  investigators,  diffusion,  osmosis 
and  filtration  are  sufficient  to  account  for  all  the  phenomena.  It  is 
assumed  tha  the  capillar}^  wall,  being  an  animal  membrane,  is  freely 
permeable  to  water  and  crystalloid  bodies  generally ;  less  so,  however, 


ABSORPTION.  229 

to  colloid  bodies,  such  as  the  proteids  of  the  blood-plasma;  moreover, 
it  is  further  assumed  that  the  physiologic  conditions  of  the  capillary 
walls  are  such  as  not  only  to  permit  of  the  passage  of  the  constituents 
of  the  blood  into  the  tissue  spaces,  but  also  the  passage  of  the  con- 
stituents of  the  inter-cellular  l}Tnph  into  the  blood,  according  to 
laws  similar  at  least  to  those  determining  the  passage  of  substances 
through  animal  membranes  as  determined  experimentally.  The 
force  giving  rise  to  filtration  is  the  difference  of  pressure  between 
that  exerted  by  the  blood  within  the  capillar}^  vessels  and  that  ex- 
erted by  the  fluid  in  the  tisssue  spaces ;  hence  any  increase  or  decrease 
of  this  difference  of  pressure  is  attended  by  an  increase  or  decrease 
in  the  production  of  lymph.  Thus  compression  of  the  veins  of  a  part 
which  interferes  with  the  outflow  of  blood  from  the  capillaries,  or  a 
dilatation,  of  the  arterioles  which  increases  the  inflow  of  blood  to 
them  will  increase  the  capillary  pressure  and  therefore  the  production 
of  l\Tnph.  The  reverse  conditions  will,  of  course,  diminish  the 
intra- capillar}'  pressure  and  lymph  production.  Hemorrhages  which 
lower  the  general  blood-pressure  may  so  lower  the  capillar)^  pressure 
as  not  only  to  stop  the  flow  of  hrniph  to  the  tissues,  but  may  give 
rise  to  a  diffusion  current  from  the  tissues  into  the  blood. 

The  quantitative  composition  of  the  lymph  compared  with  that 
of  the  blood  indicates  that  it  is  produced  by  difl'usion,  osmosis,  and 
filtration.  In  the  lymph  the  concentration  of  the  inorganic  salts  is 
practically  the  same  as  in  the  blood;  the  concentration  of  the  pro- 
teids, however,  is  somewhat  less.  These  facts  are  in  accordance  with 
what  is  known  regarding  the  diffusibihty  of  both  crystalloids  and 
colloids  through  animal  membranes. 

According  to  other  investigators,  the  production  of  lymph  is  not 
so  much  due  to  intracapillary  pressure  as  it  is  to  the  speciahzed 
activities  of  the  endothelial  cells,  activities  which  indicate  that  lymph 
is  a  secretion  the  composition  of  which  varies  in  different  situations 
by  virtue  of  a  difference  in  the  molecular  structure  of  the  endothehal 
cells.  As  is  the  case  with  many  of  the  secreting  cells  of  the  body, 
the  injection  of  various  substances  into  the  blood  apparently  increases 
the  activity  of  the  endothehal  cells,  as  shown  by  an  increased  lymph 
production  without  any  appreciable  increase  of  intracapillary  pressure. 
Thus  it  has  been  shown  that  after  the  injection  into  the  blood  of 
sugar,  sodium  chlorid,  sodium  sulphate,  urea,  etc.,  there  is  an  increase 
in  the  flow  of  lymph  from  the  thoracic  duct.  The  lymph,  however, 
under  these  circumstances  is  richer  in  water  than  is  normally  the 
case.  As  the  blood  at  the  same  time  increases  its  percentage  of 
water,  it  is  assumed  that  the  water  is  extracted  from  the  tissues,  by 
reason  of  an  increased  percentage  of  salts  in  the  tissue  spaces  due  to 
increased  activity  of  the  endothehal  cells.     A  higher  percentage  of 


230  TEXT-BOOK  OF  PHYSIOLOGY. 

salts  in  the  lymph  than  in  the  blood  is  difficult  to  account  for  on  the 
diffusion-filtration  theory.  The  injection  of  peptones,  albumin,  the 
extract  of  the  muscles  of  the  leech,  crab,  mussel,  etc.,  is  also  followed 
by  an  increase  in  the  amount  of  lymph  discharged  from  the  thoracic 
duct;  but  in  this  instance  the  lymph  possesses  a  higher  degree  of  con- 
centration, being  richer  not  only  in  inorganic  but  also  organic  constit- 
uents. The  cause  of  this  increase  in  both  the  quantity  and  quality 
of  the  lymph  is  believed  to  be  an  increased  activity  in  the  secreting 
power  of  the  endothelial  cells.  The  more  recent  experiments  of 
Starhng  indicate  that  in  addition  to  the  difference  of  pressure  be- 
tween the  blood  in  the  capillaries  and  the  l3miph  in  the  tissue  spaces, 
a  new  factor  must  be  considered  and  that  is,  the  permeability  of  the 
capillary  wall.  This  he  finds  to  vBiVy  considerably  in  different  parts 
of  the  vascular  apparatus,  being  greatest  in  the  capillaries  of  the  liver, 
less  in  the  capillaries  of  the  intestines  and  least  in  the  capillaries  of 
the  extremities.  It  also  varies  doubtless  in  all  other  situations.  The 
increase  in  the  production  of  lymph  by  the  injection  of  peptones, 
extract  of  muscles  of  the  leech,  the  crab,  etc..  Starling  explains  by  the 
assumption  that  these  substances  alter  the  properties  of  the  capillary 
wall  and  thus  increase  its  permeabihty.  The  difference  of  pressure, 
therefore,  between  blood  and  lymph  taken  in  connection  with  the 
degree  of  permeability  of  the  capillary  wall  will  account  for  the  pro- 
duction of  lymph  in  all  regions  of  the  body.  It  is  possible  that  all 
these  facts  may  be  otherwise  interpreted;  the  subject  is  yet  a  matter 
of  investigation. 

The  Functions  of  Intercellular  Lymph. — The  origin  and 
composition  of  lymph,  its  situation  and  relation  to  the  tissue  cells 
indicate  that  its  function  is  to  provide  the  tissue  cells  with  those 
nutritive  materials  necessary  to  their  growth,  repair,  and  functional 
activities,  and  to  receive  from  the  tissue  cells  their  waste  products 
prior  to  their  removal  by  the  blood-  and  lymph-vessels. 

The  necessity  for  the  production  of  lymph  becomes  apparent 
when  the  chemic  changes  which  the  tissues  undergo  at  all  times  are 
considered.  Thus  whether  in  a  state  of  relative  rest  or  in  a  state  of 
activity,  disintegrative  changes  are  constantly  taking  place  and  al- 
ways in  direct  proportion  to  the  degree  and  continuance  of  the  activity. 
If  the  tissues  are  to  continue  in  the  performance  of  their  customary 
activities,  it  is  essential  that  repair  and  restoration  be  at  once  estab- 
lished. This  is  made  possible  by  the  presence  of  lymph,  and  by  the 
power  which  living  material  possesses  of  absorbing  from  the  lymph 
the  necessary  nutritive  materials,  of  assimilating  them  and  transform- 
ing them  into  material  like  unto  itself  and  endowing  them  with  their 
own  physiologic  properties. 

Coincidently  with  the  loss  of  nutritive  material,  the  lymph  receives 
the  waste  products  of  the  tissues  and  hence  changes  in  composition. 


ABSORPTION.  231 

Should  this  change  in  composition  continue  for  any  length  of  time, 
the  lymph  would  lose  its  restorative  character  and  become  destruc- 
tive to  tissue  vitality.  Therefore  it  is  essential  that  the  nutritive 
material  be  renewed  as  rapidly  as  consumed  and  the  waste  products 
be  carried  away  as  rapidly  as  produced.  Both  these  conditions 
are  fulfilled  by  the  blood-  and  lymph- vessels. 

The  Absorption  of  Intercellular  Lymph. — From  the  fact  that 
lymph  is  being  discharged  from  the  thoracic  duct  into  the  blood, 
more  or  less  continually,  it  is  evident  that  hnnph  is  being  absorbed 
from  the  intercellular  spaces;  from  which  fact  it  may  be  inferred 
that  the  production  of  lymph  is  a  continuous  process  and  that  it  is 
passing  through  the  capillaries  in  amounts  greater  than  is  necessary 
for  the  immediate  needs  of  the  tissues.  Should  this  excess  accumu- 
late there  would  soon  arise  the  condition  of  edema  and  an  interference 
with  the  functional  activities  of  the  tissues.  Therefore  so  soon  as 
the  accumulation  attains  a  certain  volume  it  is  absorbed  in  large 
measure  by  the  lymph  capillaries  and  transmitted  to  the  lymph- 
vessels  and  thoracic  duct.  Because  of  the  general  behef  that  the 
lymph  capillaries  were  in  open  communication  with  the  tissue  spaces 
it  was  assumed  that  the  absorption  of  lymph  and  its  flow  through 
the  lymph-vessels  was  the  result  of  a  difference  of  pressure  between 
the  lymph  in  the  tissue  spaces  and  the  blood  in  the  innominate  veins. 
But  if  the  lymph  capillaries  are  closed  vessels,  as  recent  investiga- 
tions indicate,  then  additional  factors,  in  explanation  of  lymph  ab- 
sorption, must  be  sought  for. 

It  is  quite  possible  under  even  normal  conditions  of  pressure  in 
the  tissue  spaces  that  some  of  the  more  dilfusible  constituents  of  the 
lymph  should  be  absorbed  by  the  capillary  blood-vessels.  As  to 
whether  the  relatively  feebly  diffusible  colloids  should  be  so  resorbed 
is  as  yet  a  matter  of  investigation. 


ABSORPTION  OF  FOODS. 

The  most  important  of  the  absorbing  surfaces,  especially  in  its 
relation  to  the  absorption  of  new  material,  is  the  mucous  membrane 
of  the  ahmentary  canal,  and  more  particularly  that  portion  lining  the 
small  intestine,  provided  as  it  is  with  specialized  absorbing  structures 
— the  villi.  Though  certain  substances  can  be  absorbed  from  the 
mouth,  it  is  not  probable  that  any  food  is  so  absorbed.  From  the 
changes  which  the  food  principles  undergo  in  the  stomach  it  might 
naturally  be  inferred  that  their  absorption  would  promptly  follow. 
Experimental  researches  have  demonstrated,  however,  that  this  takes 
place,  if  at  all,  but  to  a  shght  extent.  If,  however,  solutions  of  inor- 
ganic salts,  sugars,  and  peptones  possessing  a  concentration  of  at 
least  5  per  cent. — a  degree  of  concentration  seldom  realized  under 


232 


TEXT-BOOK  OF  PHYSIOLOGY. 


normal  conditions — are  introduced  into  the  stomach,  their  absorption 
will  be  effected,  the  rate  of  absorption  following  in  a  general  way  the 
increase,  within  limits,  in  concentration.  Water  is  practically  not 
absorbed  from  the  stomach.  The  absorption  of  the  products  of 
digestion — i.  e.,  dextrose,  levulose,  peptones,  soaps,  glycerin,  fatty 
acids,  salts,  along  with  water,  in  which  for  the  most  part  they  are 


Fig.  91. — Longitudinal  Sec- 
tion OF  A  Villus  from  In- 
testine OF  the  Dog,  Highly 
Magnified.  a.  Columnar 
epithelium  containing  goblet- 
cells  (b)  and  migratory  leuko- 
cytes (h).  c.  Basement  mem- 
brane, d.  Plate-like  connec- 
tive-tissue elements  of  core. 
e,  e.  Blood-vessels.  /.  Ab- 
sorbent radical  or  lacteal. — 
(Piersol.) 


Fig.  92. — Section  of  Injected  Small 
Intestine  of  Cat.  a,  b.  Mucosa. 
g.  Villi,  i.  Their  absorbent  vessels. 
h.  Simple  follicles,  c.  Muscularis  mu- 
cosae. 7.  Submucosa.  g,  e'.  Circular 
and  longitudinal  layers  of  muscle. 
/.  Fibrous  coat.  All  the  dark  lines 
represent  blood-vessels  filled  with  the 
injection  mass. — (Piersol.) 


held  in  solution — is  therefore  hmited  very  largely  to  the  small  intes- 
tine, and  is  accomphshed  by  the  villous  processes  projecting  from  the 
surface  of  the  mucous  membrane. 

Structure  of  the  Villi. — The  vilh  are  small  fiHform  or  conical 
processes,  from  0.5  to  i  mm.  in  length,  and  from  0.2  to  0.5  mm.  in 


ABSORPTION. 


233 


within  the  basement 


breadth,  covering  the  surface  of  the  mucous  membrane  from  the 
pyloric  orifice  to  the  upper  surface  of  the  ileo-cecal  valve.  Each 
villus  consists  of  a  basement  membrane  (see  Fig.  91)  supporting  tall 
columnar  epithelial  cells.  Each  cell  is  composed  of  granular  bio- 
plasm containing  a  distinct  nucleus.  At  its  free  extremity  a  narrow 
border  of  the  cell  presents  a  striated  appearance,  as  if  it  were  com- 
posed of  small  rods  embedded  in  some  cement  substance.  Goblet 
or  mucin-holding  cells  are  also  to  be  found  among  the  columnar 
cells.  The  body  of  the  villus,  that  portion 
membrane,  consists  of  a  reticu- 
lated connective  tissue  support- 
ing arteries,  capillaries,  veins, 
and  lymphoid  corpuscles.  In 
the  center  of  the  villus  there  is 
usually  a  single  though  at  times 
a  double  club-shaped  lymph- 
capillary,  the  walls  of  which  are 
composed  of  epithelioid  cells 
with  sinuous  margins.  This 
capillary  probably  begins  by  a 
blind  extremity  and  opens  at 
the  base  of  the  villus  into  the 
subjacent  lymph -vessels.  The 
communicating  orifice  is  guard- 
ed by  a  valve.  It  is  also  sur- 
rounded by  a  layer  of  non-stri- 
ated muscle-fibers,  arranged 
longitudinally,  derived  from  the 
muscularis  mucosae  and  at- 
tached to  the  apex  of  the  body 
of  the  villus. 

The  arteries  which  penetrate 
the  villi  are  derived  from  those 
of  the  submucous  coat  of  the 
intestine,  which  are  the  ultimate 
branches  of  the  intestinal  artery, 
and  serve  the  purpose  of  dehv- 

ering  nutritive  material  to  the  capillary  plexus  (Fig.  92).  While 
passing  through  the  latter  a  portion  of  the  blood-plasma  transudes 
through  the  capillary  walls  into  the  spaces  of  the  reticulated  tissue, 
constituting  lymph.  At  the  same  time  products  of  tissue  metab- 
olism pass  through  the  capillary  walls  into  the  blood.  The  blood 
then  passes  into  the  venules,  which,  leaving  the  villus  at  its  base, 
unite  with  the  veins  of  the  submucous  coat  to  form  the  intestinal 
veins.     These  finally  unite  with  the  gastric  and  splenic  veins  to  form 


Fig, 


93. — Diagram  of  the  Portal 
Vein  {pv)  arising  in  the  Alimen- 
tary Tract  and  Spleen  (5),  and 
Carrying  the  Blood  from  These 
Organs  to  the  Liver. — (  Yeo's 
"  Text-book  of  Physiology.")     . 


234  TEXT-BOOK  OF  PHYSIOLOGY. 

the  portal  vein,  which  enters  the  hver  at  the  transverse  fissure  (Fig. 
93).  The  excess  of  lymph  within  the  villus  passes  into  the  club- 
shaped  lymph-capillary,  to  be  finally  carried  by  the  lymphatics 
of  the  mesentery  into  the  thoracic  duct.  During  the  intervals  of 
digestion  and  in  the  absence  of  food  from  the  intestine  there  is,  of 
course,  no  absorption  of  food  nor  the  removal  from  the  villus  of 
anything  but  the  excess  of  lymph  and  metabohc  products. 

Function  of  the  Villi. — The  viUi,  and  especially  the  epithelial 
cells  covering  them,  are  the  essential  agents  in  the  absorption  of  the 
products  of  digestion.  It  is  by  the  activity  of  these  cells  that  the 
new  materials  are  taken  out  of  the  ahmentary  canal  and  transferred 
into  the  lymph-spaces,  in  the  body  of  the  vilh,  from  which  they  are 
subsequently  removed  by  the  blood-vessels  and  lymphatics.  As  to 
the  mechanism  by  which  the  epithelial  cells  accomphsh  this  result, 
nothing  definite  can  be  asserted.  Inasmuch  as  the  absorption  of 
food  does  not  take  place  in  accordance  with  the  laws  of  osmosis  as  at 
present  understood,  it  has  been  suggested  that  the  cells  possess  a 
"selective  action"  dependent  on  their  organization  and  living  con- 
dition, an  action  which  is  to  a  great  extent  conditioned  and  limited 
by  the  degree  of  diffusibihty  of  the  substances  to  be  absorbed. 

Absorption  0}  Water  and  Inorganic  Salts. — Water  and  inorganic 
salts  after  their  absorption  from  the  intestine  and  transference  into 
the  lymph-spaces  of  the  villi  pass  through  the  walls  of  the  capillary 
blood-vessels  and  are  carried  by  the  way  of  the  portal  vein  into  the 
liver.  Unless  water  be  present  in  excessive  amounts,  there  is  no 
appreciable  absorption  of  water  by  the  lymphatics. 

Absorption  0}  Sngar. — As  previously  stated,  all  the  carbohydrates, 
with  the  exception  possibly  of  lactose,  are  transformed  by  the  diges- 
tive fluids  into  either  dextrose  or  levulose,  under  which  forms  they 
are  absorbed  by  the  epithelial  cells.  It  is  possible,  however,  that 
soluble  dextrin  may  also  be  absorbed.  Whatever  the  form  under 
which  the  carbohydrates  are  absorbed,  they  never  leave  the  epithe- 
lial cells  except  as  dextrose  and  levulose.  Direct  experimentation 
has  shown  that  the  sugars  are  taken  up  by  the  capillary  blood-vessels 
and  carried  direct  to  the  liver.  Analysis  of  the  blood  of  the  portal 
vein  after  the  ingestion  of  large  quantities  of  sugar  may  reveal  an 
increase  of  0.2 5  per  cent.;  while  after  the  injection  of  sugar  into  the 
intestine  the  percentage  may  rise  as  high  as  0.4.  As  chemic  analysis 
of  lymph  obtained  from  the  thoracic  duct  shows  no  increase  in  the 
percentage  of  sugar  beyond  that  normally  present  (o.i  per  cent.),  it 
is  assumed  that  sugar  is  not  removed  from  the  vilh  by  the  lymphatics. 

Absorption  of  Proteids. — Since  most  of  the  proteidsare  transformed 
through  hydration  and  cleavage  by  the  action  of  the  gastric  and  pan- 
creatic enzymes  into  peptones,  there  is  reason  to  believe  that  this 
change  is  necessary  to  their  complete  and  rapid  absorption.     Never- 


ABSORPTION.  235 

theless  it  has  been  shown  by  the  resuhs  of  experimentation  that 
unchanged  native  proteids,  such  as  egg-albumin,  and  partially 
digested  proteids,  such  as  acid  and  alkah  albumin,  albumoses,  may 
hkewise  be  absorbed  from  the  smah  intestine,  though  in  far  less 
amounts.  It  has  also  been  demonstrated  that  native  proteids  can 
be  absorbed  from  the  large  intestine.  Inasmuch  as  chemic  analysis 
has  failed  to  detect  more  than  a  trace  of  either  peptone  or  native  pro- 
teid  in  the  portal  blood  or  in  the  lymph  of  the  thoracic  duct,  it  must 
be  assumed  that  the  epithehum  after  absorbing  must  also  synthetize 
them  into  some  form  of  coagulable  proteid  (serum-albumin)  which  is 
readily  assimilable  by  the  blood.  That  such  a  reconversion  is  neces- 
sary would  appear  from  the  fact  that  the  introduction  of  peptones 
even  in  small  amounts  into  the  blood  is  followed  by  their  elimination 
unchanged  in  the  urine.  When  injected  into  the  blood  in  large 
amounts,  they  act  as  toxic  agents,  giving  rise  to  a  fall  of  blood-pres- 
sure, a  diminished  coagulability  of  the  blood,  coma,  and  death. 

After  passing  through  the  epithelium  into  the  spaces  of  the  villi 
they  are  removed  by  the  blood-vessels  and  carried  direct  to  the  liver. 
Even  though  there  is  no  appreciable  increase  in  the  amount  of  pro- 
teid in  the  portal  blood  during  digestion,  there  is  every  reason  to 
think  that  this  is  the  route  by  which  it  reaches  the  general  circulation. 
Ligation  of  the  thoracic  duct  does  not  interfere  with  proteid  absorp- 
tion nor  with  the  normal  elimination  of  urea  nor  with  the  weight  of 
the  animal. 

Absorption  0}  Fat. — As  previously  stated,  there  are  two  views  as 
to  the  changes  which  fats  undergo  during  digestion.  According  as 
the  one  or  the  other  is  accepted  will  depend  the  view  as  to  the  nature 
of  the  absorptive  process.  If  it  be  assumed  that  the  final  stage  in 
the  digestion  of  fat  is  a  purely  physical  one,  the  production  of  an 
emulsion  in  which  the  fats  present  themselves  as  fine  granules,  it  is 
diflficult  to  give  any  satisfactory  explanation  of  the  mechanism  by 
which  the  epithehal  cells  take  them  up.  Various  theories  have  been 
advanced  to  explain  the  process,  but  none  are  free  from  serious  ob- 
jections. This  view  of  fat  absorption  has  largely  been  based  on  the 
observation  that  during  digestion  fatty  granules  can  be  seen  in  all 
portions  of  the  cell  apparently  passing  toward  the  interior  of  the 
villus.  If,  on  the  contrary,  it  be  admitted  that  the  final  stage  in  the 
digestion  of  fats  is  the  formation  of  soaps  and  glycerin,  both  of  which 
are  soluble,  their  absorption  can  more  readily  be  accounted  for. 
According  to  this  view,  the  soaps  and  glycerin  are  again  synthetized 
by  a  process  the  reverse  of  that  which  is  produced  by  the  pancreatic 
enzyme,  with  the  appearance  of  minute  granules  of  fat.  That  this 
is  the  more  probable  view  as  to  the  mechanism  of  fat  absorption  is 
evident  from  the  fact  that  when  animals  are  fed  with  alkaline  soaps 


236  TEXT-BOOK  OF  PHYSIOLOGY. 

and  glycerin,  or  with  fatty  acids  alone,  globules  of  fat  are  found  in 
the  epithehal  cells  and  in  the  interior  of  the  villus. 

With  the  passage  of  the  fat-granules  into  the  interior  of  the  villus 
they  at  once  enter  the  lymph-radicle  and  become  constituents  of 
the  lymph-stream,  to  which  they  impart  a  white,  milky  appearance. 
If  the  abdomen  of  an  animal  in  full  digestion  be  opened,  the  lymph- 
vessels  of  the  mesentery  present  themselves  as  distinct  white  threads. 
An  examination  of  the  fluid  they  contain,  known  as  chyle,  shows  the 
presence  of  fat-granules  of  microscopic  size.  With  the  passage  of 
the  chyle  into  the  thoracic  duct  it  also  presents  the  same  milky  ap- 
pearance. For  this  reason  the  lymphatics  of  the  mesentery  were 
erroneously  termed  lacte.als.  The  chyle  as  obtained  from  these 
lymph- vessels  possesses  the  same  qualitative  though  not  quantitative 
composition  as  lymph,  the  difference  being  mainly  in  the  large  excess 
of  fat  in  the  former.  Indeed,  chyle  may  be  regarded  as  lymph  plus 
fat. 

Routes  for  the  Absorbed  Food. — Physiologic  experiments  have 
demonstrated  that  the  agents  concerned  in  the  removal  of  the  products 
of  digestion  after  their  absorption  from  the  interior  of  the  villus  are: 

1,  The  blood-vessels  of  the  gastro-intestinal  tract,  which  unite  to 

form  the  portal  vein. 

2.  The  lymph- vessels  of  the  small  intestine,  which  converge  to  empty 

into  the  thoracic  duct. 

The  products  of  digestion  find  their  way  into  the  general  circu- 
lation by  these  two  routes,  as  follows : 

The  water,  inorganic  salts,  proteids,  and  sugar  after  entering  the 
blood-vessels  of  the  villus  are  carried  by  the  blood  directly  into  the 
hver  by  the  portal  vein;  after  circulating  through  the  capillaries  of 
the  Hver  and  being  influenced  by  the  hver  cells,  they  are  discharged 
by  the  hepatic  veins  into  the  ascending  vena  cava. 

The  fats  after  entering  the  lymph-radicle  of  the  villus  are 
carried  by  the  lymph-stream  into  the  thoracic  duct,  by  which  they  are 
poured  into  the  blood  at  the  junction  of  the  left  subclavian  and  in- 
ternal jugular  veins. 

Forces  Aiding  the  Movement  of  Lymph  and  Chyle. — The 
force  which  primarily  determines  the  movement  of  the  lymph  has  its 
origin  in  the  beginnings  of  the  lymph- vessels,  the  lymph- spaces, 
and  depends  on  a  difference  in  pressure  here  and  at  the  termination 
of  the  thoracic  duct.  The  rise  of  pressure  in  the  lymph-spaces  is 
due  to  the  continual  production  of  lymph,  either  by  filtration  or 
secretory  activity  of  the  capillary  walls.  As  soon  as  the  pressure 
rises  above  that  in  the  thoracic  duct  a  forward  movement  of  lymph 
takes  place.  Other  things  being  equal,  the  rate  of  movement  will  be 
proportional  to  the  difference  of  pressure.  The  first  movement  of 
the  chyle,  its  passage  from  the  lymph-capillary  in  the  villus  into 


ABSORPTION.  237 

the  subjacent  lymph- vessel,  has  been  attributed  to  a  shortening 
of  the  villus  and  a  compression  of  the  capillary  by  the  contraction  of 
the  non-striated  muscle-fibers  by  which  it  is  surrounded.  With 
the  entrance  of  the  chyle  into  the  subjacent  lymph-vessel  there  is 
a  distention  of  the  vessel  and  a  rise  in  pressure.  When  the  muscle- 
fibers  relax,  regurgitation  is  prevented  by  the  closure  of  the  valves  at 
the  base  of  the  villus.  The  elastic  tissue  of  the  lymph-vessel  now  re- 
coils and  forces  the  chyle  toward  the  thoracic  duct.  After  the  empty- 
ing of  the  lymph-capillary  the  conditions  as  far  as  pressure  is  con- 
cerned are  favorable  to  the  absorption  of  new  material.  The 
rhythmic  contractions  of  the  intestinal  wall  undoubtedly  aid  in  the 
movement  of  lymph  and  chyle. 

It  is  quite  possible  that  the  walls  of  the  general  lymphatic  system 
aid  the  forward  movement  of  lymph  by  more  or  less  rhythmic  con- 
tractions of  their  contained  muscle-fibers. 

Inasmuch  as  the  lymph- vessels  lie  in  situations  in  which  they 
are  subject  to  compression  by  muscles  during  contraction,  it  is  prob- 
able that  the  fluid  in  the  vessels  will  be  forced  onward  toward  the 
thoracic  duct  at  each  compression,  a  backward  movement  being 
prevented  by  the  closure  of  the  valves  which  are  everywhere  present 
in  the  vessels.  Experimental  observations  have  demonstrated  the 
truth  of  this  supposition.  Alternate  contraction  and  relaxation  of 
the  muscles  of  the  leg  will,  in  an  animal  at  least,  increase  considerably 
the  flow  as  well  as  the  production  of  lymph  from  the  thoracic  duct. 
Massage  has  a  similar  influence.  The  respiratory  movements  also 
aid  the  flow  of  both  lymph  and  chyle  from  the  thoracic  duct  and 
larger  lymph-vessels  into  the  venous  blood.  During  inspiration  the 
negative  pressure  of  the  thorax  is  increased,  the  increase  being  pro- 
portional to  the  extent  of  the  inspiration.  The  positive  pressure  of 
the  air  within  the  lungs  on  the  thoracic  structures,  venae  cavae,  thoracic 
duct,  etc.,  being  at  the  same  time  diminished,  there  is  an  expansion  of 
and  a  fall  of  pressure  in  the  thoracic  duct  and  venae  cavae.  As  the 
lymph  in  the  abdominal  portion  of  the  thoracic  duct  is  subjected  to 
the  higher  intra-abdominal  pressure,  its  contents  are  forced  ener- 
getically toward  the  end  of  the  thoracic  duct.  During  expiration 
the  reverse  conditions  obtain.  As  the  negative  pressure  diminishes 
and  the  positive  intrapulmonary  pressure  increases  the  upper  part  of 
the  thoracic  duct  is  compressed  and  the  lymph  is  forced  into  the 
subclavian  vein  at  its  junction  with  the  internal  jugular.  Regurgita- 
tion here  is  prevented  by  a  closure  of  the  valves. 


CHAPTER  XL 
THE  BLOOD. 

The  blood  is  a  highly  complex  nutritive  fluid,  the  presence  and 
proper  circulation  of  which  in  the  living  organism  are  essential  to  the 
maintenance  and  activity  of  all  physiologic  processes.  The  escape 
of  the  blood  from  the  vessels,  especially  in  the  higher  animals,  is 
followed  by  a  loss  of  the  physiologic  properties  of  all  the  tissues 
within  a  short  period  of  time.  The  immediate  dependence  of  the 
functional  activities  of  the  tissues  and  organs  on  the  presence  of  the 
blood  can  be  demonstrated  by  the  following  experiment :  If  the  nozzle 
of  a  syringe,  adapted  to  the  size  of  the  animal,  be  introduced  through 
the  jugular  vein  into  the  right  side  of  the  heart  and  the  blood  be  sud- 
denly withdrawn,  there  is  an  immediate  cessation  in  the  activity  of 
all  the  organs;  the  return  of  the  blood  to  the  vessels  within  a  limited 
period  of  time  is  promptly  followed  by  a  renewal  of  their  activity. 

Though  contained  within  a  practically  closed  system  of  vessels, 
the  blood  is  brought  into  intimate  relation  with  all  the  tissue  elements 
through  the  intermediation  of  the  capillaries.  As  the  blood  flows 
through  these  delicate  vessels,  portions  of  its  soluble  nutritive  con- 
stituents, including  oxygen,  are  given  up  to  the  tissues,  by  which  they 
are  utilized  for  growth,  repair,  and  functional  activity.  At  the  same 
time  the  tissues  yield  up  to  the  blood  a  series  of  decomposition  prod- 
ucts, resulting  from  their  activity,  which  vary  in  quantity  and  quality 
according  as  the  blood  traverses  the  muscles,  nerves,  glands,  or 
other  tissues. 

The  blood  may  be  regarded,  therefore,  as  a  reservoir  of  nutritive 
materials  prepared  by  the  digestive  apparatus  and  absorbed  from 
the  food  canal;  of  oxygen,  absorbed  from  the  respiratory  surface  of 
the  lungs;  of  decomposition  products,  produced  by  and  absorbed 
from  the  tissues.  Though  the  blood  varies  in  composition  in  different 
parts  of  the  body  in  consequence  of  the  introduction  of  both  nutritive 
material  and  decomposition  products,  it  nevertheless  presents  certain 
average  physical,  morphologic,  and  chemic  properties  which  dis- 
tinguish it  as  an  individual  tissue. 

Constituents  of  Blood. — A  microscopic  examination  of  the  blood 
as  it  flows  through  the  capillary  vessels  of  the  web  of  the  frog  or  the 
mesentery  of  the  rabbit  shows  that  it  is  not  a  homogeneous  fluid,  but 
that  it  consists  of  two  distinct  portions:  viz.,  (i)  a  clear,  transparent, 
slightly  yellow  fluid,  the  plasma  or  liquor  sanguinis;  (2)  small  par- 

238 


THE  BLOOD.  239 

tides  termed  corpuscles  floating  in  it,  of  which  there  are  two  varieties, 
the  red  and  white.  By  appropriate  methods  it  can  be  shown  that  a 
third  corpuscle,  colorless  in  appearance  and  smaller  in  size  than  the 
ordinary  white  corpuscle,  is  present  in  the  blood  stream  and  known 
as  the  blood-plate  or  plaque.  The  different  constituents  can  be 
roughly  separated  by  appropriate  means  when  the  blood  is  with- 
drawn from  the  body.  If  the  blood  of  the  horse  is  allowed  to  flow 
directly  into  a  tall  cylindric  glass  vessel,  surrounded  by  ice,  it  sep- 
arates in  the  course  of  a  few  hours  into  three  distinct  layers  in  ac- 
cordance with  their  specific  gravities.  The  lower  layer  is  dark  red 
and  consists  of  the  red  corpuscles ;  the  middle  layer  is  grayish  in  color 
and  consists  of  the  white  corpuscles;  the  upper  layer  is  clear  and 
transparent  and  consists  of  the  plasma.  The  red  corpuscles  occupy 
almost  one-half,  the  white  one-fortieth,  the  plasma  a  trifle  more  than 
one-half  of  the  height  of  the  entire  blood-column,  which  indicates 
approximately  the  different  volumes  of  each.  The  same  result  can 
be  obtained  with  human  blood  by  the  use  of  the  centrifuge  or  hema- 
tocrit. 

PHYSICAL  PROPERTIES  OF  BLOOD. 

1.  Color. — Within  the  blood-vessels  two  kinds  of  blood  are  dis- 
tinguished— the  arterial,  the  color  of  which  is  a  bright  scarlet,  and 
the  venous,  the  color  of  which  is  a  dark  bluish-red  or  purple.  The 
cause  of  the  color  as  well  as  the  difference  in  color  is  the  presence  in  the 
red  corpuscles  of  a  coloring-matter,  hemoglobin,  in  different  degrees  of 
combination  with  oxygen.  The  intensity  of  the  color  in  either  kind 
of  blood  is  dependent  on  the  thickness  of  the  blood-stream,  for  in  the 
finest  capillaries,  as  seen  under  the  microscope,  there  is  an  almost 
total  absence  of  color.  As  the  arterial  blood  passes  into  and  through 
the  systemic  capillaries,  the  hemoglobin  yields  up  a  portion  of  its 
oxygen  to  the  tissues  and  changes  in  color,  though  the  change  is 
not  appreciable  by  the  eye.  On  passing  into  the  veins,  however, 
the  blood-stream  soon  presents  its  characteristic  dark  bluish  color, 
which  deepens  as  it  approaches  the  lungs.  On  passing  into  and 
through  the  capillary  vessels  of  the  lungs  the  hemoglobin  absorbs  a 
new  volume  of  oxygen,  changes  in  color,  and  on  emerging  from  the 
lungs  the  blood  presents  its  characteristic  scarlet  color. 

2.  Opacity. — Owing  to  the  fact  that  the  corpuscles  have  a  re- 
fracting power  different  from  the  plasma,  the  blood,  even  in  thin 
layers,  is  opaque.  The  repeated  refractions  and  reflections  which 
light  undergoes  in  passing  through  plasma  and  corpuscles  is  attended 
by  such  a  dissipation  that  it  is  impossible  to  see  printed  matter  through 
it.  That  the  opacity  is  due  to  the  shape  of  the  corpuscles  rather  than 
to  their  contained  coloring-matter  is  evident  from  the  fact  that  when 
the  hemoglobin  is  caused  to  separate  from  the  corpuscles  by  the 


240  TEXT-BOOK  OF  PHYSIOLOGY. 

addition  of  chemic  reagents,  the  blood,  though  it  deepens  in  color, 
becomes  at  once  transparent. 

3.  Odor. — When  freshly  drawn  the  blood  possesses  a  peculiar 
characteristic  odor  which  has  been  attributed  to  the  presence  of  a 
volatile  fatty  acid  in  combination  with  an  alkaline  base.  The  in- 
tensity of  the  odor  may  be  increased  by  the  addition  of  concentrated 
sulphuric  acid,  by  means  of  which  the  volatile  acid  is  set  free. 

4.  Specific  Gravity. — The  specific  gravity  of  blood  lies  within 
the  limits  of  1.051  and  1.059,  averaging  in  man  1.056  and  in  woman 
1 .053.  Normally,  variations  from  these  values  are  only  temporary  and 
are  connected  with  variations  in  physiologic  processes.  The  specific 
gravity  is  diminished  by  the  ingestion  of  liquids  and  abstinence  from 
solid  food.  It  is  increased  by  abstinence  from  liquids,  by  the  inges- 
tion of  dry  food,  and  by  the  elimination  of  large  quantities  of  water 
by  the  lungs,  skin,  and  kidneys. 

5.  Alkalinity. — The  reaction  of  the  blood  is  alkaline  from  the 
presence  of  the  disodium  phosphate  (Na2HPO^)  and  the  sodium  car- 
bonate, NajCOa.  The  alkalinity  can  be  readily  shown  by  allowing 
the  blood  to  remain  for  a  few  seconds  on  slightly  reddened  glazed 
litmus  paper.  On  washing  off  the  blood  a  distinct  blue  color  pre- 
sents itself  against  a  red  or  violet  background.  The  alkalinity  varies 
within  narrow  limits  in  consequence  of  variations  in  physiologic 
processes.  It  is  increased  in  the  early  stages  of  digestion  and  de- 
creased in  the  later  stages.  It  is  decreased  after  muscular  exercise 
in  consequence  of  the  increased  production  and  absorption  of  acids. 
According  to  v.  Jaksch,  the  alkalinity  corresponds  to  from  260  to  300 
milligrams  of  sodium  hydrate,  NaOH,  for  every  100  c.c.  of  blood; 
according  to  Lowy,  from  300  to  325  milhgrams.  The  hitherto  un- 
avoidable error  in  these  estimates  is  about  30  milligrams. 

6.  Temperature. — The  temperature  varies  from  36.78°  C. 
(98.2°  F.)  in  the  superior  vena  cava  to  39.7°  C.  (103.4°  F.)  in  the 
hepatic  vein,  the  mean  being  about  38°  C.  (100°  F.). 

Coagulation  of  the  Blood. — Within  a  few  minutes  after  the 
blood  is  withdrawn  from  the  vessels  of  a  hving  animal  it  begins  to 
lose  its  fluidity,  becomes  somewhat  viscid,  and  if  left  undisturbed 
passes  rapidly  into  a  semi-solid  or  jelly-like  state.  To  this  change 
in  the  physical  condition  of  the  blood  the  term  coagulation  has  been 
applied.  The  blood,  during  the  progress  of  coagulation,  not  only 
assumes  the  shape  of  the  vessel  in  which  it  is  contained,  but  becomes 
so  firmly  adherent  to  its  walls  that  it  may  be  inverted  without  the 
coagulum  becoming  dislodged.  If  a  portion  of  such  a  jelly-like  mass 
be  examined  microscopically,  it  will  be  found  to  be  penetrated  in  all 
directions  by  a  felt-work  of  extremely  fine  delicate  fibrils,  which, 
having  made  their  appearance  before  the  corpuscles  had  time  to 


THE  BLOOD. 


241 


settle  to  the  bottom  of  the  fluid,  have  entangled  them  in  the  meshes 
so  that  the  entire  mass  retains  its  characteristic  color.  These  fibrils 
are  collectively  known  as  fibrin  (Fig.  94). 

If  the  coagulated  blood  be  allowed  to  remain  undisturbed,  a 
clear,  transparent,  sHghtly  yellowish  fluid  makes  its  appearance  on 
the  surface  of  the  mass,  which  as  it  accumulates  forms  a  layer  of 
varying  degrees  of  thickness.  Within  a  few  hours  the  blood-mass 
detaches  itself  from  the  sides  of  the  vessel  in  consequence  of  the  re- 
traction of  the  fibrils,  while  at  the  same  time  the  clear  fluid  increases 
in  amount  and  accumulates  along  the  sides  and  bottom  of  the  vessel. 
The  shrinkage  in  the  volume  of  the  red  coagulum  and  the  increase 
of  the  volume  of  the  clear  fluid  which  is  expressed  from  its  meshes 
continue  for  a  period  varying  from  ten  to  fifteen  hours,  according 
to  certain  external  conditions.  The  blood  has  now  become  separated 
into  two  distinct  portions:  viz.,  a  sohd  contracted  red  mass,  termed 
clot,  and  a  clear  fluid,  termed  serum.  The  clot  consists  of  the  fibrin 
containing  in  its  meshes  the  red  and  white  corpuscles;  the  serum 


Fig.  94. — Diagram  to  Illustrate  the  Process  of  Coagulation,  i.  Fresh  blood, 
plasma,  and  corpuscles.  2.  Coagulating  blood  (birth  of  fibrin).  3.  Coagulated 
blood  (clot  and  serum). — {Waller.) 

consists  of  all  the  constituents  of  the  plasma  except  the  antecedents 
of  the  fibrin.     The  stages  of  coagulation  are  shown  in  Fig.  94. 

If  the  blood  coagulates  slowly  the  red  corpuscles,  owing  to  their 
greater  specific  gravity,  subside  to  the  bottom  of  the  blood-mass, 
giving  to  it  a  deeper  color;  the  white  corpuscles,  owing  to  their  lesser 
specific  gravity,  remain  near  the  surface  of  the  clot  and  give  to  it  a 
more  or  less  whitish  appearance,  producing  the  so-called  hufjy  coat. 
In  certain  inflammatory  conditions  the  coagulating  power  of  the  blood 
is  much  diminished,  and  the  corpuscles,  having  time  to  subside,  a  well- 
developed  buffy  coat  is  formed  which  at  one  time  had  much  interest 
for  pathologists.  As  the  contraction  of  the  fibrin  takes  place  more 
actively  in  the  center,  there  being  here  less  resistance  than  at  the 
sides  of  the  coagulum,  the  upper  surface  usually  becomes  depressed 
or  cupped. 

Coagulation  of  Plasma. — Clear  plasma  may  be  obtained  by 
means  of  the  centrifuge  from  blood  to  which  sufficient  magnesium 
sulphate  has  been  added  to  prevent  coagulation,  or  from  horse's 
16 


242  TEXT-BOOK  OF  PHYSIOLOGY. 

blood  which  has  been  allowed  to  How  into  a  tall  vessel  surrounded  by 
a  cooling  mixture  so  as  to  prevent  coagulation  and  thus  permit  the 
red  corpuscles  to  subside.  If  such  plasma  be  subjected  to  room-tem- 
perature, it  very  shortly  undergoes  coagulation,  exhibiting  practically 
the  same  phenomena  as  blood  itself.  After  a  variable  length  of  time 
it  also  separates  into  a  soft,  colorless  coagulum  or  clot  consisting  of 
fibrin,  and  a  clear  fluid,  the  serum.  The  presence  of  the  red  cor- 
puscles is  therefore  not  essential  to  the  process  of  coagulation. 

Rapidity  of  Coagulation. — The  rapidity  with  which  the  blood 
coagulates  varies  in  dilferent  classes  of  animals  under  the  same  con- 
ditions: e.  g.,  the  blood  of  the  pigeon  coagulates  immediately;  that  of 
the  dog,  in  from  one  to  three  minutes ;  that  of  the  horse,  in  from  five 
to  thirteen  minutes;  that  of  man,  in  from  four  to  seven  minutes. 
The  time,  however,  can  be  lengthened  or  shortened  by  either  chang- 
ing the  external  conditions  or  by  altering  temporarily  the  normal 
composition  of  the  blood. 

Coagulation  is  retarded  or  prevented  by  the  following  agents, 
viz.:  (i)  A  low  temperature,  especially  that  of  melting  ice.  (2)  The 
addition  of  magnesium  sulphate  Cr  volume  of  a  25  per  cent,  solution 
to  3  volumes  of  blood);  of  sodium  sulphate  (i  volume  of  a  saturated 
solution  to  7  volumes  of  blood).  (3)  The  addition  of  potassium 
oxalate  (i  volume  of  a  i  per  cent,  solution  to  3  volumes  of  blood). 
(4)  The  injection  into  the  blood  of  commercial  peptone.  (5)  The 
mouth  secretion  of  the  leech. 

Coagulation  is  hastened  by  the  following  agents,  viz.:  (i)  A 
gradually  increasing  temperature  from  38°  C.  to  50°  C.  (2)  The 
addition  of  water  in  not  too  large  amounts.  (3)  The  presence  of 
foreign  bodies.     (4)  Agitation  of  the  blood — e.  g.,  stirring. 

Fibrin  and  Defibrinated  Blood. — If  freshly  drawn  blood  is 
stirred  with  a  bundle  of  twigs  or  glass  rods  for  a  few  minutes,  the 
fibrin  collects  on  them  in  the  form  of  thick  bundles* or  strands;  after 
washing  it  with  water  the  entangled  corpuscles  are  removed,  when 
the  fibrin  assumes  its  natural  white  appearance.  The  strands  can 
be  resolved  into  a  large  number  of  dehcate  fibers  which  possess  ex- 
tensibility and  retractibility,  and  are  therefore  elastic.  The  chemic 
features  of  fibrin  have  already  been  considered  (see  page  34).  The 
remaining  fluid,  similar  in  its  physical  appearance  to  the  blood,  is 
termed  defibrinated  blood.  When  such  blood  is  allowed  to  remain 
at  rest  for  a  few  days,  the  remaining  red  corpuscles  gradually  sink 
to  the  bottom  of  the  fluid,  above  which  will  be  found  the  clear  serum. 


COMPOSITION  OF  PLASMA  AND  SERUM. 

Plasma.— The  plasma  obtained  by  any  of  the  methods  previously 
described  is  a  clear,  colorless,  transparent,  slightly  viscid  fluid,  with 


THE  BLOOD.  243 

a  specific  gravity  of  1.026  to  1.029.  ^^  ^s  composed  largely  of  water 
holding  in  solution  proteids,  sugar,  fatty  matter,  inorganic  salts,  urea, 
cholesterin,  lecithin,  etc.  In  composition  it  is  quite  complex,  con- 
taining as  it  does  not  only  the  nutritive  materials  derived  from 
the  digestion  of  the  food,  but  also  the  substances  resulting  from 
the  disintegration  of  the  tissues  consequent  on  their  functional  ac- 
tivity. 

Serum. — The  serum  is  the  clear,  transparent,  slightly  yellow 
fluid  expressed  from  the  coagulated  blood  during  the  contraction  of 
the  fibrin.  It  consists  practically  of  the  ingredients  of  the  plasma, 
with  the  exception  of  those  substances  which  entered  into  the  for- 
mation of  fibrin.  The  average  composition  of  plasma  is  shown  in 
the  following  table: 

COMPOSITION  OF  PLASMA. 

Water,    90.00 

[Serum-albumin,   4.50 

Proteids  ^  Paraglobulin,    3.40 

(  Fibrinogen, 0.30 

Fatty  matters, 0.25 

Sugar, CIO 

Extractives, 0.60 

Inorganic  salts, 0.85 

100.00 

Serum-albumin. — Of  the  proteid  constituents  of  the  blood, 
serum-albumin  is  the  most  abundant,  existing  to  the  extent  of  from 
4  to  5  per  cent.  From  its  similarity  to  egg-albumin  it  is  regarded  as 
holding  an  important  position  as  a  nutritive  agent,  for  it  is  out  of  this 
common  proteid  that  in  all  probability  each  individual  tissue  elabor- 
ates the  special  proteid  characteristic  of  it,  since  during  starvation 
the  albumin  steadily  diminishes  in  amount.  As  it  passes  through 
the  walls  of  the  capillary  vessels  it  is  found  in  the  hmiph,  pericardial 
fluid,  and  similar  secretions  in  various  parts  of  the  body,  as  well  as  in 
various  pathologic  transudates.  It  is  also  present  in  serum.  While 
circulating  in  the  lymph-spaces  the  serum-albumin  is  utilized  in 
replacing  the  proteids  which  have  undergone  disintegration  during 
tissue  metabolism.  Its  supply  in  the  blood  is  maintained  by  the 
absorption  of  peptones  which  are  formed  from  the  proteids  of  the  food 
and  which  during  the  time  of  absorption  are  changed  in  some  unknown 
way  into  serum-albumin.  It  is  readily  obtained  from  plasma  or 
serum  by  saturating  either  of  these  fluids  with  magnesium  sulphate, 
when  all  the  proteids  except  serum-albumin  are  precipitated.  After 
their  removal  the  remaining  fluid  is  subjected  to  a  temperature  of 
from  70°  to  75°  C,  when  the  serum-albumin  is  precipitated  in  a 
coagulable  form,  after  which  it  can  be  removed  and  its  chemic 
features  determined. 


244  TEXT-BOOK  OF  PHYSIOLOGY. 

Paraglobulin. — This  proteid,  though  present  in  plasma,  is  best 
obtained  from  scrum  when  this  fluid  is  saturated  with  magnesium 
sulphate.  As  the  line  of  saturation  is  approached  the  fluid  becomes 
turbid,  and  after  a  few  minutes  a  fine  white  precipitate  occurs.  It 
can  then  be  collected  on  a  filter,  dried,  and  its  chemic  properties 
determined.  In  its  reactions  it  resembles  the  various  members  of  the 
globulin  class.  The  amount  varies  from  2  to  4  per  cent,  in  the  blood 
of  man.  As  to  the  physiologic  importance  or  antecedents  of  para- 
globulin nothing  is  definitely  known.  Its  constant  presence  in  the 
blood  would  indicate  that  it  plays  an  equally  important,  though  per- 
haps different,  part  with  serum-albumin  in  the  nutrition  of  the  body. 

Fibrinogen. — This  proteid  can  be  obtained  from  plasma,  lymph, 
pericardial,  and  peritoneal  fluids,  as  well  as  from  hydrocele  fluid. 
It  is,  however,  not  to  be  obtained  from  serum,  as  it  is  removed  from 
the  blood  during  the  formation  of  sohd  fibrin.  It  is  normally  present 
in  the  blood  in  very  small  quantity,  amounting  to  not  more  than  2.2 
to  T,.T,  parts  per  thousand.  Fibrinogen  may  be  obtained  from  plasma 
which  has  been  prevented  from  coagulating,  by  the  addition  of  mag- 
nesium sulphate  in  certain  quantities  or  by  the  addition  of  a  satu- 
rated solution  of  sodium  chlorid.  In  a  few  minutes  a  flaky  precipitate 
occurs.  By  repeated  washing  and  precipitation  with  sodium  chlorid 
solutions  of  varying  strength  the  fibrinogen  may  be  obtained  in  a 
pure  state.  The  history  of  fibrinogen  is  unknown.  Beyond  the  fact 
that  it  contributes  to  the  formation  of  fibrin  there  is  no  positive 
knowledge  either  as  to  its  origin,  its  nutritive  value,  or  its  final  dis- 
position in  the  blood  under  normal  conditions. 

Fat. — The  plasma  as  well  as  the  serum  contains  a  very  small 
quantity  of  fat  in  the  form  of  microscopic  globules.  Though  the 
percentage  is  normally  not  above  0.25,  yet  just  after  a  meal  rich  in 
fatty  matter  the  amount  may  be  so  great  as  to  give  to  the  blood 
a  milky  or  opalescent  appearance.  Within  a  few  hours,  however, 
this  excess  of  fat  disappears  from  the  blood,  though  its  immediate 
disposition  is  unknown.  Soaps  or  alkaline  salts  of  the  fatty  acids, 
though  formed  during  the  digestion  of  fats,  are  not  present  in  the 
blood.     Lecithin  and  cholesterin  are  present  in  very  small  quantities. 

Sugar. — Sugar  is  present  in  the  blood  in  the  form  of  dextrose, 
and  is  now  regarded  as  a  normal  constituent.  The  amount  is 
about  I  part  per  thousand,  though  it  may  be  present  to  the  extent 
of  3  parts  per  thousand.  Beyond  this,  the  excess  soon  appears  in 
the  urine. 

Extractives. — The  blood  contains  a  series  of  nitrogenized 
bodies,  such  as  urea,  uric  acid,  creatin,  creatinin,  xanthin,  etc.,  which 
result  from  the  katabolic  changes  in  nerve-  and  muscle-tissues  as 
well  as  from  subsequent  chemic  combinations  and  decompositions. 
Though  constantly  absorbed  from  the  tissues,  they  seldom  accumu- 


THE  BLOOD. 


245 


late  beyond  a  small  amount,  since  they  are  constantly  being  elimi- 
nated from  the  blood  by  the  various  excretory  organs. 

Inorganic  Salts. — The  inorganic  salts  of  the  plasma  are  chiefly 
sodium  and  potassium  chlorids,  sulphates,  and  phosphates,  together 
with  calcium  and  magnesium  phosphates.  Of  the  salts,  sodium 
chlorid  is  the  most  abundant,  amounting  to  5.5  parts  per  thousand. 
Some  of  the  salts  are  alkahne  and  impart  to  the  blood  its  alkalinity. 
Calcium  phosphate  is  present  in  small  quantity — 2  parts  per  1000. 
This  salt  is  wanting  in  serum  for  the  reason  that  it  became  a  constitu- 
ent of  fibrin  at  the  time  of  coagulation.  In  other  respects  serum 
differs  but  slightly  from  plasma  in  the  proportions  of  its  sahne  con- 
stituents. 

HISTOLOGY  OF  THE  RED  CORPUSCLES  OR  ERYTHROCYTES. 

The  histologic  features  of  the  red  corpuscles  are  readily  observed 
in  a  drop  of  freshly  drawn  blood  when  examined  microscopically. 
The  field  of  the  microscope  will  be  seen  to  be  crowded  with  red 
corpuscles  floating  in  a  clear  transparent  fluid — the  plasma.  Here 
and  there  will  also  be  seen 
a  white  corpuscle,  round 
or  irregular  in  shape,  and 
granular  in  appearance. 
Within  a  short  time  a  char- 
acteristic phenomenon  takes 
place:  viz.,  the  arranging 
of  the  corpuscles  in  the 
form  of  columns  of  var}'- 
ing  length,  resembling  rolls 
of  coins.  These  rolls  in- 
terlace with  each  other  at 
all  angles  and  form  a  net- 
work in  the  meshes  of 
which  lie  individual  red 
and  white  corpuscles.  (See 
F^g-  95-)  The  cause  of 
this  tendency  of  the  cor- 
puscles to  adhere  to  one 
another  is  not  definitely 
known.     Since  it  does  not 

take  place  in  circulating  blood,  and  since  it  is  to  a  great  extent 
prevented  by  defibrinating  the  blood,  it  has  been  supposed  to  be 
dependent  on  the  formation  of  some  adhesive  substance  connected 
with  the  formation  of  fibrin. 

Color. — When  viewed  by  transmitted  light,  a  single  corpuscle 
is  slightly  yellow  or  greenish  in  color;  but  when  a  number  are  grouped 


Fig.  95. — Corpuscles  from  Human  Sub- 
ject. A  few  colorless  corpuscles  are 
seen  among  the  colored  discs,  many  of 
which    are      arranged      in     rouleaux. — 

(Ftmke.) 


246  TEXT-BOOK  OF  PHYSIOLOGY. 

together,  the  color  deepens  and  the  corpuscles  appear  red.  In  either 
case  the  color  is  due  to  the  presence  in  the  corpuscle  of  a  specific 
coloring-matter,  hemoglobin. 

Shape. — The  red  corpuscle  is  a  circular,  flattened  disk  with 
rounded  edges.  Each  surface  is  perfectly  smooth  and  presents  a 
shallow  depression  in  its  center,  so  that  it  is  also  biconcave.  A 
longitudinal  section  of  a  corpuscle  would  present,  when  viewed 
edgewise,  an  outline  similar  to  that  of  Fig.  96.  This  difference  in 
the  thickness  of  the  peripheral  and  central  portions  of  the  corpuscle 

gives  rise  to  differences  in  optical 

e-.-.j!-^^^;^,. — ,.  appearances  when  examined  micro- 

• Jb        scopically.     At  a  certain  distance  of 
-—- i — --^^.^^^^     y.         the  object-glass  the  corpuscle  pre- 

""a ""  sents    in    its    peripheral    portion   a 

Fig.  96.-IDEAL  Transverse  Sec-      \^^\aVs.\.  rim,  and  in  its  central  por- 
TiON  OF  A  Human  Red  Corpus-        .    °         ,      '  tt     i        1  •       • 

CLE.    (Magnified    5000    times.)      tion  a  dark  spot.     If  the  objective 
a,  h.  Diameter,   c,  d.  Thickness,     be  brought  nearer  and  the  center 

accurately  focused,  the  reverse  ap- 
pearance obtains ;  the  central  portion  becomes  bright  and  the  periph- 
eral portion  dark.  The  cause  of  this  difference  in  optical  appear- 
ance is  the  unequal  distribution  of  the  transmitted  hght  in  conse- 
quence of  the  shape  of  the  corpuscle. 

.Size. — The  diameter  of  a  typical  well-developed  red  corpuscle 
under  normal  conditions  is  0.0075  mm.;  its  greatest  thickness  is 
0.0019  mm.  Though  this  may  be  assumed  as  the  average  diameter, 
there  is  a  small  percentage  of  distinctly  smaller  and  a  small  per- 
centage of  distinctly  larger  corpuscles.  The  following  table  shows 
the  results  of  measurement  made  by  different  observers: 

Normal  Limits.  Average  Diameter. 

Welcker, diameter  0.0045-0.0095 0.0070 

Hayem, "         0.0060-0.0088 0.0075 

Gram, "         0.0067-0.0093 0.0078 

Melassez, "  0.0076 

0.00747 
(32V0  inch) 

Structure. — The  red  corpuscle  of  man  as  well  as  all  other  mam- 
mals possesses  neither  a  nucleus  nor  a  hmiting  membrane,  but  appears 
to  consist  of  a  homogeneous  substance  more  or  less  semisolid  in  con- 
sistence. Under  the  influence  of  certain  reagents  the  corpuscle 
separates  into  two  distinct  portions:  viz.,  a  colorless  protoplasmic 
stroma  and  a  coloring-matter  which  diffuses  into  the  surrounding 
liquid.  The  presence  of  the  former  can  be  demonstrated  by  the 
addition  of  iodin,  which  imparts  to  it  a  faint  yellow  color.  The 
stroma  is  elastic,  and  determines  not  only  the  shape  of  the  corpuscle 
but  gives  to  it  the  properties  of  extensibility  and  retractibility. 


THE  BLOOD. 


247 


Number  of  Red  Corpuscles.— In  any  given  specimen  of  blood 
the  corpuscles  are  so  numerous  and  the  spaces  between  them  so 
small  that  it  seems  almost  impossible  to  determine  their  number. 
This,  however,  has  been  accomphshed  for  a  cubic  miUimeter  of 
blood  by  various  observers  employing  different  methods  with  compara- 
tively uniform  results.  The  average  normal  number  of  corpuscles  in 
one  cubic  milhmeter  of  blood  is,  for  men,  5,000,000;  and  for  women, 
4,500,000.  This  value,  however,  will  vary  within  shght  hmits,  with 
variations  in  the  activity  of  physiologic  processes  and  to  a  large  extent 
at  times  in  pathologic  states  of  the  blood  or  body.  The  number  is 
increased  in  the  cutaneous  veins  by  all  influences  which  cause  a 
diminution  in  the  quantity  of  water  in  the  blood — e.  g.,  copious 
•sweating,  acute  watery  diarrhea,  fasting,  abstinence  from  Hquids; 
the  number  is  diminished  by  influences  which  dilute  the  blood — e.  g., 
the  ingestion  of  hquids,  the  absorption  of  fluids  from  the  tissue  spaces, 
etc.  But  it  is  well  to  remember  that  these  influences  which  produce 
changes  in  the  number  of  corpuscles  per  cubic  millimeter  do  not 
necessarily  produce  corresponding  changes  in  the  total  number  of 
red  corpuscles  in  the  body.  In  women  lactation,  menstruation,  and 
the  act  of  parturition  diminish  the  number.  High  altitudes  appar- 
ently increase  the  number  of  corpuscles,  as  shown  by  their  increase  in 
the  blood  of  the  peripheral  vessels.  Whether  this  is  an  indication 
that  there  is  a  corresponding  increase  of  the  total  number  in  the 
general  volume  of  the  blood  is  uncertain.  The  following  table  will 
show  the  increase  in  the  count  per  cubic  miUimeter  at  different 
altitudes: 


Place. 

Height  above  Sea-level. 

Red  Cells. 

4.974.000 
5,225,000 
5,322,000 
5,752,000 
5,748,000 
5,900,000 
7,000,000 
8,000,000 

Author. 

Christiania, 

Gottingen,    

Xiibingen,  __ 

0  meter 
148  meters 

314 

414 

425 

700  " 
1800  " 
4.392 

Laache. 
Schaper. 
Reinert. 

Zurich, 

Auerbach, 

Reibaldsgriin,    

Arosa, 

The  Cordilleras, 

Moro  cocha. 

Steirlin. 
Koppe. 

Egger. 
Viault. 

(Koppe.) 

This  increase  in  the  number  of  corpuscles  takes  place,  according 
to  Viault's  observations,  within  two  or  three  weeks,  and  is  apparently 
not  connected  with  either  diet  or  mode  of  life,  but  rather  with  dimin- 
ished atmospheric,  if  not  oxygen,  pressure.  On  returning  to  sea- 
level  there  is  a  gradual  reduction,  without  any  apparent  destruction 
of  the  corpuscles,  to  their  normal  number.  The  reason  for  these 
variations  is  not  clear. 

The  method  of  counting  corpuscles  introduced  by  Vierordt  and 


248 


TEXTBOOK  OF  PHYSIOLOGY. 


Welckcr  has  been  modified  by  different  observers,  and  especially  by 
Thoma  and  Zeiss.  On  account  of  the  great  number  of  corpuscles 
in  I  cubic  millimeter  of  blood,  it  becomes  necessary  for  purposes  of 
enumeration  that  the  blood  be  diluted  a  definite  number  of  times  and 
that  the  diluted  mixture  be  placed  in  a  counting  chamber  possessing 
a  definite  capacity.  By  means  of  the  pipette  or  melangeur  of  Potain 
and  the  counting  chamber  of  Thoma  both  these  objects  are  attained. 

^  The  pipette  consists  of  a  capillary  tube  (Fig.  gy) 

provided  with  an  enlargement  containing  a  freely  mov- 
able small  glass  ball,  a.  One  end  of  the  tube  is  pointed, 
vdiile  to  the  other  end  is  attached  a  rubber  tube  for  the 
purpose  of  facilitating  the  introduction  of  the  blood 
and  the  diluting  fluid.  The  capillary  tube,  which  is 
accurately  calibrated,  carries  marks,  ^,  i,  loi,  which 
signify  that  if  the  tube  be  filled  with  blood  up  to  the 
mark  i  and  the  diluting  fluid  be  sucked  into  the 
tube  up  to  the  mark  loi,  the  blood  will  be  diluted 
loo  times.  If  the  blood  be  sucked  up  to  the  mark 
J  and  the  diluting  fluid  to  loi,  then  the  blood  will 
be  diluted  200  times.  In  using  the  pipette  the  point 
is  introduced  into  a  drop  of  blood  derived  from  a 
small  wound  in  the  skin  of  the  lobe  of  the  ear  or 
finger  and  sucked  into  the  tube  by  introducing  the 
rubber  tube  into  the  mouth.  The  tube  is  then 
quickly  inserted  into  a  solution,  similar  in  specific 
gravity  to  the  plasma,  which  will  preserve  the  shape 
and  size  of  the  corpuscles,  such  as  Gowers's  sodium 
sulphate  solution,  sp.  gr.  1.025,  or  a  3  per  cent, 
sodium  chlorid  solution,*  and  the  fluid  sucked  into 
the  tube  up  to  the  mark  loi.  On  shaking  the 
pipette  for  a  few  minutes,  the  admixture  will  take 
place,  aided  by  the  movements  of  the  glass  ball. 

Fig.  98  shows  both  a  section  view.  A,  and  a 
surface  view,  C,  of  the  counting  chamber.     This  con- 
sists of  an  oblong  glass  plate  on  which  are  cemented 
two  small  pieces  of  glass,  one  of  which  has  in  the 
center  a  circular  opening  in  which  is  placed  the  other,  a  circular  disc  or 


Fig.  97.  —  Melan- 
geur OR  Pipette. 
— (Landois  and 
Stirling. ) 


*  Various  solutions  have  been  devised  for  diluting  blood,  any  one  of  which  may 
be  employed,  e.  g.: 

Toisson's  Fluid: 

Aquae  destillat., 160.00  parts. 

Glycerins, 30.00     " 

Sodium  sulphate,  __  8.00     " 

Sodium  chlorid, i.oo     " 

Methyl-violet, 0.025  part. 


Hayem's  Fluid: 

Hydrarg.  bichior., 0.5  gm 

Sodium  sulphate, 5.0    " 

Sodium  chlorid, 2.0    " 

Aquae  destillat., 200.0    " 


Gowers's  Fluid: 

Sodium  sulphate,    gr.  104 

Acid,  acetic,    .^j 

Aquae  dest., q.  s.  ad    ^^iv. 


THE  BLOOD. 


249 


stage.  Their  relation  is  such  that  a  narrow  groove  or  moat  separates  the 
one  from  the  other,  the  floor  of  which  is  formed  by  the  glass  plate.  The 
surface  of  the  circular  stage  is  exactly  o.i  mm.  lower  than  that  of  the 
cover-glass,  a.  On  the  surface  of  the  glass  stage  a  series  of  small  squares 
is  engraved,  each  one  of  which  has  a  side  length  of  ^^  mm.  and  an  area 
of  :f^o  square  mm.,  B.  To  faciHtate  counting,  a  group  of  i6  squares  is 
surrounded    by    a    heavy 

dark  line.     This  group  is  a  b 

separated  from  adjoining 
groups,  also  enclosed  by 
dark  lines,  by  an  inter- 
mediate light  line,  which 
serves  as  a  guide  in  pass- 
ing from  one  group  to 
another.  When  a  cover- 
glass  is  accurately  applied 
to  the  glass,  b,  each  one 
of  the  small  squares  will 
have  a  cubic  capacity  of 
,:-^^_X  O.I,  or  4oVo  cubic 
millimeter,  and  every  four 
such  squares  will  have 
therefore  a  capacity  of 
yoVtt  cubic  millimeter. 

Before  placing  the  di- 
luted blood  on  the  coimt- 
ing  stage,  the  fluid  in  the 
tube  of  the  pipette  should 
be  blown  out  and  dis- 
carded, as  it  contains  no 
portion  of  the  blood.  A 
few  drops  are  then  placed 
on  the  glass  stage  and 
covered  with  the  cover- 
glass.  After  a  few  min- 
utes the  corpuscles  settle 
over  the  ruled  spaces  and 
are  ready  for  counting. 
The  number  of  corpuscles 
in  a  horizontal  series  of  4 
squares  is  then  counted;  this  number  is  then  multiplied  by  1000  in  order 
to  get  the  number  in  i  cubic  millimeter  of  the  diluted  blood,  and  this 
product  by  100  or  200  according  to  the  extent  of  the  dilution:  e.  g.,  four 
squares  contain  50  corpuscles;  multiplied  by  1000  and  then  by  100  = 
5,000,000.  The  accuracy  of  the  comiting  is  proportional  to  the  number 
of  squares  counted.  If  200  squares  are  counted  and  the  average  taken, 
the  probable  limit  of  error  will  not  be  more  than  2  per  cent. 

Effects  of  Reagents  on  the   Red  Corpuscles. — Within  the 
blood-vessels  the  composition  of  the  plasma  is  such  that  both  the 


Fig. 


98. — Apparatus  of  Thoma  and  Zeiss 
FOR  Counting  the  Corpuscles.  A.  In 
section.  C.  Surface  view  without  cover- 
glass.  B.  Microscopic  appearance  with  the 
blood-corpuscles. — {Landois  aiid  Stirling.) 


250  TEXT-BOOK  OF  PHYSIOLOGY. 

form  and  composition  of  the  corpuscles  arc  maintained  under  normal 
physiologic  conditions.  This  fluid,  therefore,  is  preservative  of  the 
structure  and  function  of  the  corpuscle.  When  examined  micro- 
scopically with  a  view  of  determining  their  histologic  features,  the 
plasma  must  be  diluted,  and  in  consequence  they  rapidly  undergo 
physical  and  chemic  changes  from  the  absorption  or  loss  of  water. 
To  prevent  these  elTects  the  corpuscles  must  be  immersed  in  a  fluid 
containing  a  percentage  of  salts  approximating  that  of  the  plasma. 
Under  such  circumstances  they  will  neither  absorb  nor  give  up 
water.  Such  a  fluid  is  found  in  the  physiologic  salt  solution,  which 
contains  0.64  per  cent,  sodium  chlorid.  This  fluid  maintains  the 
chemic  equilibrium  of  the  corpuscles,  and  is  therefore  said  to  be 
isotonic  to  the  corpuscle. 

If  distilled  water  be  added  to  the  drop  of  blood,  the  corpuscle 
absorbs  it,  swells,  and  assumes  a  more  or  less  spheric  form,  some- 
times cup-shaped.  The  hemoglobin  dissolves  out  and  the  stroma 
becomes  almost  invisible.  Its  presence  can  be  detected  by  the 
addition  of  iodin.  The  addition  of  salt  solutions, — e.  g.,  sodium 
chlorid,  sodium  sulphate,  ammonium  chlorid,  etc.,— which  increase 
the  density  of  the  plasma,  cause  a  shrinkage  of  the  corpuscles  so  that 
they  assume  a  crenated  or  notched  appearance.  Dilute  solutions  of 
acetic  acid,  of  alkalies,  especially  potassium  and  sodium  hydrate, 
cause  the  corpuscles  to  swell,  to  lose  their  color,  dissolve,  and  en- 
tirely disappear.  Many  other  agencies  of  a  physical  and  chemic 
nature,  such  as  heat  60°  C,  electricity,  bile  salts,  the  vapor  of 
chloroform,  ether,  ammonium  sulphocyanid,  etc.,  also  destroy  the 
integrity  of  the  corpuscles,  and  cause  the  hemoglobin  to  separate 
from  the  stroma  and  diffuse  into  the  plasma  without  itself  under- 
going any  appreciable  change  in  composition.  The  blood  at  the  same 
time  will  become  transparent  and  cliange  to  a  dark  red  color,  to 
which  the  term  "lake  color"  has  been  given. 

The  Corpuscles  of  Other  Vertebrated  Animals. — In  all  mam- 
mals, with  the  exception  of  the  camel,  llama,  and  dromedary,  the 
red  corpuscles  present  the  same  shape  and  structure  as  the  corpuscles 
of  man,  and  may  be  described  as  circular,  flattened,  biconcave  disks. 
In  the  animals  excepted  the  corpuscles  are  oval.  The  size,  however, 
varies  in  different  animals  from  0.0092  mm.  (2TTT  inch)  in  the  ele- 
phant to  0.0023  nim.  (y-jiTT  inch)  in  the  musk-deer,  while  in  most 
animals  the  average  lies  between  0.0084  nim.  and  0.0050  mm.  Inas- 
much as  the  question  may  arise  as  to  whether  the  corpuscles  of  any 
given  specimen  of  blood  are  those  of  a  human  being  or  of  some  other 
mammal,  a  knowledge  of  the  size  of  the  corpuscles  becomes  a  matter 
of  medicolegal  as  well  as  of  physiologic  interest.  Though  the 
differences  in  size  are  slight,  yet  it  is  possible  for  skilled  microscopists, 
when  examining  fresh  blood,  to  make  a  diagnosis  between  the  cor- 


THE  BLOOD. 


251 


puscles  of  man  and  those  of  the  domesticated  animals,  with  the  ex- 
ception, perhaps,  of  the  guinea-pig.  Tlie  diagnosis  of  the  corpuscles 
of  dried  blood  which  have  been  altered  by  the  action  of  various  ex- 
ternal agents,  even  though  capable  of  a  certain  degree  of  restoration, 
is  most  difficult,  and  should  not  be  attempted  in  criminal  cases  with- 
out large  experience  in  microscopy,  in  measurements  and  methods 
of  preparation  of  all  kinds  of  blood-corpuscles,  and  a  proper  per- 
ception of  corpuscular  forms  and  sizes.  In  the  following  table  the 
average  results  of  the  measurements  of  the  corpuscles  in  different 
classes  of  animals  are  given  (abstracted  from  Formad's  compilation) : 


Gulliver. 

WORMLEY. 

C.  Schmidt. 
Malunin. 

French  Medico- 
legal Soc. 
Welcker. 

Form AD. 

Inch. 

1  3200 
1-3538 
1-3532 
1-3607 
1.4267 
1.4230 
1.4600 
1.4404 
1.5300 
1.6366 

Mm. 

Inch. 

Mm. 

Inch. 

Mm. 

Inch. 

Mm. 

Inch. 

Mm. 

Man 

Guiaea-pig.. 

Dog 

Rabbit, 

Ox 

Pig 

Horse 

Cat 

Sheep 

Goat 

0.0079 
0.0071 
0.0071 
0.0070 
0.0060 
0.0060 
0.0057 
0.0058 
0.0048 
0.0040 

3250 
3223 
3561 
3653 
4219 
4268 
4243 
4372 
4912 
6189 

0.0078 
0.0079 
0.0071 
0.0070 
0.0060 
0.0059 
0.0059 
0.0058 
00031 
0.0041 

1.3300 

1.3300* 

1.3636 

1-3968 

1-4354 

1 .4098 

1.4464 

1-4545 

1.5649 

1.6369 

0.0077 
0.0077 
0.0070 
0.0064 
0.0058 
0.0062 
0.0057 
0.0056 
0.0045 
0.0040 

3257 

3213T 

348s 

3653 

4545 

4098 

4545 

3922 

5076 

SS2S 

0.0078 
0.0079 
0.0073 
0.0069 
0.0056 
0.0062 
0.0056 
0.0065 
0.0059 
0.0046 

1.3200 
1.3400 
1-3580 
1.3662 
1.4200 
1.4250 
1. 43 10 

1.5000 
1. 6100 

0.0079 
0  0075 
0.0071 
0.0069 
0.0060 
0.0060 
0.0059 

0.0051 
0.0042 

In  birds,  reptiles,  and  amphibians  the  corpuscles  are  larger  than 
in  mammals,  are  oval  in  shape,  and  nucleated. 
(See  Figs.  99  and  100.)  As  the  scale  of 
animal  life  is  descended  the  corpuscles  in- 
crease in  size,  until  in  the  proteus  and  am- 
phiuma  the  long  diameter  attains  an  average 
length  of  0.058  mm.  and  0.077  mm.  respec- 
tively. In  fish  the  corpuscles  are  smaller,  oval, 
and  nucleated,  with  the  exception  of  the  lam- 
prey eels,  in  which  they  are  circular,  biconcave, 
and  nucleated,  though  the  nucleus  is  gener- 
ally concealed  in  the  peripheral  portion  of 
the  corpuscle.  As  in  these  animals,  the  cor- 
puscles are  almost  twice  the  size  of  the  human 
red  corpuscles,  they  can,  notwithstanding  the 
similarity  of  shape,  be  readily  distinguished  from  them. 

Function  of  the  Red  Corpuscles. — The  red  corpuscles,  in 
virtue  of  the  capacity  of  their  contained  hemoglobin  for  oxygen 
absorption,  may  be  regarded  as  carriers  of  oxygen  from  the  lungs  to 
the  tissues,  and  therefore  important  factors  in  the  general  respiratory 


Fig.  99.  Fig.  100. 
Amphibian  Colored 
b  l  o  o  d-c  o  r  p  u  s  cles. 
Fig.  99,  on  the  flat; 
Fig.  100,  on  edge. — 
(Landois  and  Stirling.) 


*  Masson. 


t  Woodward. 


252  TEXT-BOOK  OF  PHYSIOLOGY. 

process.  The  size  as  well  as  the  number  of  the  corpuscles  in  different 
classes  of  animals  appears  to  be  directly  related  to  the  activity  of  the 
respiratory  process.  In  those  animals  in  which  the  corpuscles  are 
small  and  numerous  and  the  total  superficial  area  large,  respiration 
is  active,  the  quantity  of  oxygen  absorbed  is  large,  and  the  energy 
evolved  through  oxidation  great.  In  those  animals,  on  the  contrary, 
in  which  the  corpuscles  are  large  and  relatively  few  in  number,  the 
reverse  conditions  obtain.  This  is  in  accordance  with  the  fact  that 
the  superficial  area  of  any  given  volume  of  substance  is  increased  in 
proportion  to  the  extent  to  which  it  is  subdivided. 

The  superficial  area  of  a  single  human  red  corpuscle  has  been 
estimated  at  0.000128  sq.  mm.  If  the  number  of  corpuscles  in  i 
cubic  milhmeter  of  blood  averages  5,000,000,  the  superficial  area 
would  amount  to  640  square  miUimeters;  and  if  the  amount  of  blood 
in  the  body  of  a  man  weighing  75  kilos  is  taken  as  one-thirteenth  of 
this  weight, — that  is,  5769  grams  (5463  c.c), — the  total  area  of  the 
corpuscular  surface  will  amount  to  3496  square  meters. 

Life-history  of  Red  Corpuscles. — In  the  performance  of  their 
functions  the  red  corpuscles  undergo  more  or  less  disintegration  and 
finally  destruction;  but  as  the  average  number  is  maintained  under 
normal  physiologic  conditions,  there  must  be  a  constant  renewal  of 
corpuscles  from  day  to  day.  The  evidence  of  destruction  of  red 
corpuscles  is  furnished  by  the  presence  in  the  blood,  in  various  situ- 
ations of  the  body,  of  a  pigment  containing  iron  and  the  presence  of 
pigments  in  the  bile  and  urine,  all  of  which  are  believed  to  be  deri- 
vatives of  effete  hemoglobin.  The  blood-pigment  (hematin),  which 
contains  the  iron  of  the  hemoglobin,  is  found  in  the  capillaries  of 
the  fiver,  in  the  cells  of  the  splenic  pulp,  and  in  the  marrow  of  the 
bones.  Whether  the  presence  of  the  pigment  in  these  organs  is  a 
proof  that  the  corpuscles  are  destroyed  here,  or  whether  they  are  to 
be  regarded  merely  as  agents  concerned  in  the  further  reduction 
and  efimination  of  the  hematin,  is  uncertain.  The  genetic  rela- 
tionship between  bile-pigment  and  hemoglobin  is  shown  by  the  fact 
that  any  artificial  destruction  of  hemoglobin  or  its  injection  into  the 
blood  is  attended  by  an  increase  in  the  quantity  of  bile-pigment 
eliminated.  It  appears  also  from  chemic  considerations  that  the 
hemoglobin  will  undergo  cleavage  into  a  globuHn  body  and  hematin, 
which  by  the  loss  of  its  iron  is  readily  converted  into  the  bile-pig- 
ment, bihrubin.  The  amount  of  this  latter  pigment  may  therefore  be 
taken  as  an  index  of  the  extent  of  corpuscular  destruction. 

This  gradual  decay  of  corpuscles  as  well  as  the  losses  occasioned 
by  hemorrhages  necessitate  a  continuous  formation  of  new  cor- 
puscles, so  that  the  normal  number  may  be  maintained.  The  rapidity 
with  which  corpuscles  may  be  renewed,  in  the  woman  at  least,  is 
shown  by  a  computation  of  Mr.  Charles  L.  Mix.     A  woman  loses 


THE  BLOOD.  253 

during  a  menstrual  period  150  c.c.  of  blood.  At  the  end  of  twenty- 
eight  or  thirty  days  this  volume  is  restored,  so  that  in  one  day  5  c.c, 
or  5000  c.mm.,  of  blood  must  be  formed,  or  208  c.mm.  per  hour  and 
3^  c.mm.  per  minute.  That  is,  during  a  certain  number  of  years 
15,750,000  corpuscles  must  be  formed  every  minute,  and  this  inde- 
pendent of  the  daily  loss  due  to  functional  activity. 

At  the  present  time  there  is  a  general  agreement  among  histolo- 
gists  that  in  adult  life  the  red  corpuscles  are  derived  from  embryonic 
forms,  the  so-called  erythroblasts,  which  are  found  chiefly  in  the 
marrow  of  the  long  bones.*  In  this  situation  both  arterial  and  venous 
capillaries  are  relatively  large  and  the  blood  is  separated  from  the 
surrounding  marrow  by  extremely  thin  walls.  In  the  passages  of  this 
capillary  network  the  erythroblasts  make  their  appearance  most 
probably  by  a  transformation  of  preexisting  marrow  cells.  At  first 
they  are  large,  homogeneous,  colorless,  perhaps  slightly  tinged  with 
hemoglobin  and  distinctly  nucleated.  They  increase  in  number  by 
karyokinesis  and  at  the  same  time  increase  in  their  hemoglobin  con- 
tent. The  nucleus  is  finally  extruded,  carrying  with  it  a  portion  of 
the  perinuclear  cytoplasm,  after  which  the  remainder  of  the  cor- 
puscle assumes  the  shape  and  size  of  the  adult  corpuscle  and  is  carried 
out  into  the  general  circulation.  After  severe  hemorrhage  the  forma- 
tive processes  in  the  marrow  may  become  so  active  that  erythroblasts 
make  their  appearance  in  the  blood-stream  before  the  extrusion  of 
the  nucleus  has  taken  place. 

CHEMIC  COMPOSITION  OF  RED  CORPUSCLES. 

Hemoglobin. — The  red  corpuscle  consists  of  a  stroma  and  a 
coloring-matter,  hemoglobin.  In  the  normal  condition  the  latter  is 
amorphous  and  in-  some  unknown  way  combined  with  the  former 
and  not  merely  diffused  in  its  meshes.  The  amount  of  hemoglobin 
per  corpuscle  is  estimated  at  90  per  cent.,  so  that  the  corpuscle  may 
be  conceived  of  as  a  mass  of  hemoglobin  supported  and  enclosed  by  a 
protoplasmic  stroma. 

If  blood  w^hich  has  been  rendered  laky,  by  water  or  any  other 
of  the  known  agencies,  be  allowed  to  slowly  evaporate,  the  dissolved 
hemoglobin  undergoes  crystaUization.  The  rapidity  with  which  the 
crystals  form  varies  in  the  blood  of  different  animals  under  similar 
conditions.  According  to  the  ease  with  which  crystallization  takes 
place,  Preyer  has  classified  various  animals  as  follows:  (i)  Very 
difficult — calf,  pigeon,  pig,  frog;    (2)  difficult — man,  monkey,  rab- 


*  For  an  admirable  resume  of  the  various  views  regarding  the  origin  and  formation 
of  red  corpuscles  see  the  paper  of  Mr.  Charles  L.  Mix,  Boston  Med.  and  Surg.  Journal, 
1892,  Nos.  II  and  12;  also  paper  by  Prof.  W.  H.  Howell,  Journal  of  Morphology, 
vol.  IV,  1802. 


2  54 


TEXT-BOOK  OF  PHYSIOLOGY. 


bit,  sheep;  (3)  easy — cat,  dog,  mouse,  horse;  (4)  very  easy — guinea- 
pig,  rat. 

The  hemoglobin  crystals  vary  in  shape  according  to  the  blood 
from  which  they  are  obtained  (Fig.  loi).     Those  obtained  from  the 

guinea-pig  are  tetrahedral;  those 
from  man  and  most  mammals 
are  prismatic  rhombs;  those 
from  the  squirrel  are  in  the  form 
of  hexagonal  plates.  Notwith- 
standing these  shght  differences, 
all  forms  belong  to  the  same 
crystal  system,  with  the  excep- 
tion of  those  from  the  squirrel. 

A  simple  but  very  effective 
method  of  obtaining  blood-crys- 
tals suggested  by  Reichert  is  to 
lake  defibrinated  blood,  espe- 
cially that  of  the  dog,  rat,  guinea- 
pig,  and  horse,  with  acetic  or 
ethyhc  ether  and  then  add  a 
solution,  I  to  5  per  cent.,  of 
ammonium  oxalate.  A  drop  of 
this  mixture  placed  under  the 
microscope  will  show  crystal  for- 
mation in  a  very  few  minutes. 

Chemic  Composition  of 
Hemoglobin. — By  appropriate 
methods  hemoglobin  can  be  ob- 
tained in  a  practically  pure  form, 
and  when  subjected  to  a  tem- 
perature of  100°  C.  its  water  of 
crystallization  is  driven  off,  after 
which  it  can  be  analyzed.  In 
the  subjoined  table  the  results  of  several  analyses  are  given  for  100 
parts  of  hemoglobin. 


Fig.    ioi. — Crystallized    Hemoglobin. 
a,  b.  Crystals  from  venous   blood  of 


man.  c.  From  blood  of  cat. 
Guinea-pig.  e.  Of  marmot, 
squirrel. — (Gaulier). 


Of 
Of 


c, - 
o,. 

H,. 

N,. 
S,  . 
Fe, 


THE  BLOOD.  255 

The  elementary  composition  of  hemoglobin  is  thus  seen  lo  vary 
slightly  in  different  animals,  suggesting  that  there  may  be  different 
kinds  of  hemoglobin.  The  rational  molecular  formula  is  not  known. 
On  the  assumption  that  each  molecule  contains  one  atom  of  iron, 
Preyer  suggested  the  following  empirical  formula:  CgooHgggNjj^Oj^j,- 
SgFe,  with  a  molecular  weight  of  13,332;  Jaquet  has  suggested  a 
different  formula:  viz.,  Cj5gHj2,,3Nj9502,8S3Fe,  with  a  molecular  weight 
of  16.669.  It  is  very  evident  from  this  that  the  molecule  is  of  enor- 
mous size  and  exceedingly  complex. 

Quantity  of  Hemoglobin. — The  quantity  of  hemoglobin  in  blood 
as  determined  by  chemic,  chromometric,  and  spectro-photometric 
methods  amounts  to  about  14  per  cent,  in  man  and  13  per  cent,  in 
woman.  Of  the  chemic  methods,  that  based  on  the  amount  of  iron 
is  the  most  familiar.  Chemic  analysis  has  shown  that  hemoglobin 
contains  0.43  per  cent,  and  blood  0.056  per  cent,  of  iron;  with  these 
two  factors  the  quantity  of  hemoglobin  can  be  determined  by  the 
following  formula:  x  =  '°°  ^  ° °^^  =  13.33  P^^  cent.  The  total 
quantity  of  hemoglobin  in  the  blood,  assuming  the  latter  to  be  about 
5769  grams  (one-thirteenth  of  the  body-weight,  75  kilos)  will  therefore 
amount  to  769  grams;  e.  g.,  x  =  5769 x  13.33  ^  ^5^^     'pj^g  |-Q^g^[  amount 

of  iron  in  the  blood  is  obtained  by  the  following  formula:  viz.,  x  = 
s769j^o^s6  _  grams. 

100  "-'       "JO 

Under  normal  physiologic  conditions  the  percentage  of  hemo- 
globin undergoes  but  slight  variation.  In  pathologic  states  there  is 
frequently  a  great  diminution  in  the  amount,  especially  in  chlorosis, 
splenic  leukemia,  and  pernicious  anemia,  diseases  in  w^hich  it  dimin- 
ishes to  2^  per  cent,  in  many  instances.  For  the  determination  of 
these  variations  in  the  hemoglobin  for  clinical  purposes  two  chromo- 
metric methods  are  at  present  largely  employed,  that  of  Gowers  and 
V.  Fleischl.  All  chromometric  methods  are  based  on  the  principle 
that  if  two  equally  thick  and  equally  well-illuminated  solutions  pre- 
sent the  same  intensity  of  color,  their  richness  in  coloring-matter  is 
the  same.  There  are  two  methods  by  which  this  can  be  done:  (i)  By 
diluting  the  blood  to  be  examined  wdth  water  until  the  shade  of  color 
corresponds  to  that  of  a  solution  of  hemoglobin  of  known  strength 
(Gowers).  (2)  Diluting  a  given  quantity  of  blood  with  a  given 
quantity  of  water  and  then  finding  an  identical  color  which  repre- 
sents a  previously  determined  quantity  of  hemoglobin  (v.  Fleischl). 

Gowers'  hemoglobinometer  consists  (Fig.  102)  of  two  glass  tubes 
of  exactly  the  same  size.  One,  A,  contains  glycerin  jelly  colored 
with  picro-carmine  the  shade  of  which  corresponds  to  that  of  normal 
blood  diluted  100  times,  20  c.mm.  in  2000  c.mm.  of  water  repre- 
senting a  I  per  cent,  solution.  The  other  tube,  B,  is  ascendingly 
graduated  with  120  divisions,  each  one  of  which  corresponds  to  20 


256 


TEXT-BOOK  OF  PHYSIOLOGY. 


c.mm.  With  a  graduated  pipette  20  cubic  millimeters  of  blood  are 
accurately  measured  and  blown  into  the  bottom  of  the  tube  B,  in 
which  a  few  drops  of  distilled  water  have  been  placed  so  as  to  prevent 
coagulation.  Water  is  then  added  drop  by  drop  until  the  color  of 
the  diluted  blood  is  exactly  that  of  the  standard.  The  division  of 
the  scale  reached  by  the  dilution  will  represent  the  relative  per- 
centage of  hemoglobin.  If  this  tint  is  not  obtained  until  the  dilu- 
tion reaches  100  divisions,  the  quantity  of  hemoglobin  is  normal. 
If  more  water  must  be  added,  it  is  in  excess;  if  less,  it  is  diminished. 
If,  for  example,  the  20  cubic  miUimeters  of  blood  from  an  anemic 
patient  gave  the  standard  tint  at  60  divisions,  the  blood  contained 
but  60  per  cent,  of  the  normal  amount  of  hemoglobin. 


Fig.  102. — GowERs'  Hemoglobinometer.  A.  Pipette  bottle  for  distilled  water. 
B.  Capillary  pipette.  C.  Graduated  tube.  D.  Tube  with  standard  dilution. 
F.  Lancet  for  pricking  the  finger. — {Landois  and  Stirling.) 


Von  Fleischl's  hemometer  consists  of  a  metallic  cell  divided  into 
two  compartments,  a  and  a',  by  a  vertical  partition  (Fig.  103).  In 
the  former  a  definite  quantity  of  blood  is  placed  and  diluted  with  a 
known  quantity  of  water.  Beneath  the  compartment  a'  is  placed 
a  glass  wedge  stained  with  the  golden  purple  of  Cassius  or  simi- 
lar pigment,  the  color  of  which  passes  from  a  deep  red  at  one  end  to 
clear  glass  at  the  other  (Fig.  104).  To  the  side  of  this  wedge  is 
placed  a  scale  ranging  from  o  to  120.  By  means  of  the  screw, 
R  T,  the  glass  wedge  is  moved  until  the  color  of  the  glass  and 
diluted  blood  is  identical.  The  illumination  of  the  blood  and  glass 
wedge  is  accompanied  by  lamplight  reflected  from  the  white  reflect- 
ing surface  beneath.     The  depth  of  color  of  the  glass  opposite  100  on 


THE  BLOOD. 


257 


the  scale  corresponds  to  that  of  normal  blood.  If,  therefore,  the 
colors  are  identical  at  75  divisions,  the  blood  contains  but  75  per 
cent,  of  hemoglobin. 

Very  frequently  the  diminution  of   corpuscles   and  hemoglobin 


Fig.  103. — Von  Fleischl's  Hemometer.  K.  Red  colored  wedge  of  glass  moved 
by  R.  G.  Mixing  vessel  vs^ith  two  compartments,  a  and  a'.  M.  Table  with 
hole  to  read  off  the  percentage  of  hemoglobin  on  the  scale  P.  T.  To  move  K. 
S.  Mirror  of  plaster-of-Paris. 


proceeds  along  parallel  lines,  in  which  case  the  amount  of  hemoglobin 
per  corpuscle  is  supposed  to  be  normal  and  the  color-index  =  i. 
If  the  hemoglobin  diminution  is  greater  than  the  corpuscles,  as  is 
the  case  in  many  pathologic  conditions,  the  color-index  is  less  than 
unity.  If  the  percentage  of  corpuscles  is  determined  by  the  method 
of  counting  to  be  80  per  cent. 
(4,000,000  per  cubic  milli- 
meter) and  the  percentage  of 
hemoglobin  60,  the  color- 
index  is  obtained  by  dividing 
the  latter  by  the  former;  e.  g., 
•f^  =  0.75.  In  other  words, 
each  corpuscle  has  but  0.75 
per  cent,  of  the  normal 
amount  of  hemoglobin. 

Absorption  Spectra. — Both  oxyhemoglobin  and  reduced  hemo- 
globin, Hke  other  soluble  pigments,  have  an  absorbing  influence  on 
certain  waves  of  light,  and  hence  give  rise  to  absorption  bands  which 
17 


Fig.  104. — Tinted  Glass  Wedge  of  the 
VON  Fleischl  Hemometer.  —  {Da 
Costa's  Hematology.) 


258 


TEXT-BOOK  OF  PHYSIOLOGY. 


can  be  studied  with  the  spectroscope,  and  which  are  so  character- 
istic as  to  serve  for  their  identification. 

In  principle  a  spectroscope  consists  of  a  prism  which  decomposes 
the  hght  from  a  narrow  sht  into  a  band  of  all  the  spectral  colors. 
A  form  of  spectroscope  in  common  use  is  that  shown  in  Fig.  105. 
It  consists  of  a  tube,  B,  which  has  at  one  end  a  sht  that  can  be 
narrowed  or  widened  by  means  of  a  screw.  The  light,  having 
passed  through  it,  falls  on  an  achromatic  convex  lens  (called  the 
colhmator)  at  the  opposite  end  of  the  tube  which  renders  the  diver- 
gent rays  of  light  parallel.  These  parallel  rays  subsequently  fall 
on  the  prism,  by  which  they  are  dispersed  and  directed  into  the 


F13.  105. — The  Spectroscope.  A.  Telescope.  B.  Tube  for  the  admission  of 
light  and  carrying  the  collimator.  C.  Tube  containing  a  scale,  the  image  of 
which  when  illuminated  is  reflected  above  the  spectrum.  D.  The  fluid  exam- 
ined.— {Landois  and  Stirling.) 


tube.  A,  which  is  nothing  more  than  a  small  telescope.  On  looking 
into  it  at  the  ocular  end  the  spectral  colors  are  seen  If  the  light 
has  been  derived  from  the  sun,  the  spectrum  will  present  vertical  dark 
hues,  the  so-called  Fraunhofer's  lines.  They  are  given  from  A  to  F 
in  Fig.  106.  If  a  colored  medium  be  held  in  front  of  the  sht  so  that 
the  light  has  to  pass  through  it  first,  certain  dark  bands  will  appear 
in  the  spectrum,  owing  to  the  absorption  of  certain  rays. 

Dilute  solutions  of  arterial  blood  show  two  absorption  bands 
between  the  Fraunhofer  fines,  D  and  E,  in  the  green  and  yellow 


THE  BLOOD. 


259 


portion  of  the  spectrum.  (See  Fig.  io6.)  The  band  nearest  D 
frequently  designated  as  alpha  is  dark  in  the  center  and  sharply 
defined.  The  band  which  lies  toward  E  is  broader  and  less  sharply 
defined. 

As  the  amount  of  light  absorbed  varies  with  the  concentration  of 
the  solution  as  well  as  its  thickness,  and  gives  rise  to  absorption  bands 
of  different  widths  and  intensities,  it  becomes  necessary,  in  order  to 
obtain  the  characteristic  bands,  to  employ  only  dilute  solutions. 

The  absorption  spectra,  as  seen  with  different  strengths  of  solu- 
tion one  centimeter  thick,  are  shown  grapically  in  Fig.  107.  It  will  be 
observed  that  solutions  varying  in  strength  from  o.i  per  cent,  to  0.6 
per  cent,  give  rise  to  the  two  characteristic  bands,  but  with  gradually 


Red.     Orange.  Yellow. 


Green. 


Cyan  Blue. 


A    a      B     C 

40  50 


Fig.  106. — Spectra  of  Hemoglobin  and  Some  of  its  Compol^nds. 

Stirlhig.) 


Reduced 
Hemoglobin 


-{Landois  and 


increasing  breadths.  With  a  percentage  greater  than  0.65  per  cent, 
the  light  between  D  and  E,  the  yellow-green,  becomes  extinguished 
and  the  two  bands  fuse  together,  forming  a  single  band  overlapping 
slightly  the  fines  D  and  E.  At  the  same  time  there  is  a  progressive 
darkening  of  the  violet  end  of  the  spectrum.  At  0.85  per  cent.,  all 
the  fight  is  absorbed  with  the  exception  of  a  small  amount  of  the  red. 
Solutions  less  than  o.oi  per  cent,  to  0.003  P^^  cent,  show  but  a  single 
absorption  band — that  nearest  D. 

A  solution  of  venous  blood  or  of  reduced  hemoglobin  shows  but  a 
single  absorption  band  (see  Fig.  106),  frequently  designated  as 
gamma,  broader  and  less  marked  between  the  lines  D  and  E,  but 
extending  sfightly  beyond  D.     Fig.  108  shows  in  the  same  graphic 


26o 


TEXT-BOOK  OF  PHYSIOLOGY. 


manner  the  increasing  breadth  of  the  absorption  band  with  increas- 
ing strengths  of  solution,  as  well  as  the  simultaneous  absorption  of 
light  at  both  the  red  and  violet  ends  of  the  spectrum. 

Compounds  of  Hemoglobin. — The  .coloring-matter  of  the 
blood  is  characterized  by  the  property  of  combining  with  and  of 
again  yielding  up  oxygen.  The  union  is  a  chemic  one,  taking  place 
under  certain  pressure  conditions.  It  therefore  may  exist  in  two 
states  of  oxidation,  distinguished  by  a  difference  in  color  and  their 
absorption  spectra.  If-  hemoglobin  either  in  blood  or  in  solution 
be  shaken  with  air,  it  at  once  combines  with  oxygen  and  is  con- 
verted into  oxyhemoglobin,  which  imparts  to  the  blood  or  solution  a 
bright  red  or  scarlet  color.     If  the  blood  or  solution  be  now  deprived 


Fig.  107. — Graphic  Representation 
OF  THE  Absorption  of  Light  in 
A  Spectrum  by  Solutions  of 
Oxyhemoglobin  of  Different 
Strengths.  The  shading  indi- 
cates the  amount  of  absorption  of 
the  spectrum,  and  the  numbers  at 
the  side  the  strength  of  the  solu- 
tion. 


aCB    D 


Fig.  108. — Graphic  Representation 
OF  THE  Absorption  of  Light 
in  a  Spectrum  by  Solutions 
OF  Hemoglobin  of  Different 
Strengths.  The  shading  indi- 
cates the  amount  of  absorption  of 
the  spectrum,  and  the  numbers  at 
the  side  the  strength  of  the  solu- 
tion. 


of  oxygen,  the  oxyhemoglobin  is  converted  into  reduced  hemoglobin, 
which  imparts  to  the  blood  or  solution  a  dark  bluish  or  purple  color. 
The  quantity  of  oxygen  absorbed  by  i  gram  of  hemoglobin  is 
estimated  at  1.56  c.c.  measured  at  0°  C.  and  760  mm.  of  mercury. 
The  compound  formed  by  the  union  of  oxygen  and  hemoglobin  is  a 
very  feeble  one ;  for  when  the  pressure  is  lowered  the  union  becomes 
less  stable,  and  as  the  zero  point  is  approached,  as  in  the  Torricelhan 
vacuum,  a  rapid  dissociation  of  the  oxygen  takes  place.  This,  how- 
ever, is  not  due  entirely  to  a  fall  of  pressure  but  partly  to  the  dis- 
sociating force  of  heat,  which  increases  in  power  as  the  pressure  falls. 
The  same  dissociation  of  oxygen  can  be  brought  about  by  passing 
through  blood  indifferent  gases,  such  as  hydrogen,  nitrogen,  carbon 


THE  BLOOD.  261 

dioxid,  which  lower  oxygen  pressure,  or  by  the  addition  of  reducing 
agents,  such  as  ammonium  sulphid  or  Stokes'  fluid. 

These  experimental  determinations  of  the  relation  of  oxygen  to 
hemoglobin  partly  explain  the  oxidation  and  deoxidation  of  the 
hemoglobin  in  the  lungs  and  tissues.  As  the  blood  passes  through 
the  lungs  and  is  subjected  to  the  oxygen  pressure  there,  the  hemoglo- 
bin combines  with  a  definite  quantity  of  oxygen,  and  on  emerging 
from  the  lungs  exhibits  a  bright  red  or  scarlet  color;  as  the  blood 
passes  through  the  systemic  capillaries  where  the  oxygen  pressure 
in  the  surrounding  tissues  is  low,  the  oxyhemoglobin  yields  up  a  por- 
tion of  its  oxygen,  becoming  deoxidized  or  reduced,  and  the  blood 
on  emerging  from  the  tissues  exhibits  a  dark  bluish  color.  The  portion 
of  oxygen  given  up  to  the  tissues  is  termed  respiratory  oxygen.  In 
100  parts  of  arterial  blood  the  coloring-matter  presents  itself  almost 
exclusively  in  the  form  of  oxyhemoglobin.  In  passing  through  the 
capiharies  about  5  per  cent,  only  gives  up  its  oxygen  and  becomes 
reduced,  so  that  both  kinds  are  present  in  venous  blood.  In  asphyx- 
iated blood  only  reduced  hemoglobin  is  present.  It  is  this  capa- 
bihty  of  combining  with  and  of  again  yielding  up  oxygen,  that 
enables  hemoglobin  to  become  the  carrier  of  oxygen  from  the  lungs 
to  the  tissues. 

Carbon  Monoxid  Hemoglobin. — Carbon  monoxid  is  a  con- 
stituent of  both  coal-gas  and  water-gas.  From  either  source  it  is 
likely  to  accumulate  in  the  air,  and  if  inspired  for  any  length  of  time 
produces  a  series  of  effects  which  may  eventuate  in  death.  If  blood 
be  brought  into  contact  with  this  gas,  it  assumes  a  bright  cherry-red 
color,  which  is  quite  persistent  and  due  to  the  displacement  of  the 
loosely  combined  oxygen  and  the  union  of  the  carbon  monoxid  with 
the  hemoglobin.  The  compound  thus  formed  is  very  stable  and  resists 
the  action  of  various  reducing  agents.  The  passage  of  air  or  of  some 
neutral  gas  through  the  blood  for  a  long  period  of  time  will  gradually 
displace  the  carbon  monoxid  and  enable  the  hemoglobin  to  again 
absorb  oxygen.  It  is  for  this  reason  that  partial  poisoning  with  the 
gas  is  not  fatal.  It  is  to  its  power  of  displacing  oxygen  and  form- 
ing a  stable  compound  with  hemoglobin  and  thus  interfering  with 
its  respiratory  function  that  carbon  monoxid  owes  its  poisonous 
properties.  Examined  spectroscopically,  solutions  of  carbon  mon- 
oxid hemoglobin  exhibit  two  absorption  bands  closely  resembling 
in  position  and  extent  those  of  oxyhemoglobin;  but  careful  examina- 
tion shows  that  they  are  slightly  nearer  the  violet  end  of  the  spectrum 
and  closer  together.  (See  Fig.  106.)  A  useful  test  for  CO  blood 
is  the  addition  of  caustic  soda,  which  produces  a  cinnabar  red  pre- 
cipitate. 

Methemoglobin. — This  is  a  pigment,  closely  related  to  oxy- 
hemoglobin, found  in  the  blood  after  the  administration  of  various 


262  TEXT-BOOK  OF  PHYSIOLOGY. 

drugs,  in  cysts  and  in  the  urine  in  hematuria  and  hemoglobinuria. 
It  is  also  produced  when  a  solution  of  hemoglobin  is  exposed  to  the 
air  and  becomes  acid  in  reaction  and  brown  in  color.  The  spectrum 
shows  two  absorption  bands  similar  to  oxyhemoglobin,  but  in  addition 
a  new  and  quite  distinct  band  near  the  hne  C,  in  the  red.  If  the 
acid  solution  be  rendered  alkahne  by  the  addition  of  ammonia,  this 
band  disappears  and  another  makes  its  appearance  near  the  line  D. 
The  addition  of  ammonium  sulphid  develops  reduced  hemoglobin, 
which,  on  the  absorption  of  oxygen,  produces  again  oxyhemoglobin, 
as  shown  by  the  spectroscope. 

Hematin. — Boiling  hemoglobin  or  adding  to  it  acids  or  alka- 
lies decomposes  it  and  develops  one  or  more  proteid  bodies  to 
which  the  general  term  globulin  has  been  given,  and  an  iron-holding 
pigment  termed  hematin.  This  is  regarded  as  an  oxidation  product 
of  hemoglobin  and  constitutes  about  4  per  cent,  of  its  composition. 
When  obtained  in  a  pure  state,  it  is  a  non-crystalHzable  blue-black 
powder  with  a  metalHc  luster.  According  as  it  is  treated  with  acids  or 
alkahes,  two  forms  of  hematin  can  be  obtained  (acid  and  alkahne), 
each  of  which  has  special  properties,  giving  rise  to  different  absorp- 
tion bands. 

Hemochromogen. — This  pigment  is  derived  from  hemoglobin, 
of  which  it  constitutes  about  96  per  cent.,  during  decomposition  in 
the  absence  of  oxygen.  In  solution  it  produces  a  purple  color,  but 
soon  absorbs  oxygen  and  is  converted  into  hematin. 

Hemin. — This  pigment  is  a  derivative  of  hematin,  presenting 
itself  in  the  form  of  microscopic  rhombic  plates  or  rods  (Teichmann's 
crystals),  which  are  so  characteristic  as  to  serve  as  tests  for  blood- 
stains in  medico-legal  inquiries.  These  crystals  are  readily  obtained 
by  adding  to  a  small  quantity  of  dried  blood  on  a  glass  shde  a  few 
drops  of  glacial  acetic  acid  and  a  crystal  of  sodium  chlorid;  after 
heating  gently  for  a  few  minutes  over  a  spirit  lamp  and  then  allowing 
the  mixture  to  cool,  crystallization  of  the  hemin  soon  takes  place. 

Hematoidin. — This  term  has  been  applied  to  a  pigment  which 
occurs  in  the  form  of  yellow  crystals  in  old  blood-clots  or  in  blood 
which  has  been  extravasated  into  the  tissues.  In  its  chemic  com- 
position and  in  its  reactions  it  closely  resembles  bilirubin,  the  pigment 
of  the  bile,  exhibiting  the  same  characteristic  play  of  colors  on  the 
addition  of  nitric  acid. 

The  Stroma. — The  stroma  of  the  red  corpuscles  obtained  by  the 
methods  which  dissolve  out  the  hemoglobin  has  been  shown  by 
analysis  to  consist  of  from  60  to  70  per  cent,  of  water  and  40  to  30 
per  cent,  of  solid  material,  containing  a  proteid  resembling  cell- 
globulin,  lecithin,  cholesterin,  and  inorganic  salts,  among  which 
potassium  phosphate  is  especially  abundant. 


THE  BLOOD.  263 

HISTOLOGY  OF  THE  WHITE  CORPUSCLES  OR  LEUKOCYTES. 

The  histologic  features  of  the  white  corpuscles  can  readily  be 
observed  under  the  same  conditions  as  in  the  case  of  the  red  corpuscles. 
Within  the  smaller  blood-vessels  they  are  seen  adhering  to  the  walls 
of  the  vessel ;  in  a  drop  of  freshly  drawn  blood  they  are  found  in  the 
spaces  between  the  rouleaux  of  red  corpuscles.     (See  Fig.  96,  p.  236.) 

Shape  and  Size. — In  the  resting  condition,  whether  seen  in  the 
vessel  or  on  the  stage  of  the  microscope,  the  white  corpuscle,  as  its 
name  implies,  is  grayish  in  color,  round  or  globular  in  form,  though 
often  presenting  a  more  or  less  irregular  surface.  Its  diameter  varies 
from  0.0004  to  0.0013  mm.,  though  the  average  is  about  o.ooii  mm. 
or  about  -jtwo  inch. 

Structure. — A  typical  white  corpuscle  consists  of  a  ground- 
substance  uniformly  transparent  and  apparently  homogeneous,  in 
which  are  embedded  a  number  of  granules  of  varying  size,  some  of 
which  are  very  fine,  while  others  are  larger.  By  various  reagents  it 
has  been  demonstrated  that  the  granules  are  fatty,  proteid,  and 
carbohydrate  (glycogen)  in  character.  In  the  fresh  cells  the  ex- 
istence of  a  nucleus  is  difficult  of  detection,  though  its  presence  can 
be  demonstrated  by  the  addition  of  acetic  acid,  which  renders  the 
perinuclear  cytoplasm  more  transparent  and  makes  the  nucleus 
conspicuous  and  sharply  defined.  From  its  structure  it  is  apparent 
that  the  white  corpuscle  belongs  to  the  group  of  undifferentiated 
tissues  and  resembles  the  cells  of  the  embryo  in  its  earhest  stages  as 
well  as  the  unicellular  organism,  the  amoeba. 

Number  of  White  Corpuscles. — The  number  of  white  cor- 
puscles per  cubic  millimeter  of  blood  is  much  less  than  the  number 
of  red  corpuscles,  the  ratio  being  in  the  neighborhood  of  i  white  to 
700  red.  This  ratio,  however,  varies  within  wide  hmits  in  different 
portions  of  the  body  and  under  normal  variations  in  physiologic 
conditions.  In  the  blood  of  the  splenic  artery  there  is  but  i  white 
to  2260  red,  while  in  the  splenic  vein  there  is  i  white  to  every  60  red; 
or  about  thirty-eight  times  as  many  as  in  the  artery.  In  the  portal 
vein  there  is  i  white  to  740  red,  while  in  the  hepatic  vein  there  is  i 
white  to  170  red. 

The  total  number  of  white  corpuscles  per  cubic  millimeter  has 
been  estimated  at  from  5000  to  10,000,  though  the  average  is  about 
7500.  The  number,  however,  is  influenced  by  a  variety  of  physio- 
logic conditions.  The  ingestion  of  food  rich  in  proteid  material 
raises  the  count  from  30  to  40  per  cent.,  as  compared  with  the 
count  before  the  meal.  Fasting  for  a  few  days  always  lowers  the 
count,  and  in  a  case  of  total  abstinence  of  food  for  a  week,  reported 
by  Luciani,  the  count  fell  to  861  per  cubic  milhmeter,  after  which 
it  rose  to  1530,  where  it  practically  remained  for  the  succeeding  three 


264 


TEXT-BOOK  OF  PHYSIOLOGY. 


Fig.  109. — Human  Leukocytes  show- 
ing Ameboid  Movements. — (Frey.) 


weeks  of  the  fasting  period.  In  the  new-born  the  number  is  greater 
than  in  adults — 17,000  to  20,000  per  cubic  milHmeter.  Cabot  states 
that  30,000  is  never  a  high  count  after  a  meal  in  infants  under  two 
years.  In  the  later  months  of 
pregnancy,  especially  in  primi- 
parae,  the  number  increases  to 
16,000  to  18,000.  Many  patho- 
logic conditions  of  the  body  also 
influence  the  count  very  con- 
siderably. 

The  method  for  counting  the 
white  corpuscles  is  similar  to  that 
used  in  counting  the  red.  The 
given  volume  of  blood  should, 
however,  be  diluted  with  10  vol- 
umes of  a  one-third  of  one  per 
cent,  solution  of  acetic  acid, 
which  renders  the  red  corpuscles 
invisible  and  thus  facilitates  the 
counting  of  the  white.  The  pip- 
ette should  have  a  larger  bore 

than  that  used  for  the  red,  and  a  greater  number  of  squares  in  the 
counting  chamber  should  be  counted,  so  as  to  diminish  the  percent- 
age of  error. 

Physiologic    Properties. — The   white   corpuscles   possess   the 

characteristic  property  of 
exhibiting  movements  simi- 
lar to  those  observed  in  the 
amoeba,  and  are  therefore 
termed  ameboid.  These 
movements  consist  in  alter- 
nate protrusions  and  re- 
tractions of  portions  of  the 
cell  body,  as  a  result  of 
which  they  exhibit  a  great 
variety  of  forms.  (See  Fig. 
109.)  The  protruded  pro- 
cess can  also  attach  itself 
to  some  point  of  the  sur- 
face on  which  it  rests,  and 
then  draw  the  body  of  the 
corpuscle  after  it.  By  a 
repetition  of  this  process 
the  corpuscle  can  slowly 
creep  about  and  change  its  position  in  space.     In  virtue  of  these 


Fig.  iio. — Small  Vessel  of  a  Frog's  Mesen- 
tery   SHOWING    DiAPEDESIS.      W,    W.   Vas- 

cular  walls,  a,  a.  Poiseuille's  space,  r, 
r.  Red  corpuscles.  /,  /.  Colorless  cor- 
puscles adhering  to  the  wall,  and,  c,  c,  in 
various  stages  of  extrusion.  /,  /.  Ex- 
truded corpuscles. — {Landois  and  Stirling.) 


{Triacid  Stain.) 
I,  2,  3,  4.  Small  Lymphocytes. 

Contrast  the  faintly  coIoilcI  protoplasm  of  these  cells  in  the  triple  stained  specimen 
with  their  intensely  basic  protoplasm  in  the  film  stained  with  eosin  and  methylene- 
blue,  17  and  18.  The  cell  body  of  1  is  invisible.  Note  the  kidney-shaped  nucleus 
in  4. 

5,  6.  Large  Lymphocytes. 

With  this  stain  the  nucleus  reacts  more  strongly  than  the  protoplasm;  with  eosin 
and  methylene-blue  (19,  20),  on  the  contrary,  the  protoplasm  is  so  deejjly  stained 
that  the  nucleus  appears  pale  by  contrast.  This  peculiarity  is  also  observed  in 
the  smaller  forms  of  lymphocytes. 

7,  8.  Transitional  Forms. 

Note  the  moderately  basic  and  indented  nucleus,  and  the  almost  hyaline  non- 
granular protoplasm.  Compare  8  with  the  myelocyte,  7,  Plate  I,  these  cells 
differing  chiefly  in  that  the  myelocyte  contains  neutrophile  granules. 

9,  10,  II.  Polynuclear  Neutrophiles. 

These  ceils  are  characterized  by  a  polymorphous  or  polynuclear  nucleus,  sur- 
rounded by  a  cell  body  filled  with  fine  neutrophile  granules.  In  11  the  nuclear 
structure  is  obviously  separated  into  four  parts;  in  9  it  is  moderately,  and  in  10 
markedly,  polymorphous. 

12,  13.  Eosinophiles. 

The  nuclei  are  not  unlike  those  of  the  polynuclear  neutrophile,  except  that  they 
are  somewhat  less  convoluted,  and  poorer  in  chromatin,  staining  less  intensely. 
The  protoplasm  is  filled  with  coarse  eosinophile  granules,  the  characteristics  of 
which  are  clearly  illustrated  by  13,  a  "fractured"  eosinophile. 

14.  Eosinophilic  Myelocyte. 
Compare  with  15 

15,  16.  Myelocytes.     {Neutrophilic.) 

These  cells  are  morphologically  similar  to  14,  except  that  they  contain  neutrophile 
instead  of  eosinophile  granules.  Note  that  the  granules  of  the  myelocyte  are 
identical  with  those  of  the  polynuclear  neutrophile.  A  dwarf  form  of  myelocyte 
is  represented  by  16. 

{Eosin  and  Methylene-blue.) 

i-j,  18.  Small  Lymphocytes. 

Note  the  narrow  rim  of  pseudo-granular  basic  protoplasm  surrounding  the  nucleus, 
and  the  pale  appearance  of  the  latter. 

19,  20.  Large  Lymphocytes. 

Budding  of  the  basic  zone  of  protoplasm  is  represented  by  20.  Both  of  these 
cells  belong  to  the  same  type  as  5  and  6. 

2£,  22.  Large  Mononuclear  Leukocytes. 

Compared  with  19  and  20,  these  cells  have  a  decidedly  less  basic  protoplasm,  but 
a  somewhat  more  basic  nucleus.  In  the  triple  stained  film  these  differences  can- 
not be  detected,  so  that  they  must  be  classed  as  large  lymphocytes. 

23.  Transitional  Form. 

The  distinction  between  this  cell  and  24  is  not  marked;  the  nucleus  of  the  latter 
simply  being  somewhat  more  basic  and  convoluted. 

24,  25,  26,  27.  Polynuclear  Neutrophiles. 

With  this  stain  these  cells  show  a  feebly  acid  protoplasm,  and  lack  granules. 
Note  that  the  more  twisted  the  nucleus  the  deeper  it  is  stained.     Compare  with 
9,  10,  and  II. 
28,  29.  Eosinophiles. 

Compare  with  12  and  13. 

30.  Eosinophilic  Myelocyte. 
Compare  with  14. 

31.  Basophile.     {Finely  granular.) 

This  cell  is  characterized  by  the  presence  of  exceedingly  fine  ''-granules,  staining 
the  pure  color  of  the  basic  dye.  The  nucleus  is  markedly  convoluted  and  deficient 
in  chromatin.     The  cell  here  shown  was  found  in  normal  blood. 

32.  ^T„  34,  35,  36.  Mast  Cells.  . 

The  granules  take  a  modified  basic  color,  as  shown  by  their  royal-purple  tint  in 
this  illustration.  Note  their  unusually  large  size  and  ovoid  shape  in  35,  their 
pecuHar  distribution  in  35  and  36,  and  their  irregularity  in  size  in  32  and  36. 
With  the  triacid  mixture  these  granules,  as  well  as  those  of  the  finely  granular 
basophile,  31,  remain  unstained,  showing  as  dull-white  stippled  areas  in  the  cell 
body.  The  nuclear  chromatin  of  the  mast  cell  is  so  delicate  and  so  feebly  stained 
that  it  is  barely  visible.  These  cells  were  found  in  the  blood  of  a  case  of  spleno- 
medullary  leukemia. 


PLATE   I. 


Q 


••:■  •?.•*•  13 


'°§o<4»J'>'« 


17      18      ^..^■■~-      ^■^^x 

oo(  )0 


26  27 


34 


•3'5|?»' 


35 


f* 


36 


'%.#«.«•• 


The  Leukocytes. 

(i-i6,  Triacid  Stain;   17-36,  Eosin  and  Melhylene-blue.) 

(E.  F.  Faber,  lee.) 

(From   DaCosta's  '-Clinical  Hematology.") 


THE  BLOOD.  265 

ameboid  movements  the  corpuscle  can  appropriate  small  particles 
of  pigment,  such  as  indigo  or  carmine,  and  after  a  short  time 
eliminate  them  from  various  parts  of  the  surface.  It  is  also  capable 
of  thrusting  a  process  into  and  through  the  wall  of  the  capillary 
vessel,  after  which  the  remainder  of  the  corpuscle  follows  (Fig. 
no).  This  continues  until  the  corpuscle  is  outside  the  vessel  and  in 
the  lymph-space,  where  it  resumes  its  original  shape  and  movement. 
This  process  is  best  observed  in  inflammatory  conditions,  when  the 
blood  has  come  to  rest  and  the  vessels  are  occluded  with  both  red  and 
white  corpuscles.  To  this  passage  of  the  white  blood-corpuscles 
through  the  capillary  wall  the  term  diapedesis  is  given.  The  move- 
ments of  the  white  corpuscles  are  increased  by  a  rise  in  temperature 
up  to  40°  C,  beyond  which  they  cease,  owing  to  the  coagulation  of 
the  cell-substance.  A  low  temperature  also  arrests  the  movements. 
Induced  electric  currents  also  cause  contraction  and  death  of  the  cell. 
Moisture  and  oxygen  are  necessary  to  their  activity.  From  their 
similarity  to  lower  organisms  the  white  corpuscles  may  be  regarded 
as  independent  organisms  living  in  the  animal  fluids,  just  as  the 
amoeba  Uves  in  its  natural  Hquid  medium. 

Classification. — With  the  aid  of  the  tricolor  staining  fluid  of 
Ehrhch  four  distinct  forms  of  white  corpuscles  or  leukocytes  can  be 
demonstrated  to  be  present  in  the  blood,  viz.: 

1.  Small  lymphocytes,  so  called  from  their  resemblance  to  the  cor- 

puscles of  the  lymph-glands,  consisting  of  a  small  dark  nucleus 
surrounded  by  a  very  thin  layer  of  cytoplasm. 

2.  Large  mononuclear  lymphocytes,  which  represent  the  preceding  type 

at  a  later  stage  of  development  and  in  the  possession  of  a  large 
amount  of  perinuclear  cytoplasm  more  or  less  hyahne  and  devoid 
of  granules.  The  nucleus  is  often  deeply  notched,  resembhng  a 
horseshoe  in  shape.  This  cell  is  capable  of  executing  ameboid 
movements. 

3.  Polymorphonuclear  leukocytes   or   neutrophiles,  which   represent 

the  adult  condition  of  the  cell.  The  nucleus  is  irregular  and 
assumes  a  variety  of  shapes  in  different  cells,  a  feature  which 
has  suggested  the  name  given  to  the  cell.  The  perinuclear  cyto- 
plasm contains  a  number  of  granules  which  are  made  evident 
when  stained  with  the  neutral  mixture  of  Ehrlich.  These 
cells  exhibit  active  ameboid  movements.  They  make  up  about 
60  to  70  per  cent,  of  the  whole  number  of  the  white  blood-cells. 

4.  Eosinophile  cells,  the  granules  of  which  stain  most  readily  with 

acid  stains  like  eosin.  The  granules  are  spheric  and  larger 
than  in  the  previous  cell.  The  nucleus  is  pale  and  irregular 
in  shape.  The  eosinophile  cell  is  regarded  as  the  old  or  "over- 
ripe" cell  and  is  the  most  actively  ameboid  of  all  the  cells.  It 
is  present  to  the  extent  of  from  |  to  4  per  cent. 


266  TEXT-BOOK  OF  PHYSIOLOGY. 

Origin  of  White  Corpuscles. — The  white  corpuscles  which  are 
present  in  the  blood  are  believed  to  be  derived  from  the  lymphocytes 
or  lymph-corpuscles  which  find  their  way  into  the  blood  at  the  points 
where  the  lymph-ducts  discharge  their  lymph:  viz.,  at  the  junc- 
tions of  the  internal  jugular  and  subclavian  veins.  Along  the  course 
of  the  lymph-vessels  are  to  be  found,  in  different  regions  of  the 
body,  numerous  lymph-glands  the  meshes  of  which  are  filled  with 
small,  colorless,  nucleated  cells,  which  arise  by  self-division  and  rep- 
resent the  early  stages  in  the  development  of  lymphocytes.  Similar 
corpuscles  are  found  in  the  mucous  membranes,  skin,  spleen,  and  the 
fluids  of  the  tissues.  As  the  lymph  flows  through  the  glands  these  cells 
are  washed  out  and  carried  direct  to  the  blood.  In  their  passage  they 
grow  in  size  by  increasing  the  amount  of  their  cytoplasm  and  even- 
tually become  normal  adult  leukocytes.  After  an  unknown  period 
of  life  they  undergo  dissolution  and  disappear. 

Chemic  Composition. — The  chemic  composition  of  the  white 
corpuscles  has  been  inferred  from  an  analysis  of  pus-corpuscles, 
with  which  they  are  practically  identical,  and  of  lymph-corpuscles 
from  the  lymph-glands.  Of  the  corpuscle  about  90  per  cent,  is 
water  and  the  remainder  soHd  matter  consisting  mainly  of  proteids, 
of  which  nuclein,  nucleo-albumin,  and  cell  globulin  are  the  most 
abundant.  The  two  former  are  characterized  by  the  presence  of  a 
considerable  quantity  of  phosphorus,  amounting  to  as  much  as  10 
per  cent.  Lecithin,  fat,  glycogen,  and  earthy  and  alkahne  phos- 
phates are  also  present. 

Functions. — The  functions  of  the  white  corpuscles  are  but  im- 
perfectly known,  and  at  present  no  positive  statements  can  be  made. 
It  has  been  suggested  that  wherever  found  in  the  body,  whether  in 
blood  or  tissues,  they  are  engaged  in  the  removal  of  more  or  less  in- 
soluble particles  of  disintegrated  tissues,  in  attacking  and  destroying 
more  or  less  effectively  various  forms  of  invading  bacteria  and  thus 
protecting  the  body  against  their  deleterious  actiVity.  This  they  do 
by  surrounding,  enveloping,  and  incorporating  either  the  tissue  par- 
ticle or  bacterium  and  digesting  it.  On  account  of  this  swallowing 
action  these  cells  were  termed  by  Metchnikoff  phagocytes  and  the 
process  phagocytosis.  He  regards  them  as  the  general  scavengers  of 
the  body.  It  has  been  suggested  that  they  are  also  engaged  in 
the  absorption  of  fat  from  the  lymphoid  tissue  of  the  intestine.  In 
their  dissolution  they  contribute  to  the  blood-plasma  certain  proteid 
materials  which  assist  under  favorable  circumstances  in  the  coagu- 
lation of  the  blood. 

HISTOLOGY  OF  THE  BLOOD-PLATES. 

The  blood-plates  or  plaques  are  small  histologic  elements  circu- 
lating in  the  blood-plasma.     They  were  discovered  by  Hayem,  who 


THE  BLOOD.  267 

applied  to  them  the  term  hematoblasts,  on  the  supposition  that  they 
were  the  early  stages  in  the  development  of  the  red  corpuscles.  This 
is  now  known  to  be  erroneous.  On  account  of  their  specific,  distinct 
characters,  and  their  constant  presence  in  the  blood  of  hving  animals 
(guinea-pig  and  bat),  they  are  now  regarded  as  normal  constituents 
of  the  blood  and  designated  as  the  third  corpuscle.  When  blood  is 
freshly  drawn  from  the  body,  the  plaques  rapidly  undergo  disintegra- 
tion and  disappear;  but  by  treating  the  blood  with  osmic  acid,  the 
form  and  structure  of  the  plaque  may  be  retained. 

The  blood-plaque  may  be  defined  as  a  colorless,  grayish-white, 
homogeneous  or  finely  granular  protoplasmic  disk,  varying  in  diam- 
eter from  1.5  to  3.5  micro-millimeters.  The  edges  are  rounded  and 
well  defined,  but  it  is  not  certain  whether  they  are  only  flattened  or 
are  shghtly  biconcave.  There  is,  however,  no  nucleus.  The  ratio  of 
the  plaques  to  the  red  corpuscles  is  i  to  18  or  20,  and  the  total  number 
per  cubic  miUimeter  has  been  estimated  to  be  250,000  to  300,000. 

When  blood  is  shed  they  tend  to  adhere  to  each  other  and  form 
irregular  masses  known  as  Schultze's  granular  masses.  If  threads 
are  suspended  in  blood,  the  plaques  accumulate  in  enormous  numbers 
upon  them  and  appear  to  form  a  center  from  which  fibrin  filaments 
radiate  as  coagulation  proceeds.  The  white  thrombi  which  form  in 
blood-vessels  in  consequence  of  diseased  states — e.  g.,  endocarditis, 
atheromatous  ulceration,  etc. — are  composed  very  largely  of  blood- 
plaques  and  fibrin  threads.  The  function  of  the  blood-plaques  is 
unknown,  but  it  has  been  surmised  that  in  some  way  they  are,  like 
the  leukocytes,  concerned  in  the  coagulation  of  the  blood.  When- 
ever they  are  diminished  in  number,  as  in  purpura  and  hemophilia, 
coagulation  takes  place  very  slowly. 

The  blood-plaques  can  be  seen  with  high  powers  of  the  micro- 
scope in  the  blood-vessels  of  the  omentum  of  the  guinea-pig  and  rat, 
especially  when  the  blood-stream  begins  to  slow.  They  are  also 
readily  seen  in  the  blood-vessels  of  subcutaneous  connective  tissue 
of  various  animals,  and  especially  in  that  of  the  new-born  rat.  A 
small  quantity  of  this  tissue  moistened  with  normal  saline  and  exam- 
ined microscopically  with  suitable  powers  will  show  large  numbers 
of  plaques  within  the  blood-vessels. 


THE  TOTAL  QUANTITY  OF  THE  BLOOD;  ITS  GENERAL  COMPOSITION. 

The  determination  of  the  total  quantity  of  the  blood  in  an  animal 
is  best  made  by  the  chromometric  method,  somewhat  modified  at 
present,  of  Welcker.  This  consists,  first,  in  bleeding  an  animal, 
collecting  all  the  blood  it  yields,  and  weighing  it ;  second,  in  washing 
out  the  vessels  with  a  normal  saline  solution  until  the  fluid  comes 
from  the  veins  clear  and  free  from  blood ;  third,  in  mincing  the  tissues 


268  TEXT-BOOK  OF  PHYSIOLOGY. 

of  the  body,  after  removal  of  the  contents  of  the  alimentary  canal, 
soaking  them  in  water  for  twenty-four  hours,  and  then  expressing 
them.  All  the  washings  are  collected  and  weighed.  A  given 
volume  of  the  normal  defibrinatcd  blood,  treated  with  carbon 
monoxid  so  as  to  give  it  uniform  color,  is  then  diluted  with  water 
until  its  tint  is  identical  with  that  of  the  washings  similarly  treated 
with  carbon  monoxid.  From  the  quantity  of  water  necessary  to 
dilute  the  blood  the  quantity  of  blood  in  the  washings  is  readily 
determined.  The  animal  having  been  previously  weighed  and  the 
weight  of  the  contents  of  the  alimentary  canal  deducted,  the  ratio 
of  the  total  weight  of  the  blood  to  the  weight  of  the  body  at  once 
becomes  apparent.  By  this  method  it  has  been  shown  that  the  ratio 
of  blood  to  body- weight  in  a  human  adult  is  i  :  13;  in  an  infant, 
1 :  19;  in  a  dog,  i  :  13;  in  a  cat,  i  :  21.  Thus  an  adult  man  of  75 
kilos  weight  would  have  5769  grams  of  blood. 

The  amount  of  blood  in  the  different  organs  has  been  determined 
by  hgating  the  blood-vessels  in  the  living  animal,  removing  the  organ, 
and  after  allowing  the  blood  to  escape  subjecting  the  tissues  to  the 
chromometric  methods  described  above.  According  Jo  Ranke,  the 
volume  of  the  blood  is  distributed  as  follows :  Heart,  lungs,  arteries, 
and  veins,  l;  liver,  I;  muscles,  J;  other  organs,  ^. 

General  Composition. — The  results  of  the  analyses  of  the  blood 
will  vary  with  the  animal  and  the  methods  employed.  The  following 
table,  taken  from  Gad,  shows  the  average  composition,  expressed  in 
whole  numbers,  of  horse's  blood.  In  essential  respects  the  ratio  of 
the  constituents  in  human  blood  would  not  be  materially  different. 

One  thousand  parts  of  blood  contain: 

{Water, 200 200 

f  Hemoglobin, 116 

Solids,    128  ^  Other  organic  matter, 10 

(Salts,    2 

fWater,    604 604 

Plasma...... 672 -j  [ ISumin7::::i::.":::-":"i:-i:i  5I 

^Solids,    68-^^.,'  '         "7 

'  Other  orgamc   matter, 3 

I  Potassium  and  sodium  salts, 4 

[Calcium  and  magnesium  salts, i 

CHEMISTRY  OF  COAGULATION. 

The  changes  which  eventuate  in  the  formation  of  fibrin,  and 
hence  all  the  subsequent  phenomena  of  coagulation,  are  chemic  in 
character;  but  as  these  changes  take  place  in  organic  compounds  the 
composition  of  which  is  but  imperfectly  known,  the  intimate  nature 
of  the  process  is  quite  obscure.  All  the  theories  which  have  been 
advanced  in  explanation,  though  approximating  the  truth,  are  more 
or  less  incomplete  and  in  some  respects  contradictory.     Since  the 


THE  BLOOD.  269 

coagulation  is  coincident  with  the  appearance  of  the  fibrin,  the  ante- 
cedents of  this  substance,  the  physical  and  chemic  conditions  which 
condition  its  development,  and  the  succession  of  chemic  changes  in- 
volved must  be  determined,  before  any  consistent  theory  can  be 
established. 

Extra-vascular  Coagulation. — At  present  it  is  generally  be- 
lieved that  the  immediate  factors  concerned  in  extra-vascular  coagu- 
lation are  fibrinogen,  a  calcium  salt,  and  a  ferment-body.  As  to  the 
manner  in  which  these  three  bodies  react  one  with  another  there  is  a 
diversity  of  opinion.  At  least  five  different  theories  are  current  at 
the  present  time,  all  of  which  have  some  features  in  common,  though 
presenting  points  of  difference. 

Alexander  Schmidt  long  contended  that  fibrin  was  the  result  of 
a  union  of  fibrinogen  and  paraglobuhn ;  that  the  union  was  brought 
about  by  a  ferment-body;  that  the  presence  of  the  neutral  salts  of  the 
plasma  was  necessary  to  the  activity  of  the  ferment.  Previous  to  his 
death  in  1893  Schmidt  modified  his  view  as  follows:  The  insoluble 
fibrin  is  developed  out  of  a  soluble  fibrin  derived  from  paraglobuhn, 
which  in  turn  is  a  product  of  general  cell  disintegration;  the  conver- 
sion of  the  fibrinogen  into  fibrin  is  due  to  the  activity  of  a  ferment, 
thrombin,  a  derivative  of  pro-thrombin,  a  product  of  the  disintegra- 
tion of  leukocytes,  lymph-cells,  etc.;  that  the  production  of  thrombin 
is  conditioned  by  the  presence  of  the  neutral  salts  of  the  plasma  in 
normal  percentages;  that  no  one  of  these  salts,  calcium  included,  acts 
in  a  specific  manner;  finally,  that  fibrin  is  not  a  compound  of  a  pro- 
teid  and  calcium. 

Hammersten,  as  a  result  of  many  years  of  investigation,  believes 
that  paraglobuhn  is  not  necessary  to  the  process,  fibrinogen  alone 
being  transformed  into  fibrin  under  the  influence  of  the  ferment,  in  the 
presence  of  a  neutral  salt,  especially  calcium,  which  acts  specifically 
in  a  manner  different  from  the  sodium  salts.  Inasmuch  as  the  quan- 
tity of  fibrin  produced  is  always  less  than  the  quantity  of  fibrinogen 
previously  present,  Hammersten  concludes  that  the  latter  substance, 
under  the  influence  of  the  ferment,  undergoes  a  cleavage  into  two 
unequal  portions,  one  of  which  remains  in  solution,  the  other  solidify- 
ing as  fibrin.  While  admitting  that  the  calcium  salts  act  specifically, 
he  believes  that  they  are  concerned  rather  with  the  production  of  the 
ferment  than  the  fibrin,  for  if  the  ferment  is  present  in  sufficient 
quantity  coagulation  takes  place  in  a  typical  manner  even  in  the  total 
absence  of  calcium. 

Arthus  and  Pages  conclude  that  for  the  transformation  of  fibrin- 
ogen into  fibrin  the  calcium  salts  are  absolutely  essential  and  act  in  a 
specific  manner;  that  the  ferment  causes  a  cleavage  of  fibrinogen  into 
two  substances,  one  of  which  remains  in  solution,  the  other  com- 
bines with  calcium  to  form  fibrin.     They  offer  in   support  of   this 


270  TEXT-BOOK  OF  PHYSIOLOGY. 

view  the  fact  that  if  a  i  per  cent,  solution  of  potassium  oxalate  be 
added  to  blood  in  quantity  sufficient  to  precipitate  the  calcium,  coagu- 
lation will  not  take  place;  but  if  calcium  is  restored  coagulation 
proceeds  in  the  usual  manner.  They  transfer  the  sphere  of  influence 
of  calcium  to  the  formation  of  the  fibrin  rather  than  to  the  formation 
of  the  ferment. 

Pekelharing's  researches  led  him  to  the  conclusion  that  there 
arises  from  the  disintegration  of  the  leukocytes  a  nucleo-proteid, 
pro-thrombin,  which  combining  with  the  calcium  salt  forms  the 
ferment  thrombin.  This  compound  then  transfers  the  calcium  to 
the  fibrinogen,  which  in  turn  becomes  fibrin;  the  latter  is  therefore 
a  proteid-calcium  compound. 

Lilienfeld  asserts  that  fibrin  formation  is  a  cleavage  process  by 
which  fibrinogen  is  separated  into  two  bodies,  one  an  albumose  which 
remains  in  solution,  the  other  a  proteid  to  which  he  has  given  the 
name  thromhosin.  This  cleavage  is  attributed  to  the  action  of  the 
usual  ferment,  a  product  of  the  disintegration  of  leukocytes.  Throm- 
bosin  combines,  according  to  Lilienfeld,  with  calcium  to  form  fibrin. 

In  a  critical  examination  of  these  different  theories  Hammersten 
denies  that  fibrin  is  a  compound  of  a  proteid  and  calcium ;  for  chemic 
analysis  of  both  fibrinogen  and  fibrin  shows  that  the  former  contains 
as  much  calcium  as  the  latter,  and  that  therefore  the  view  of  coagu- 
lation according  to  which  fibrinogen  unites  with  calcium  to  form 
fibrin  is  without  foundation.  On  the  contrary,  he  maintains  that  the 
specific  influence  of  the  calcium  is  directed  toward  the  production  of 
the  ferment,  for  if  this  be  present  in  sufficient  quantity  coagulation 
takes  place  in  a  typical  manner,  no  matter  whether  the  blood  has 
been  decalcified  by  potassium  oxalate  or  not. 

Intra-vascular  Coagulation. — So  long  as  the  relations  of  the 
blood  and  the  vascular  system  remain  physiologic  no  coagulation 
occurs  in  the  vessels.  The  reason  assigned  for  this  is  that  the  fer- 
ment, though  continually  being  produced,  is  as  rapidly  being  de- 
stroyed, and  hence  never  accumulates  in  amount  sufficient  to  develop 
fibrin.  This  view  is  supported  by  the  fact  that  if  a  solution  of  cell- 
protoplasm,  leukocytes,  lymph-corpuscles,  etc.,  presumably  contain- 
ing a  large  amount  of  the  ferment,  be  injected  into  the  blood-vessels, 
extensive  intra-vascular  coagulation  promptly  follows.  It  is  also 
believed  that  the  lining  of  the  blood-vessel  in  some  unknown  way 
restrains  the  coagulation  process  even  though  the  circulation  has 
come  to  rest. 

Under  pathologic  conditions  of  the  circulatory  apparatus,  espe- 
cially of  the  internal  lining,  intra-vascular  coagulation  frequently 
arises,  though  the  process  can  not  be  considered  as  identical  with 
extra-vascular  coagulation.  Many  pathologists  assert  that  in  its 
origin,  mode  of  formation,  and  structure  the  intra-vascular  coagulum 


THE  BLOOD.  271 

or  thrombus  is  not  a  true  coagulum  as  ordinarily  understood,  but 
rather  a  conglutination  of  blood-plaques  and  leukocytes.  Whenever 
the  integrity  of  the  internal  wall  of  the  vessel  is  impaired  by  disease 
or  by  the  introduction  of  foreign  bodies,  there  is  primarily  a  de- 
position and  accumulation  of  blood-plaques  at  the  injured  area  or 
on  the  foreign  body  which  constitutes  to  a  large  extent  the  mass  of 
the  thrombus  which  at  once  forms.  The  thrombi  which  form  on 
the  surface  of  atheromatous  ulcers,  on  the  valves  of  the  heart,  and 
in  the  veins  in  consequence  of  diseased  states,  on  threads  or  needles 
passed  through  the  vessels,  at  the  orifices  of  torn  blood-vessels, 
consist  largely  of  blood-plaques.  A  thrombus  so  formed  may  con- 
tain a  number  of  dehcate  fibrin  threads,  which,  however,  present 
a  different  appearance  from  the  fibrin  of  the  extra- vascular  clot.  In 
the  thrombi  which  form  around  foreign  bodies  there  is  always  a 
larger  quantity  of  fibrin  than  in  those  originating  from  causes  wholly 
within  the  vessel. 


CHAPTER   XII. 
THE  CIRCULATION  OF  THE  BLOOD. 

Each  organ  and  tissue  of  the  body  is  the  seat  of  a  more  or  less 
active  metaboHsm,  the  maintenance  of  which  is  essential  to  its  physio- 
logic activity.  This  metabohsm  is  characterized  by  the  assimilation 
of  food  materials  and  the  production  of  waste  products;  that  it  may 
be  maintained  it  is  imperative  that  there  shall  be  a  continuous  supply 
of  the  former  and  a  continuous  removal  of  the  latter.  Both  condi- 
tions are  subserved  by  the  blood.  In  order,  however,  that  this  fluid 
may  fulfil  these  functions  it  must  be  kept  in  continuous  movement, 
must  flow  into  and  out  of  the  tissues  in  volumes  varying  with  their 
activity,  under  a  given  pressure  and  with  a  certain  velocity. 

The  apparatus  by  which  these  results  are  attained  is  termed 
the  circulatory  apparatus.  This  consists  of  a  central  organ, 
the  heart;  a  series  of  branching  diverging  tubes,  the  arteries;  a  net- 
work of  minute  passageways  with  extremely  dehcate  walls,  the  capil- 
laries ;  a  series  of  converging  tubes,  the  veins.  These  structures  are 
so  arranged  as  to  form  a  closed  system  of  vessels  within  which  the 
blood  is  kept  in  continuous  movement  mainly  by  the  pressure  pro- 
duced by  the  pumping  action  of  the  heart,  though  aided  by  other 
forces.     (See  Fig.  1 1 1 .) 

In  this  system  a  particle  of  blood  which  passes  any  given  point 
will  eventually  return  to  the  same  point,  no  matter  how  intricate  or 
tortuous  the  route  may  be  through  which  it  in  the  meanwhile  travels ; 
for  this  reason  the  blood  is  said  to  move  in  a  circle,  and  the  movement 
itself  is  termed  the  circulation. 

In  order  to  understand  the  reasons  for  the  movement  of  the  blood 
in  one  direction  only,  as  well  as  for  many  other  phenomena  connected 
with  the  circulation,  a  knowledge  of  the  structure  of  the  heart  and 
its  internal  mechanism  is  of  primary  importance. 

THE  PHYSIOLOGIC  ANATOMY  OF  THE  HEART. 

The  heart  is  a  cone  or  pyramid-shaped  hollow  muscular  organ 
situated  in  the  thorax  just  behind  the  sternum.  The  base  is  directed 
upward  and  to  the  right  side ;  the  apex  downward  and  to  the  left  side, 
extending  as  far  as  the  space  between  the  cartilages  of  the  fifth  and 
sixth  ribs.  In  this  situation  the  heart  is  enclosed  and  suspended 
in  a  fibroserous  sac,  the  pericardium,  attached  to  the  great  vessels  at 
its  base. 

272 


THE  CIRCULATION  OF  THE  BLOOD. 


273 


The  heart  is  a  double  organ,  consisting  of  a  right  and  a  left  half, 
separated  by  a  vertical  septum.  The 
general  cavity  of  each  side  is  partially 
subdivided  by  a  transverse  constriction 
into  two  smaller  cavities,  an  upper  and 
a  lower,  known  respectively  as  the 
auricle  and  the  ventricle.  The  heart 
may  therefore  be  said  to  consist  of 
four  cavities,  the  walls  of  which  are 
composed  of  muscle-tissue.  Of  these 
four  cavities,  the  right  auricle  and  the 
right  ventricle  constitute  the  venous 
heart;  the  left  auricle  and  the  left 
ventricle,  the  arterial  heart. 

The  right  auricle  is  quadrangular 
in  shape  and  presents  on  its  posterior 
aspect  two  large  openings,  the  termi- 
nations of  the  two  final  trunks  of  the 
venous  system,  the  superior  and  in- 
ferior vence  cava  (Fig.  112).  Below, 
the  auricle  communicates  with  the  ven- 
tricle by  a  large  opening  which,  from 
its  position,  is  termed  the  auriculo- 
ventricular  opening.  The  walls  of  the 
auricle  are  extremely  thin,  not  meas- 
uring more  than  two  milhmeters  in 
thickness. 

The  right  ventricle,  as  shown  on 
cross-section,  is  crescentic  in  shape 
owing  to  the  projection  of  the  ven- 
tricular septum.  It  presents  at  its 
upper  left  angle  a  cone-shaped  pro- 
longation, the  comis  arteriosus.  From 
this  prolongation,  and  continuous  with 
it,  arises  the  pulmonary  artery.  The 
wall  of  the  ventricle  measures  in  the 
middle  about  four  milhmeters  in  thick- 
ness. The  inner  surfaces  of  the  ven- 
tricle show:  (i)  a  complicated  system 
of  muscle  ridges  and  bands,  the  col- 
umned carnecB  (fleshy  columns),  and 
(2)  a  set  of  muscle  projections,  the 
musculi  papillares  (papillary  muscles), 
which  arise  by  a  broad  base  from  the 
walls  of  the  ventricle  and  project  upward  toward  the  auriculo- 
18 


Fig.  III. — Diagram  of  Circula- 
tion. I.  Heart.  2.  Lungs. 
3.  Head  and  upper  extremi- 
ties. 4.  Spleen.  5.  Intestine. 
6.  Kidney.  7.  Lower  extremi- 
ties.    8.  Liver. — (Dalton.) 


274 


TEXT-BOOK  OF  PHYSIOLOGY. 


ventricular  opening.  From  the  apex  of  each  papillary  muscle  there 
are  given  off  fine  tendinous  cords,  the  chorda  tendinccE,  which  become 
attached  above  to  the  auriculo-vcntricular  valve. 


Fig.  112. — The  Right  Auricle  and  Ventricle  Opened,  and  a  Part  of  Their 
Right  and  Anterior  Walls  Removed,  so  as  to  show  Their  Interior. 
i — I-  Superior  vena  cava.  2.  Inferior  vena  cava.  2'.  Hepatic  veins  cut  short. 
3  Right  auricle.  3'.  Placed  in  the  fossa  ovalis,  below  which  is  the  Eustachian 
valve.  3".  Is  placed  close  to  the  aperture  of  the  coronary  vein.  H — h-  Placed 
in  the  auriculo-ventricular  groove,  where  a  narrow  portion  of  the  adjacent  walls 
of  the  auricle  and  ventricle  has  been  preserved.  4,  4.  Cavity  of  the  right  ventri- 
cle; the  upper  figure  is  immediately  below  the  semilunar  valves.  4'.  Large 
columna  carnea  or  musculus  papillaris.  5,  5',  5".  Tricuspid  valve.  6.  Placed 
in  the  interior  of  the  pulmonary  artery,  a  part  of  the  anterior  wall  of  that  vessel 
having  been  removed,  and  a  narrow  portion  of  it  preserved  at  its  commencement, 
where  the  semilunar  valves  are  attached.  7.  Concavity  of  the  aortic  arch  close 
to  the  cord  of  the  ductus  arteriosus.  8.  Ascending  part  or  sinus  of  the  arch  cov- 
ered at  its  commencement  by  the  auricular  appendix  and  pulmonary  artery. 
9.  Placed  between  the  innominate  and  left  carotid  arteries.  10.  Appendi.x  of 
the  left  auricle.  11,  11.  The  outside  of  the  left  ventricle,  the  lower  figure  near 
the  apex. — {Allen  Thomson.) 

The  left  auricle,  similar  in  general  shape  to  the  right,  presents 
posteriorly  four  openings,  the  terminations  of  the  four  final  trunks 
of  the  venous  system  of  the  lungs,  the  pulmonary  veins.     Below  is 


THE  CIRCULATION  OF  THE  BLOOD. 


27s 


placed  the  corresponding  auriculo-ventricular  opening.  The  wall 
of  the  auricle  measures  about  3  mm.  in  thickness.  The  left  ventricle 
(Fig.  113)  is  conic  in  shape  from  above  downward  and  oval  or  cir- 
cular in  shape  on  cross-section.     At  its  upper  right  angle  it  presents 


Fig.  113. — The  Left  Auricle  and  Ventricle  Opened  and  a  Part  of  Their 
Anterior  and  Left  Walls  Removed.  J. — The  pulmonary  artery  has  been 
divided  at  its  commencement;  the  opening  into  the  left  ventricle  is  carried  a  short 
distance  into  the  aorta  between  two  of  the  segments  of  the  semilunar  valves; 
and  the  left  part  of  the  auricle  with  its  appendix  has  been  removed.  The  right 
auricle  is  out  of  view.  i.  The  two  right  pulmonar}'  veins  cut  short;  their  open- 
ings are  seen  within  the  auricle,  i'.  Placed  mthin  the  cavity  of  the  auricle  on 
the  left  side  of  the  septum  and  on  the  part  which  forms  the  remains  of  the  valve 
of  the  foramen  ovale,  of  which  the  crescentic  fold  is  seen  toward  the  left  hand 
of  i'.  2.  A  narrow  portion  of  the  wall  of  the  auricle  and  ventricle  preserved 
round  the  auriculo-ventricular  orifice.  3,  3'.  The  cut  surface  of  the  walls  of  the 
ventricle,  seen  to  become  ver}'  much  thinner  toward  3",  at  the  apex.  4.  A  small 
part  of  the  anterior  wall  of  the  left  ventricle  which  has  been  preserved  with  the 
principal  anterior  columna  carnea  or  musculus  papillaris  attached  to  it.  5'  5- 
Musculi  papillares.  5'.  The  left  side  of  the  septum,  between  the  two  ventricles, 
within  the  cavity  of  the  left  ventricle.  6,  6'.  The  mitral  valve.  7.  Placed  in 
the  interior  of  the  aorta,  near  its  commencement  and  above  the  three  segments  of 
its  semilunar  valve  which  are  hanging  loosely  together.     7'.  The  exterior  of  the 


276 


TEXT-BOOK  OF  PHYSIOLOGY. 


a  circular  orifice,  the  margins  of  which  give  attachment  to  the  walls 
of  the  aorta,  the  main  arterial  trunk  of  the  systemic  circulation.  The 
inner  surfaces  of  the  ventricle  show  a  similar  though  better  devel- 
oped system  of  columnae  carneas,  muscuH  papillares,  chordae  tendineae, 
etc.  The  wall  of  the  left  ventricle  measures  about  11,5  mm.  in  thick- 
ness in  the  middle. 

The  Endocardium. — The  cavities  of  both  the  right  and  left  sides 
of  the  heart  are  lined  by  a  thin  firm  connective-tissue  membrane, 

closely  adherent  to  the  muscle- 
tissue,  termed  the  endocar- 
dium. It  also  contains  elastic 
fibers  and  smooth  muscle- 
fibers.  Its  entire  surface  is 
covered  over  with  a  layer  of 
polygonal  endothelial  cells. 
This  membrane  serves  to  re- 
sist undue  distention  of  the 
heart  during  contraction  and 
to  prevent  separation  of  the 
muscle-fibers.  The  endocar- 
dium is  continuous  with  the 
lining  membrane  of  the  blood- 
vessels. 

The  Cardio- pulmonary 
Vessels. — Though  the  two 
sides  of  the  heart  are  separ- 
ated from  each  other  by  a 
vertical  septum,  they  are  ana- 
tomically and  physiologically 
connected  by  the  intermedia- 
tion of  the  pulmonary  system 
of  vessels:  viz.,  the  pulmonary 
artery,  capillaries,  and  veins 
(Fig.  114). 
The  pulmonary  artery  arises  from  the  conus  arteriosus  of  the  right 
ventricle.  After  a  short  upward  course  it  divides  into  a  right  and  a 
left  branch,  which  enter  corresponding  lungs.  The  vessel  at  once 
divides  and  subdivides  into  a  number  of  branches,  which,  after  fol- 
lowing the  bronchial  tubes  to  their  termination,  give  origin  to  capil- 
laries that  surround  the  air-cells  of  the  pulmonary  lobules. 

The  capillaries  in  this  situation  are  extremely  abundant  and  well 


Fig.  114. — Diagram  of  the  Heart  and 
Pulmonary  Circulation  in  Mamma- 
lians, a.  Right  auricle,  b.  Right  ven- 
tricle, c.  Pulmonary  artery,  d.  Lungs. 
e.  Pulmonary  vein.  /.  Left  auricle,  g. 
Left  ventricle,  h.  Aorta,  i.  Vena  cava. 
— (Dalton.) 


great  aortic  sinus.  8.  The  root  of  the  pulmonary  artery  and  its  semilunar  valves. 
8'.  The  separated  portion  of  the  pulmonary  artery  remaining  attached  to  the 
aorta  by  9,  the  cord  of  the  ductus  arteriosus.  10.  The  arteries  rising  from  the 
summit  of  the  aortic  arch.- — {Allen  Thomson.) 


THE  CIRCULATION  OF  THE  BLOOD. 


277 


developed.  They  lie  close  to  the  inner  surfaces  of  the  air-cells.  The 
blood  is  thus  brought  into  intimate  relationship  with  the  pulmonary 
air,  and  the  exchange  of  gases — the  excretion  of  carbon  dioxid  and 
the  absorption  of  oxygen — for  which  the  cardio-pulmonary  vessels 
exist,  is  readily  accomplished. 

The  pulmonary  veins  which  return  the  blood  to  the  heart  are 
formed  by  the  convergence  and  union  of  the  small  veins  which  emerge 
from  the  capillary  sys- 
tem. The  final  trunks 
thus  formed,  the  four 
pulmonary  veins,  —  two 
from  each  lung, — enter 
the  posterior  wall  of  the 
left  auricle. 

The  Course  of  the 
Blood  through  the 
Heart. — There  is  thus 
established  a  pathway 
between  the  venae  cavas 
on  the  right  side  and  the 
aorta  on  the  left  side,  by 
way  of  the  right  side  of 
the  heart,  the  cardio- 
pulmonary vessels,  and 
the  left  side  of  the  heart. 

The  venous  blood 
flowing  toward  the  heart 
is  emptied  by  the  supe- 
rior and  inferior  venae 
cavas  into  the  right  auri- 
cle, from  which  it  passes 
through  the  auriculo- 
ventricular  opening  into 
the  right  ventricle  (Fig. 
115);  thence  into  and 
through  the  pulmonar}' 
artery  and  its  branches 
to  the  pulmonary  capil- 
laries, where  it  is  arterialized  by  the  exchange  of  -gases — the  giving 
up  of  a  portion  of  carbon  dioxid  to  the  lungs  and  the  absorption 
of  oxygen — and  changed  in  color  from  bluish-red  to  scarlet.  The 
arteriahzed  blood,  flowing  toward  the  heart,  is  emptied  by  the  pul- 
monary veins  into  the  left  auricle,  from  which  it  passes  through  the 
auriculo-ventricular  opening  into  the  left  ventricle;  thence  into  the 
aorta  and  its   branches  to   the    systemic   capillaries,    where    it    is 


Fig.  1 1 5 . — Diagram  of  Course  of  Blood  through 
THE  Heart,  i,  2.  Superior  and  inferior  venae 
cavse.  3.  Right  auricle.  4.  Right  ventricle. 
5,  5,  5.  Pulmonary  artery  and  branches.  6,  6. 
Pulmonary  veins.  7.  Left  auricle.  8.  Left 
ventricle.  9.  Aorta.  10.  Innominate  artery. 
II.  Left  carotid  artery.  12.  Left  subclavian 
arter}'. — -{After  Moral  and  Doyon.) 


278  TEXT-BOOK  OF  PHYSIOLOGY 

de-arterialized  by  a  second  but  opposite  exchange  of  gases — the 
giving  up  of  a  portion  of  its  oxygen  to  the  tissues  and  the  absorption 
of  carbon  dioxid  from  the  tissues — and  changed  in  color  from  scarlet 
to  bluish-red.  The  venous  blood  is  again  returned  by  the  systemic 
veins  to  the  venas  cavae.  Though  the  blood  is  thus  described  as 
flowing  first  through  the  right  side  and  then  through  the  left  side,  it 
must  be  kept  in  mind  that  the  two  sides  fill  synchronously;  that 
while  the  blood  is  flowing  into  the  right  side  from  the  venae  cava;, 
it  is  also  flowing  from  the  pulmonary  veins  into  the  left  side  in  equal 
quantities  and  velocities. 

Though  there  is  but  one  set  of  capillaries,  as  a  rule,  between 
arteries  and  veins,  there  is  an  exception  in  the  case  of  the  arteries  and 
veins  of  the  abdominal  viscera.  Thus  the  veins  emerging  from  the 
capillaries  of  the  stomach,  intestines,  pancreas,  and  spleen,  instead 
of  passing  directly  to  the  inferior  vena  cava,  unite  to  form  a  large 
vein — the  portal  vein — which  enters  the  liver.  In  this  organ  the 
portal  vein  divides  to  form  a  second  capillary  system  which  is  in 
close  relation  to  the  liver  cells  and  from  which  arise  the  veins  which 
unite  to  form  the  hepatic  veins.  These  latter  vessels  empty  into  the 
inferior  vena  cava  just  below  the  diaphragm. 

From  the  foregoing  facts  physiologists  frequently  divide  the 
general  circulation  into: 

1.  The  pulmonary  circulation,  which  includes  the  course  of  the  blood 

from  the  right  side  of  the  heart  through  the  lungs,  to  the  left 
side  of  the  heart. 

2.  The  systemic  circulation,  which  includes  the  course  of  the  blood 

from  the  left  side  of  the  heart  through  the  aorta  and  its  branches, 
through  the  capillaries  and  veins  to  the  right  side  of  the  heart. 

3.  The  portal  circulation,  which  includes  the  course  of  the  blood  from 

the  capillaries  of  the  stomach,  intestines,  pancreas,  and  spleen 

through  the  portal  vein  to  the  liver. 

Orifices  and  Valves. — The  movement  of  the  blood  along  the 
path  of  the  circle  above  outhned  is  accomphshed  by  the  alternate 
contraction  and  relaxation  of  the  muscle  walls  of  the  heart.  That 
the  movement  may  be  a  progressive  one,  that  there  shall  be  no 
regurgitation  during  the  relaxation,  it  is  essential  that  some  of  the 
orifices  of  the  heart  be  closed.  This  is  accomphshed  by  the  heart 
valves. 

The  right  auriculo-ventricular  opening  is  surrounded  and  strength- 
ened by  a  ring  of  fibrous  tissue  to  which  is  attached  a  membrane  par- 
tially subdivided  into  three  portions  or  cusps,  which  during  the  period 
of  relaxation  are  directed  into  the  ventricle  (Fig.  116);  during  the 
period  of  contraction  they  are  raised  and  placed  in  complete  apposi- 
tion, when  they  act  as  a  valve  preventing  a  backward  flow  into  the 
auricle  (Fig.  117).     In  the  former  position  the  valve  is  open;  in  the 


THE  CIRCULATION  OF  THE  BLOOD. 


279 


Fig.  116. — Right  Cavities  of  the  Heart. 
Auriculo-ventricular  valves  open,  arte- 
rial valves  closed. — (Dai/on.) 


latter,  shut.  For  these  reasons  this  structure  is  known  as  the  tri- 
cuspid valve.  This  valve  is 
formed  of  fibrous  tissue  de- 
rived from  the  fibrous  ring, 
some  muscle-fibers,  covered 
over  by  a  reduplication  of 
the  endocardium.  To  the 
under  surface  and  to  the 
edges  of  this  valve  the  ten- 
dinous cords  of  the  papillary 
muscles  are  firmly  and  intri- 
cately attached.  These  cords 
are  just  sufficiently  long  to 
permit  closure  of  the  valve 
and  to  prevent  their  being 
floated  into  the  auricle. 

The  orifice  of  the  pul- 
monary artery  is  also  sur- 
rounded by  a  ring  of  fibrous 
tissue  to  which  are  attached 
three  semilunar  or  pocket- 
shaped  membranes,  the 
semilunar  valves.  Each  valve  is  formed  by  a  reduplication  of  the 
endocardium  strengthened  by  fibrous  tissue.     In  the  center  of  the 

free  edge  of  the  valve  there 
is  a  small  nodule  of  fibro- 
cartilage  (the  corpus  Aur- 
antius).  The  outer  edge 
of  the  valve  is  strengthened 
by  a  delicate  fibrous  band. 
A  similar  band  strengthens 
the  convex  attached  por- 
tion of  the  valve  just  where 
it  is  joined  to  the  fibrous 
ring.  A  third  set  of  fibers 
pass  toward  the  nodule,  in- 
terlacing in  all  directions. 
Two  narrow  crescentic- 
shaped  areas  (the  lunulas) 
near  the  free  edge  are  de- 
void of  these  fibers.  Dur- 
ing the  period  of  relaxation 

^  ^         ^  ,  of  the  heart  the  edges  of 

Fig.    117. — Right   C.a.vities   of  the   Heart.        ,  ,  _        .         1 

Auriculo-ventricular    valves    closed,    semi-       "-^^     val\  es      aie      m     CiOSC 

lunar  valves  open. — (Daiton.)  apposition    and   prevent    a 


28o 


TEXT-BOOK  OF  PHYSIOLOGY. 


return  of  the  blood  into  the  ventricle  (Fig.  ii6);  during  the  con- 
traction they  are  directed  into  the  artery  (Fig.  117).  In  the  former 
position  they  are  shut;  in  the  latter,  they  are  open. 

The  left  auriculo-ventricular  opening  is  provided  with  a  similar 

though  better  developed  fi- 
brous ring  and  membran- 
ous valve.  It  is,  however, 
subdivided  into  but  two 
portions  or  cusps,  and  is 
therefore  termed  the  bi- 
cuspid valve,  or,  from  its 
fancied  resemblance  to  a 
bishop's  mitre,,  the  mitral 
valve.  The  general  ar- 
rangement, connections, 
and  mode  of  action  of  this 
valve  are  similar  in  all  re- 
spects to  those  of  the  tricus- 
pid valve.  The  orifice  of 
the  aorta  is  also  surrounded 
by  a  ring  of  fibrous  tissue 
to  which  are  attached  three 
semilunar  or  pocket-shaped 
valves  (Fig.  113),  which  in 
their  arrangement,  connections,  and  mode  of  action  are  similar  in  all 
respects  to  those  at  the  orifice  of  the  pulmonary  artery.  The  anatomic 
relations  of  the  cardiac  orifices  one  to  the  other  and  the  appearance 
presented  by  the  valves  when  closed  are  repre- 
sented in  Fig.  118. 

The  Heart  Muscle-fibers  and  Their  Ar- 
rangement.— The  muscle-fibers  of  the  heart, 
though  transversely  striated  and  nucleated, 
diijfer  in  shape  and  arrangement  from  those 
found  in  any  other  situation.  The  individual 
fiber  is  short  and  broad  and  usually  divided  at 
one  or  both  ends.  By  this  means  the  fibers  are 
united  not  only  longitudinally,  but  laterally. 
(See  Fig.  119.)  The  fibers  are  devoid  of  a 
sarcolemma  and  united  one  to  the  other  by  a 
cement  material.  The  entire  musculature  is 
permeated  and  supported  by  connective  tissue 
which  is  so  arranged  as  to  group  the  fibers  in 
bundles  or  fasciculi  of  varying  size. 

The  arrangement  of  the  muscle  bundles  is  quite  comphcated  and 
in  accordance  with  the  functions  of  the  individual  portions  of  the 


Fig.  118. — Valves  of  the  He.4rt.  i.  Right 
auriculo-ventricular  orifice,  closed  by  the 
tricuspid  valve.  2.  Fibrinous  ring.  3.  Left 
auriculo-ventricular  orifice,  closed  by  the 
mitral  valve.  4.  Fibrinous  ring.  5.  Aortic 
orifice  and  valves.  6.  Pulmonic  orifice 
and  valves.  7,  8,  9.  Muscular  fibers. — 
{Bonamy  and  Bean.) 


Fig.  119. — Muscle- 
fibers  FROM  the 
Heart  of  a  Mam- 
mal. —  {Landois 
and  Sliding.') 


THE  CIRCULATION  OF  THE  BLOOD. 


heart.  In  the  auricles  the  bundles  are  arranged  in  two  sets :  an  outer 
transverse  set,  which  pass  from  auricle  to  auricle,  and  an  inner  longi- 
tudinal set,  which  pass  over  the  auricles  to  be  attached  anteriorly 
and  posteriorly  to  the  connective  tissue  of  the  auriculo-ventricular 
groove.  The  longitudinal  fibers  of  each  auricle  are  practically  in- 
dependent of  each  other.  Circularly  arranged  fibers  are  present  near 
the  terminations  of  the  vense  cavae  and  pulmonary  veins. 

In  the  ventricles  the  muscle-bundles  are  also  arranged  in 
two  sets,  a  superficial 
longitudinal  and  a  deep 
transverse,  though  their 
arrangement  is  some- 
what more  complicated 
than  that  observed  in 
the  auricles.  In  a  gen- 
eral way  it  may  be  said 
that  the  superficial  lon- 
gitudinal fibers  on  both 
the  anterior  and  poste- 
rior surfaces  from  their 
origin  in  the  connective 
tissue  of  the  auriculo- 
ventricular  groove  pass 
obliquely  downward  and 
forward  from  right  to 
left  toward  the  apex, 
where  they  turn  back- 
ward and  inward  in  a 
vortex,  after  which  they 
ascend  to  terminate  in 
the  wall  of  the  septum, 
the  columnae  carneae 
and  musculi  papillares. 
Longitudinal  fibers  are 
also  found  on  the  inner 
surface.  The  transverse 
fibers  are  very  abundant 
and  surround  each  ven- 
tricle separately.  Between  the  superficial  longitudinal  and  deep 
transverse  fibers  there  are  several  layers  of  fibers  which  possess 
varying  degrees  of  obliquity.  The  general  arrangement  of  the  fibers 
is  such  as  to  ensure  a  complete  and  simultaneous  discharge  of  blood 
from  both  auricles  and  ventricles  (Fig.  120). 


Fig. 


120. — Muscle-fibers  of  the  Ventricles. 
I.  Superficial  fibers,  common  to  both  ventri- 
cles. 2.  Fibers  of  the  left  ventricle.  3.  Deep 
fibers,  passing  upward  toward  the  base  of  the 
heart.  4.  Fibers  penetrating  the  left  ventricle. 
— {Sappey,  after  Bonamy  and  Beau.) 


282  TEXT-BOOK  OF  PHYSIOLOGY. 


THE  MECHANICS  OF  THE  HEART. 

The  immediate  cause  of  the  movement  of  the  blood  through  the 
vessels  is  the  contraction  and  relaxation  of  the  muscle -walls  of  the 
heart,  and  more  particularly  of  the  walls  of  the  ventricles,  each  of 
which  plays  alternately  the  part  of  a  force-pump,  and  to  a  shght 
extent  of  a  suction-pump.  The  motive  power  is  furnished  by  the 
heart  itself,  by  the  transformation  of  potential  energy,  stored  up 
during  the  period  of  rest,  into  kinetic  energy — /.  e.,  heat  and  mechanic 
motion. 

The  contraction  of  any  part  of  the  heart  is  termed  the  systole; 
the  relaxation,  the  diastole.  As  each  side  of  the  heart  has  two  cavities 
the  walls  of  which  contract  and  relax  in  succession,  it  is  customary  to 
speak  of  an  auricular  systole  and  diastole,  and  a  ventricular  systole 
and  diastole.  As  the  two  sides  of  the  heart  are  in  the  same  anatomic 
relation  to  each  other,  they  contract  and  relax  in  the  same  periods 
of  time. 

The  movements  of  the  heart,  as  well  as  many  phenomena  con- 
nected with  the  flow  of  blood  through  its  cavities,  have  been  deter- 
mined by  observation  of,  and  experiment  on,  the  exposed  heart  of  a 
mammal, — e.  g.,  dog,  cat,  rabbit, — supplemented  and  corrected  by 
experiments  on  the  heart  in  its  normal  relations.  V^aluable  informa- 
tion as  to  the  heart-beat  and  the  influences  which  modify  it  has  been 
obtained  from  experiments  made  on  the  isolated  heart  of  the  turtle, 
frog,  and  allied  animals. 

If  the  thorax  of  a  dog  completely  anesthetized  is  opened  and 
artificial  respiration  established,  the  heart  will  be  observed  in  active 
movement  inside  the  pericardium.  If  this  sac  is  divided  and  turned 
aside,  the  heart  will  be  fully  exposed  to  view.  At  the  normal  rate 
of  movement  characteristic  of  the  dog  it  will  be  almost  impossible 
to  determine  either  the  succession  of  events  or  their  duration.  But 
by  observing  the  heart  under  different  conditions  at  different  rates  of 
movement  and  with  instrumental  aids  physiologists  have  succeeded 
not  only  in  analyzing  the  movements,  but  in  describing  their  sequence 
and  in  estimating  their  time  duration. 

Thus  it  has  been  determined  that  the  heart  presents  two  distinct 
movements  which  alternate  with  each  other  in  quick  succession.  One 
is  the  movement  of  contraction,  or  the  systole,  by  which  the  blood 
contained  within  its  cavities  is  ejected  into  the  arteries — pulmonary 
artery  and  aorta;  the  other  is  the  movement  of  relaxation,  or  the 
diastole,  followed  by  a  pause  during  which  the  cavities  again  fill  up 
with  the  blood  from  the  venag  cavae  and  pulmonary  veins. 

Sequence  of  Events. — It  has  been  ascertained  that  the  contrac- 
tion of  the  auricles  and  ventricles  as  well  as  their  subsequent 
relaxations,  though  occurring  with   extreme  rapidity,  do  not  take 


THE  CIRCULATION  OF  THE  BLOOD.  283 

place  simultaneously  but  successively;  that  the  contraction  process 
passes  over  the  heart  in  the  form  of  a  wave;  that  it  begins,  indeed, 
at  the  terminations  of  the  great  veins,  then  passes  to  and  over  the 
auricles,  thence  to  and  over  the  ventricles  from  base  to  apex  with 
great  rapidity,  but  occupying  in  these  different  regions  unequal 
periods  of  time;  that  the  relaxation  immediately  succeeds  the  con- 
traction, in  the  same  order,  and  that  at  the  close  of  the  ventricular 
relaxation  there  is  a  period  during  which  the  whole  heart  is  in  repose, 
passively  filling  with  blood. 

Changes  in  Position  and  Form. — In  passing  from  the  diastolic 
to  the  completed  systolic  condition  the  exposed  heart  undergoes 
changes  both  of  position  and  form  as  the  contraction  rises  to  its 
maximum.  This  having  been  attained,  the  heart  undergoes  reverse 
changes  until  the  original  diastolic  condition  is  regained.  Thus 
at  the  time  of  the  ventricular  systole  the  apex  is  tilted  upward,  the 
entire  heart  is  twisted  on  its  axis  from  left  to  right  and  forced  down- 
ward by  the  expansion  and  elongation  of  the  pulmonary  artery  and 
aorta.  At  the  time  of  the  diastole,  the  reverse  movements  take 
place. 

It  is  probable,  however,  that  these  movements  are  not  permitted  to 
the  same  extent  in  the  unopened  chest,  for  the  following  reasons: 
the  heart  is  enclosed  in  the  pericardium,  is  supported  posteriorly  by 
the  expanded  lungs,  and  both  posteriorly  and  inferiorly  by  the 
diaphragm,  all  of  which  cooperate  in  keeping  the  heart,  and  more 
particularly  the  right  ventricle,  in  close  contact  with  the  chest- 
waU  and  limiting  its  movements  By  means  of  needles  inserted 
into  the  apex  of  the  heart,  through  the  chest-walls,  it  has  been 
shown  by  their  slight  movement  that  the  apex  is  practically  a  fixed 
point. 

In  the  diastolic  condition  the  shape  of  the  heart  near  the  base  is 
eUiptic  on  cross-section,  the' long  diameter  extending  from  side  to  side. 
In  the  completed  systolic  condition  the  shape  of  the  same  cross-section 
is  that  of  a  circle.  In  passing  from  the  diastolic  to  the  systolic  con- 
dition the  transverse  diameter  diminishes  while  the  antero-posterior 
diameter  increases,  while  the  whole  heart  becomes  somewhat  more 
conic  in  shape.  It  is  questionable  if  the  vertical  diameter  per- 
ceptibly shortens.  During  the  systole  the  heart  hardens,  increases 
in  convexity,  and  is  more  forcibly  pressed  against  the  chest-wall. 
As  this  takes  place  suddenly,  it  gives  rise  to  a  marked  vibration 
of  the  chest-wall,  knovv'n  as  the  cardiac  impulse.  This  is  princi- 
pally observed  in  the  space  between  the  fourth  and  fifth  ribs, 
between  the  left  edge  of  the  sternum  and  a  hne  drawn  vertically 
through  the  nipple.  The  cardiac  impulse  is  synchronous  with  the 
cardiac  svstole. 


284 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  Cardiac  Cycle. — The  entire  period  of  the  heart's  pulsation 
may  be  divided  into  three  phases,  viz. : 

1.  The  auricular  contraction. 

2.  The  ventricular  contraction. 

3.  The  pause  or  period  of  repose,  during  which  both  auricles  and 

ventricles  are  at  rest. 
These  three  phases  collectively  constitute  a  cardiac  cycle  or  a 
cardiac  revolution.  The  duration  of  a  cycle,  as  well  as  the  duration 
of  each  of  its  three  phases,  varies  in  different  animals  in  accordance 
with  the  number  of  cycles  which  recur  in  a  unit  of  time.  In  human 
beings  in  adult  hfe  there  are  about  72  cycles  to  the  minute;  the  average 
duration  therefore  is  0.83  second.  From  this  it  follows  that  the  time 
occupied  by  any  one  of  the  three  phases  must  be  extremely  short  and 
difficult  of  determination.  From  observations  made  on  human 
beings  and  from  experiments  on  animals  the  following  estimates 
have  been  made  and  accepted  as  approximately  correct: 
I.  The  auricular  systole,  0.16  second. 

2.  The  ventricular  systole,  0.32 
second. 

3.  The  period  of  rest  for  both 
auricles  and  ventricles,  c.32 
second. 

The  relations  of  these  three 
phases  to  one  another  may  be 
illustrated  by  the  following  dia- 
gram (Fig.  121),  in  which  the 
space  1-2  is  the  duration  of  a 
cardiac  cycle  divided  into  eight 
equal  spaces,  each  of  which  re- 
presents one-tenth  of  a  second. 
The  line  A  represents  the  auric- 
ular, the  line  V  the  ventricular 
phase.  The  rise  in  the  line  A 
represents  the  contraction;  the  fall  and  subsequent  continuation,  the 
relaxation  and  pause.  The  rise  in  the  line  V  and  its  continuation 
represent  the  contraction;  the  fall  and  subsequent  continuation,  the 
relaxation  and  the  pause.  From  this  it  is  apparent  that  the  auric- 
ular contraction  or  systole  has  a  brief  duration,  0.16  second,  while 
the  relaxation  or  diastole  has  a  long  duration,  0.64  second;  that  the 
ventricular  contraction  immediately  following  the  auricular  has  a 
duration  of  0.32  second,  while  the  relaxation  and  diastole  have  a 
duration  of  0.48  second;  that  the  pause  of  the  entire  heart,  that  is,  the 
period  between  the  termination  of  the  ventricular  systole  and  the  be- 
ginning of  the  next  auricular  systole,  is  only  0.32  second. 

The  frequency  of  the  heart-beat  varies  with  a  variety  of  con- 
ditions: e.  g.,  age,  sex,  posture,  exercise,  etc. 


Fig.  121.- 


-The  Phases  of  the  Heart's 
Pulsation. 


THE  CIRCULATION  OF  THE  BLOOD.  285 

Age. — The  most  important  normal  condition  which  modifies  the 
activity  of  the  heart  is  age.     Thus : 

Before  birth,  the  number  of  beats  a  minute  averages 140 

During  the  first  year  it  diminishes  to 128 

During  the  third  year  it  diminishes  to 95 

From  the  eighth  to  the  fourteenth  year  it  averages 84 

In  adult  life  it  averages 72 

Sex. — The  heart-beat  is  more  rapid  in  females  than  in  males. 
Thus  while  the  average  beat  in  males  is  72,  in  females  it  is  usually 
8  or  10  beats  more. 

Posture. — Independent  of  muscle  efforts  the  rate  of  the  beat  is 
influenced  by  posture.  It  has  been  found  that  when  the  body  is 
changed  from  the  lying  to  the  sitting  and  to  the  standing  position, 
the  heart  will  vary  as  follows — from  66  to  71  to  81  on  the  average. 

Exercise  and  digestion  also  temporarily  increase  the  number  of 
beats. 

The  Action  of  the  Valves. — As  previously  stated,  the  forward 
movement  of  the  blood  is  permitted  and  regurgitation  prevented  by 
the  alternate  action  of  the  auriculo-ventricular  and  the  semilunar 
valves.  As  a  point  of  departure  for  a  consideration  of  the  action  of 
the  valves  and  their  relation  to  the  systole  and  diastole  of  the  heart, 
the  close  of  the  ventricular  systole  may  be  conveniently  selected. 

At  this  moment,  if  the  blood  is  not  to  be  returned  to  the  ventricles, 
the  semilunar  valves  must  be  instantly  and  completely  closed.  This 
is  accomplished  in  the  following  manner:  During  the  outflow  of  blood 
from  the  ventricles  the  valves  are  pushed  outward  toward  the  walls 
of  the  vessels,  though  not  coming  into  contact  with  them;  for  behind 
them  are  the  pouches  of  Valsalva,  containing  blood,  continuous  with 
and  under  the  same  pressure  as  that  in  the  vessels  themselves.  With 
the  cessation  of  the  outflow  and  the  beginning  of  the  relaxation  the 
pressure  of  the  blood  behind  the  valves  suddenly  forces  them  inward 
until  their  free  edges,  including  the  lunulee,  come  into  complete  appo- 
sition. By  this  means  the  orifices  of  the  pulmonary  artery  and  aorta 
are  securely  closed  and  a  return  flow  prevented.  Reversal  of  the 
valves  is  prevented  by  their  mode  of  attachment  to  the  fibrous  rings 
of  the  orifices. 

During  the  ventricular  systole  the  relaxed  auricles  have  been 
filling  with  blood.  With  the  ventricular  relaxation  this  volume,  or 
its  equivalent,  flows  readily  into  the  empty  and  easily  distensible 
ventricles,  its  place  being  taken  by  an  additional  volume  of  blood 
flowing  from  the  venae  cavae  and  pulmonary  veins.  Whether  the 
ventricles  exert  a  suction  power  at  the  moment  of  their  relaxation  is 
an  undecided  question.  A  steady  stream  of  blood  into  the  auricles 
and  ventricles  continues  throughout  the  entire  period  of  rest  until 
both  cavities  are  filled.  The  tricuspid  and  bicuspid  valves  which 
hang  down  into  the  ventricular  cavities  are  now  floated  up  by  cur- 


286  TEXT-BOOK  OF  PHYSIOLOGY. 

rents  of  blood  welling  up  behind  them  until  they  are  nearly  closed. 
The  auricles  now  contract,  forcing  their  contained  volumes,  or  at 
least  the  larger  portions  of  them,  into  the  ventricles,  which  become 
fully  distended. 

With  the  cessation  of  the  auricular  systole  the  ventricular  systole 
begins.  If  the  blood  is  not  to  be  returned  to  the  auricles  at  this 
moment,  the  tricuspid  and  mitral  valves  must  be  suddenly  and  com- 
pletely closed.  This  is  readily  accomplished  by  reason  of  the  position 
of  the  valves,  which  have  been  floated  up  and  placed  almost  in  apposi- 
tion by  the  blood  itself.  With  the  beginning  of  the  ventricular  pressure 
the  blood  is  forced  upward  against  the  valves  until  their  free  edges 
are  brought  together  and  the  orifices  closed.  Reversal  of  these 
valves  is  prevented  by  the  contraction  and  shortening  of  the  papil- 
lary muscles,  which  in  consequence  exert  a  traction  on  their  under 
surfaces.  The  blood  now  confined  in  the  ventricle  between  the 
closed  auriculo- ventricular  and  semilunar  valves  is  subjected  j  to 
pressure  from  all  sides.  As  the  pressure  rises  proportionately  to  the 
vigor  of  the  contraction,  there  comes  a  moment  when  the  intra- 
ventricular pressure  exceeds  that  in  the  aorta  and  pulmonary  artery. 
Immediately  the  semilunar  valves  of  both  vessels  are  thrown  open  and 
the  blood  discharged.  Both  contraction  and  outflow  continue  until 
the  ventricles  are  practically  empty,  after  which  ventricular  relaxa- 
tion sets  in,  attended  by  a  rapid  fall  of  pressure.  Under  the  influence 
of  the  positive  pressure  of  the  blood  in  the  sinuses  of  Valsalva  the 
semilunar  valves  are  again  closed,  the  column  of  blood  supported, 
and  regurgitation  is  prevented.  With  the  accumulation  of  blood  in 
the  auricles  the  cardiac  cycle  is  completed. 

Relative  Functions  of  Auricles  and  Ventricles. — Though 
both  auricles  and  ventricles  are  essential  to  the  continuous  movement 
of  blood,  they  possess  unequal  values  in  this  respect.  The  passage 
of  the  blood  through  the  pulmonary  and  systemic  vessels  is  accom- 
plished by  the  driving  power  of  the  right  and  left  ventricles  respec- 
tively, aided,  however,  by  minor  extra-cardiac  forces.  They  may 
be  regarded  therefore  as  force-pumps. 

If  the  heart  consisted  of  ventricles  only,  the  flow  of  blood  from 
the  venae  cavae  and  pulmonary  veins  would  be  temporarily  arrested 
during  their  systole  and  their  subsequent  refilling  delayed.  This  is 
obviated,  however,  by  the  addition  of  the  auricles;  for  during  the 
ventricular  systole  the  blood  continues  to  flow  into  the  auricles,  in 
which  it  is  temporarily  stored  until  the  ventricular  relaxation  sets  in. 
With  this  event  the  accumulated  blood  passes  into  the  ventricles,  which 
are  thus  practically  filled  before  the  auricular  systole  occurs  by  which 
the  fining  is  completed.  By  this  means  there  is  no  delay  in  the  filhng 
of  the  ventricles,  and  hence  their  effective  working  as  force-pumps 
is  more  readily  secured.  The  auricles  may  therefore  be  regarded 
as  feed-pumps.     For  this  reason  it  is  probable,  notwithstanding  the 


THE  CIRCULATION  OF  THE  BLOOD. 


28: 


contraction  of  the  circular  muscle-libers  at  the  terminations  of  the 
venous  system,  the  flow  of  blood  into  the  auricles  is  never  entirely 
arrested.  Regurgitation  in  these  vessels  does  not  occur  for  the  reason 
that  the  pressure  in  the  auricles  is  not  higher  than,  if  as  high  as,  in 
the  great  veins. 

Synchronism  of  the  Two  Sides  of  the  Heart. — If  the  balance 
of  the  circulation  is  to  be  maintained,  the  two  sides  of  the  heart  must 
act  synchronously.  That  they  do  so  can  be  shown  by  attaching 
levers  to  their  walls  and  thus  recording  their  activities.  The  syn- 
chronism is  so  perfect  that  until  recently  it  was  generally  believed  to 
be  dependent  on  nerve  connections;  but  Porter  has  shown  that  if  the 
ventricles  are  cut  away  from  the  auricles,  in  which  the  nerve  mechan- 
ism seemed  to  lie,  the  synchronism  of  the  former  is  not  interfered 
with;  that  the  apical  halves  of  the  ventricles  will  beat  synchronously 
if  perfused  with  blood  through  an  artery;  that  a  very  small  bridge  of 
muscle-tissue  will  carry  the  wave  of  excitation  from  one  part  to  neigh- 
boring parts  of  the  ventricle.  It  is  therefore  probable  that  tlie  syn- 
chronism is  accomplished  through  muscle  connections  only.  The 
left  ventricle,  in  keeping  with  the  greater  work  it  has  to  do,  has  a 
greater  development  than  the  right,  and  therefore  contracts  more 
energetically.  The  ratio  between  the  energy  of  the  left  and  right 
sides  is  approximately  3  to  i. 

Intra-cardiac  Pressure. — It  has  been  stated  that  during  the 
pause  of  the  heart  when  its  cavities  are 
filling  with  blood  the  semilunar  valves 
are  kept  closed  by  the  pressure  of  the 
blood  in  the  pulmonary  artery  and 
aorta,  a  pressure  due  to  the  resistance, 
as  will  be  explained  later,  offered  to 
the  flow  of  the  blood  mainly  by  the 
smaller  arteries  and  capillaries;  that  max  valve 
they  are  opened  only  when  the  press- 
ure of  the  blood  within  the  ventricle 
exceeds  that  in  the  arteries.  It  be- 
comes, therefore,  a  matter  of  impor- 
tance to  determine  the  extent  of  this 
pressure  as  well  as  its  variations  dur- 
ing the  course  of  a  cardiac  cycle. 
This  can  be  done  by  inserting  a  long 
catheter  into  either  the  right  or  left 
ventricle,  through  the  jugular  vein  or 
the  carotid  artery  respectively,  and 
connecting  its  free  extremity  with  a 
mercurial  manometer.  By  the  inter- 
position of  a  double  valve  such  as  represented  in  Fig.  122,  it 
becomes  possible,  according  to  the  direction  the  blood  is  permitted 


to  .Tianometer 


mm  valve 


to  heart 

Fig.  122. — V.  Fr.\xk's  Valve. 
This  is  placed  in  the  course  of 
the  tube  between  heart  and 
manometer,  so  that  the  latter 
may  be  used  as  a  maximum, 
minimum,  or  ordinary  man- 
ometer according  to  the  tap 
which  is  left  open. — {Starling.) 


288  TEXT-BOOK  OF  PHYSIOLOGY. 

to  flow,  to  obtain  either  the  maximal  or  the  minimal  pressure  that 
occurs  in  the  heart  during  a  series  of  cycles.  Thus  Goltz  found  in 
the  left  ventricle  of  the  dog  a  maximal  pressure  of  114  to  135  mm.; 
in  the  right  ventricle,  a  pressure  of  35  to  62  mm.  Minimal  pres- 
sures of  — 23  to  — 52  mm.  for  the  left  ventricle  have  also  been 
obtained. 

The  maximal  pressure  in  the  ventricles  during  the  systole,  though 
always  higher  than  that  in  the  arteries,  is  not  a  fixed  or  an  invariable 
pressure,  as  it  rises  and  falls  with  the  latter  from  moment  to  moment. 
Within  hmits  the  cardiac  power,  and  therefore  the  intra-cardiac 
I)ressure,  is  capable  of  considerable  increase.  The  function  of  the 
heart  is  to  drive  the  blood  through  the  vessels  with  a  given  velocity. 
This  is  only  possible  by  first  overcoming  the  resistance  to  the  flow 
offered  by  the  vessels,  as  indicated  by  the  arterial  pressure.  As  this 
is  a  variable  factor,  rising  and  falHng  very  considerably  at  times,  the 
teart  must  meet  and  exceed  each  rise,  if  the  circulation  is  to  be  main- 
tained. The  power  put  forth  by  the  heart  is  proportional  to  the 
work  it  has  to  perform.  If  the  arterial  pressure  continues  higher 
than  the  average  for  any  length  of  time,  the  heart  meets  the  condition 
by  an  hypertrophy  of  its  walls. 

The  Intra-ventricular  Pressure  Curve. — An  accurate  interpre- 
tation of  the  play  of  the  heart  mechanism  necessitates  the  obtaining  of 
a  graphic  record  of  the  course  of  the  intra-ventricular  pressure,  its  varia- 
tions and  time  relations.  With  such  a  record  may  be  compared  the 
records  of  the  pressures  in  the  venae  cavae,  on  the  one  hand,  and  in  the 
aorta,  on  the  other  hand,  and  their  relations  one  to  another  accurately 
defined. 

The  intra-ventricular  pressure  has  been  obtained  by  specially  de- 
vised manometers  or  tonometers  or  tono graphs,  as  they  are  variously 
termed,  the  construction  of  which  is  such  as  to  enable  them  to  respond 
instantly  to  the  very  rapid  variations  of  the  pressure  which  occur 
during  the  brief  cardiac  cycle.  One  of  the  best  is  that  of  Hiirthle. 
This  consists  of  a  small  metallic  tambour  5  or  6  millimeters  in  diam- 
eter, covered  by  a  thin  rubber  membrane.  A  small  button  resting 
on  the  membrane  plays  against  an  elastic  steel  spring,  by  the  tension 
of  which  the  pressure  of  the  blood  is  counterbalanced.  The  move- 
ments of  the  membrane  are  taken  up,  magnified,  and  recorded  by  a 
suitable  lever.  A  long  cannula  is  inserted  into  the  right  ventricle 
through  the  jugular  vein  or  into  the  left  ventricle  through  the  carotid 
artery.  Both  cannula  and  tambour  are  filled  with  an  alkahne  solu- 
tion to  prevent  coagulation  of  the  blood,  and  then  joined  air-tight. 
The  pressure  of  the  blood  in  the  ventricle  is  thus  transmitted  by  a 
hquid  column  to  the  tambour  and  to  its  attached  lever.  With  such 
a  manometer  a  curve  is  registered  similar  to  that  shown  in  Fig.  123. 
To  obtain  the  absolute  value  of  this  curve  in  millimeters  of  mercury 


THE  CIRCULATION  OF  THE  BLOOD. 


y /\AMA/WWV\M/WV /V  MAA/WWWWV 


O/  2 


J4  J 


it  is  necessary  to  previously  graduate  the  instrument.  An  examination 
of  the  curve  shows  that  previous  to  the  ventricular  contraction  there 
is  a  very  shght  rise  of  pressure  above  that  of  the  atmosphere,  repre- 
sented by  the  hne  a  —  b.  This  may  be  due  to  the  inflow  of  blood 
from  the  auricle  during  the  diastole.  At  o  the  pressure  suddenly 
rises,  passes  quickly  to  its  maximum  value,  (2),  which  is  maintained 
with  slight  variations  for  some  time,  and  then  suddenly  (3)  begins 
to  fall,  and,  rapidly  reaches  the  hne  of  atmospheric  pressure,  or  even 
passes  below  it,  becoming  negative  in  fact  for  a  short  period.  The 
curve  may  also  be  taken  as  a  record  of  the  ventricular  contraction, 
for  there  are  reasons  to  beUeve  that  the  two  closely  coincide  through- 
out their  entire  course.  A  characteristic  feature  of  this  curve  is  the 
more  or  less  horizontal  portion  comprised  between  the  points  2  and  3, 
marked  by  several  elevations  and  depressions,  which  has  been  termed 
the  systolic  plateau. 

With  other  forms  of  elastic  man- 
ometers, especially  those  in  which 
the  transmission  of  the  intra-ven- 
tricular  pressure  is  effected  by  air 
or  by  a  combination  of  air  and  li- 
quid, this  portion  of  the  curve  is 
represented  by  a  single  peak,  which 
is  taken  as  an  indication  that  the 
maximum  pressure  once  reached  is 
not  maintained,  but  immediately 
begins  to  fall  to  its  original  level, 
notwithstanding  the  continued  con- 
traction of  the  ventricle.  Those 
who  adhere  to  this  view  attribute 
the  plateau  to  the  closure  of  the 
orifice  of  the  catheter  by  the  con- 
tracting and  approximating  walls  of  the  ventricle.  There  are  reasons 
for  believing,  however,  that  the  former  curve  is  the  more  correct  re- 
presentation of  the  course  of  the  intra- ventricular  pressure.  Bayliss 
and  Starhng  photographed  on  a  moving  surface  the  oscillations  of  a 
fluid,  a  solution  of  sodium  sulphate,  in  a  capillary  glass  tube  one 
end  of  whicli  was  closed,  the  other  end  placed  in  connection  with 
an  intracardiac  catheter,  the  oscillations  representing  the  variations 
in  pressure.  The  photogram  thus  obtained  resembles  the  curve 
obtained  by  Hiirthle's  membrane  manometer. 

The  Relation  of  the  Intra-ventricular  Pressure  Curve  to  the 
Intra-cardiac  Mechanisms. — By  itself  the  curve  of  the  intra- 
ventricular pressure  affords  no  indication  as  to  events  occurring 
within  the  heart:  i.  e.,  as  to  the  times  during  the  systole,  of  the 
closure  of  the  auriculo-ventricular  valves  and  the  opening  of  the 
19 


'/   -2 


a-tf  s 


Fig.  123. — V.  Curve  of  the  pressure 
in  the  ventricle  of  the  dog. — 
{Hilrthle.)  A.  Curve  of  the  pres- 
sure in  the  aorta.  The  curves 
were  taken  simultaneously.  s. 
Tuning-fork  vibrations,  loo  per 
second. 


290  TEXT-BOOK  OF  PHYSIOLOGY. 

semilunar  valves,  or  the  times  during  the  diastole,  of  the  closure  of  the 
semilunar  valves  and  the  opening  of  the  auriculo-ventricular  valves. 

By  registering  the  curve  of  pressure  in  the  aorta  simultaneously 
with  the  pressure  in  the  left  ventricle  (Fig.  123),  and  by  comparing 
these  with  the  curve  of  the  successive  differences  of  pressure  in  these 
two  cavities  as  determined  by  the  "differential  manometer,"  it  be- 
comes possible  to  mark  on  the  ventricular  pressure  curve  the  points 
at  which  the  foregoing  events  take  place. 

During  the  systoHc  plateau  the  blood  is  passing  from  the  ventricle 
into  the  aorta.  Independent  of  the  slight  elevations  and  depressions 
there  is  an  absolute  fall  of  pressure  between  the  beginning  and  the 
end  of  the  plateau.  There  is  also  a  corresponding  fall  in  the  aortic 
pressure,  corresponding  to  these  two  points.  The  curve  of  the  dif- 
ference of  pressure  shows,  however,  that  the  ventricular  pressure  is 
slightly  higher  than  the  aortic.  This  fall  in  both  ventricular  and 
aortic  pressures  is  due  to  the  escape  of  blood  from  the  arterial 
into  and  through  the  capillary  system.  At  3,  however,  whether 
completely  emptied  or  not,  the  ventricle  suddenly  relaxes,  and  its 
pressure  soon  falls  below  that  in  the  aorta.  As  soon  as  this  takes 
place  the  semilunar  valves  must  close,  if  regurgitation  into  the  ven- 
tricular cavity  is  to  be  prevented.  A  comparison  of  the  aortic  pres- 
sure curve  shows  a  shght  notch,  the  "dicrotic  notch,"  just  preceding 
a  slight  elevation,  the  "dicrotic"  wave.  This  notch  is  taken  as  the 
moment  when  the  semilunar  valves  close.  The  corresponding  point 
on  the  ventricular  pressure  curve  has  been  placed  just  where  the 
ordinate  4  cuts  the  descending  portion.  As  yet,  however,  the  pressure 
is  higher  in  the  ventricle  than  in  the  auricle,  and  so  continues  until 
near  the  line  of  atmospheric  pressure.  At  this  point  the  pressure  in 
the  auricle,  due  to  the  accumulation  of  blood  during  the  ventricular 
systole,  now  forces  open  the  mitral  valve  and  the  blood  flows  into  the 
ventricle.  The  opening  of  the  mitral  valve  occurs  about  the  point 
where  the  ordinate  5  cuts  the  curve. 

The  ventricular  pressure  curve  affords  but  shght,  if  any,  indication 
of  the  auricular  systole.  It  apparently  does  not  give  rise  to  any 
noticeable  increase  in  the  ventricular  pressure.  The  slight  rise  in 
the  pressure  curve,  which  just  precedes  the  abrupt  rise  due  to  the 
ventricular  systole,  may  be  taken  as  an  indication  of  an  increasing 
pressure  due  to  the  inflow  of  blood  from  the  auricle.  As  soon  as 
the  pressure  in  the  ventricle  exceeds  that  in  the  auricle  the  mitral 
valve  closes.  This  is  marked  on  the  curve  where  the  ordinate  cuts 
it,  at  o.  Coincident  with  this,  the  ventricular  systole  begins,  and  as 
the  blood  is  contained  within  a  closed  cavity  the  pressure  abruptly 
rises.  A  comparison  of  the  aortic  curve  shows  that  for  a  short  time 
during  the  ventricular  systole,  the  pressure  is  falling,  but  at  one  point 
it  turns  at  a  sharp  angle  and  rapidly  rises.     This  is  an  indication  that 


THE  CIRCULATION  OF  THE  BLOOD.  291 

the  semilunar  valves  are  suddenly  thrown  open  and  the  blood  begins 
to  pass  into  the  aorta.  This  event  occurs  at  a  moment  marked  on 
the  ventricular  curve  by  the  ordinate  i.  Beyond  this  point  the  pres- 
sure continues  to  rise,  for  the  aortic  pressure  must  not  only  be  ex- 
ceeded, but  a  certain  velocity  must  be  imparted  to  the  blood.  Between 
the  ordinates  i  and  4,  the  semilunar  valves  remain  open  and  the 
blood  passes  into  the  aorta. 

In  accordance  with  the  foregoing  the  ventricular  systole  may  be 
subdivided  into  two  periods : 

1.  The  period  of  rising  tension,  from  the  beginning  of  the  systole  to 

the  opening  of  the  semilunar  valves,  occupying  from  0.02  to 
0.04  second. 

2.  The  period  of  ejection,  from  the  opening  of  the  semilunar  valves 

to  the  end  of  the  systole,  occupying  about  0.2  second. 
The  ventricular  diastole  may  also  be  divided  into  two  periods : 

1.  The  period  of  falling  tension  or  relaxation,  from  the  end  of  the 

systole  to  the  time  of  lowest  pressure  in  the  ventricle,  occupying 
about  0.05  second. 

2.  The  period  of  filling,  from  the  opening  of  the  mitral  valve  to  the 

beginning  of  the  systole. 

Negative  Pressure. — As  shown  by  the  ventricular  pressure  curve 
there  is  a  moment  when  the  pressure  falls  below  atmospheric  pres- 
sure, becoming  negative  to  it.  The  extent  to  which  this  takes  place, 
its  duration  and  frequency,  have  never  been  satisfactorily  determined. 
The  cause  of  the  negative  pressure,  its  influence  on  the  opening  of 
the  auriculo- ventricular  valves,  and  on  the  entrance  of  blood  into  the 
ventricles  are  equally  unknown.  The  most  probable  cause  is  an 
expansion  of  the  base  of  the  ventricles  due  to  the  enlargement  of  the 
aorta  and  pulmonary  artery.  That  it  is  not  due  to  the  expansion 
of  the  thorax  is  evident  from  the  fact  that  it  is  occurs  when  the  thorax 
is  open  and  the  heart  exposed. 

Heart-sounds. — Two  sounds  accompany  each  pulsation  of  the 
heart,  both  of  which  may  be  heard  by  applying  the  ear  or  the  stetho- 
scope to  the  chest- walls,  especially  over  the  region  of  the  heart.  One 
of  these  sounds  is  low  in  pitch,  dull  and  prolonged;  the  other  is  high 
in  pitch,  clear  and  short.  These  sounds  can  be  approximately  repro- 
duced by  pronouncing  the  syllables  lubb-dupp,  lubb-dupp.  The 
long  dull  sound  occurs  with  the  systole,  the  first  phase  of  a  new  cardiac 
cycle,  and  is  therefore  termed  the  jirst  sound;  the  short  clear  sound 
occurs  at  the  beginning  of  the  diastole,  with  the  second  phase  of  the 
cardiac  cycle,  and  is  therefore  termed  the  second  sound.  The  first 
sound  is  the  systolic,  the  second  the  diastolic,  sound.  With  the  ear 
it  can  readily  be  determined  that  there  is  a  brief  pause  between  the 
first  and  second  sounds,  and  a  longer  pause  between  the  second  and  the 
first  sounds.     The  duration  of  the  first  sound  is  almost  equal  to  the 


292  TEXT-BOOK  OF  PHYSIOLOGY. 

duration  of  the  systole — viz.,  0.3  second;  the  duration  of  the  second 
sound  is  not  more  than  o.i  second.  The  systoHc  sound  is  heard 
most  distinctly  over  the  body  of  the  heart;  the  diastohc  sound  is 
heard  most  distinctly  in  the  neighborhood  of  the  third  rib  to  the  right 
of  the  sternum. 

The  causes  of  the  heart-sounds  have  enlisted  the  attention  of 
clinicians  and  physiologists  for  years,  and  many  factors  have  been 
assigned  for  their  production.  At  present  it  is  generally  believed 
that  the  first  sound  is  the  product  of  at  least  two,  possibly  three, 
factors:  viz.,  the  contraction  of  the  muscular  v^alls  of  the  ventricles, 
the  simultaneous  closure  and  subsequent  vibration  of  the  tricuspid 
and  mitral  valves,  and  the  sudden  increase  of  pressure  of  the  apex  of 
the  heart  against  the  chest- wall. 

That  the  contraction  of  the  ventricular  muscle  gives  rise  to  a 
sound  is  certain  from  the  fact  that  it  is  per- 
ceptible in  an  excised  heart  when  the  cavities 
are  free  from  blood  and  when  the  valves  are 
prevented  from  closing.  The  explanation 
of  this  sound  is  extremely  difficult,  as  the 
contraction,  though  prolonged,  is  not  of  the 
nature  of  a  tetanus  and  therefore  not  char- 
acterized by  rapid  variations  of  tension. 
The  apex  element  may  be  ehminated  by  plac- 
ing the  individual  in  the  recumbent  position. 
Fig.   124.— Scheme  of  a  'pj^g  second  sound  is  the  product  of  the 

Cardiac  Cycle      Trie         . 

inner  circle  shows  what  simultaneous  closure  and  subsequent  vibra- 
events  occur  in  the  tion  of  the  aortic  and  pulmonary  valves 
heart,  and  the  outer,     ^^^^  ^  ^j^     beginning  of  the  ven- 

the     relation     of     the  .  i        i  i        i 

sounds  and  silences  to     tricular  diastole  as  the  blood  surges  back 

these  events.  against   the  closed  valves.     This  has  been 

definitely  proved  by  the  fact  that  the  sound 
disappears  when  the  valves  are  destroyed  or  held  back  by  hooks 
introduced  into  the  aorta  and  pulmonary  artery.  It  is  also  possible 
that  the  vibration  of  the  column  of  blood  produces  an  additional  tone 
which  adds  itself  to  that  produced  by  the  valves. 

The  relation  of  the  sounds  to  the  systole  and  diastole  of  the  heart 
is  represented  in  Figs.  124  and  121. 

The  Blood-supply  to  the  Heart. — The  nutrition  of  the  heart, 
its  contractihty,  the  force  and  frequency  of  the  beat,  are  dependent 
on  and  maintained  by  the  introduction  of  arterialized  blood  into  and 
the  removal  of  waste  products  from  its  tissue.  This  is  accomphshed 
by  the  coronary  arteries,  on  the  one  hand,  and  the  coronary  veins,  on 
the  other.  The  arteries,  two  in  number,  the  right  and  left,  arise  from 
the  aorta  in  the  pouches  of  Valsalva  just  above  the  right  and  left 
semilunar  valves.     Turning  in  opposite  directions,  they  ultimately 


THE  CIRCULATION  OF  THE  BLOOD. 


293 


anastomose,  forming  a  circle  around  the  base  of  the  ventricles. 
From  both  the  right  and  left  artery  branches  are  given  off  which  run 
over  the  walls  of  both  auricles  and  ventricles,  the  most  important 
of  which  in  man  are  the  anterior  and  posterior  interventricular. 
These  main  vessels  lie  in  grooves  on  the  surface  of  the  heart  beneath 
the  visceral  pericardium,  surrounded  by  connective  tissue  and  fat. 
Small  branches  penetrate  the  heart-muscle  in  which  they  break  up 
into  capillaries.  From  the  capillary  areas  small  veins  arise  which, 
passing  backward,  converge  to  form  the  coronary  veins.  These 
follow  the  course  of  the  arteries  and  finally  terminate  in  the  coronary 
sinus,  located  in  the  auriculo-ventricular  groove  on  the  posterior 
surface  of  the  heart.  This  sinus  opens  into  the  right  auricle  between 
the  opening  of  the  inferior  vena  cava  and  the  auriculo-ventricular 
opening.  Its  orifice  is  guarded  by  a  valve,  which  is  usually  single, 
though  sometimes  double. 

While  by  far  the  larger  portion  of  the  blood  is  returned  by  the 
coronary  veins,  it  is  also  certain  that  some  of  it  is  returned  by  small 
veins  which  open  into  little  pits  or  depressions  on  the  inner  surface  of 
the  heart-walls,  known  as  the  foramina  Thebesii.  It  has  been  lately 
shown  by  Pratt  that  these  foramina  are  present  not  only  in  the  auricular 
wall,  as  generally  stated,  but  in  the  walls  of  all  the  cavities.  These 
foramina  communicate  through  a  capillary  plexus  with  both  arteries 
and  veins,  and  by  special  large  passages  with  the  veins  alone. 

During  the  systole  the  intra-mural  vessels  are  compressed  and  the 
blood  driven  out  of  the  capillaries  into  the  veins;  during  the  diastole, 
the  vessels  again  dilate  and  permit  the  blood  to  re-enter  freely  from 
the  arteries.  The  greater  the  force  and  frequency  of  the  beat,  the 
greater  the  volume  of  blood  passing  through  the  coronary  system. 

The  period  of  time  in  the  cardiac  cycle  during  which  the  coronary 
arteries  are  filled  with  blood,  whether  during  the  systole  or  the  dias- 
tole, has  been  a  subject  of  much  discussion.  At  present,  however, 
as  the  result  of  many  experiments  it  is  generally  believed  that  they 
are  filled  at  the  time  of  the  systole.  A  comparison  of  the  tracings  of 
the  pulse-wave  taken  simultaneously  in  the  carotid  and  coronary 
arteries  shows  that  the  pressure  rises  and  falls  simultaneously  in  both 
vessels;  that  there  is  a  complete  agreement  between  the  two  trac- 
ings, and  as  a  corollary  both  vessels  are  filled  during  the  systole. 
But  because  of  the  pressure  which  the  heart  muscle  must  exert  upon 
the  smaller  arteries  and  veins  within  its  own  substance  during  systole, 
it  is  probable  that  there  is  a  freer  circulation  in  the  coronary  vessels 
during  the  period  of  diastolic  repose. 

In  mammals  the  nutrition  of  the  heart-muscle,  its  irritabihty  and 
contractihty,  depend  on  the  blood-supply  derived  from  the  coronary 
vessels.  This  is  shown  by  the  effects  which  follow  its  withdrawal. 
Ligation  of  both  coronary  arteries  in  the  dog  is  followed  by  a  diminu- 


294  TEXT-BOOK  OF  PHYSIOLOGY. 

tion  in  the  force  and  frequency  of  the  heart-beat,  and  in  a  few  minutes 
by  complete  cessation.  Ligation  of  even  a  single  branch  of  a  coro- 
nary artery,  provided  it  supply  a  sufficiently  large  territory, — e.  g., 
the  arteria  circumflcxa, — is  sufficient  to  cause  arrest  in  at  least  80 
per  cent,  of  animals  (Porter).  With  the  ligation  of  this  vessel  there 
occurs  a  gradual  diminution  in  the  force  and  frequency  of  the  systole. 
As  the  power  of  coordinate  contraction  ceases  the  heart-muscle 
frequently  exhibits  a  series  of  independent  contraction  of  individual 
fibers  and  cells  known  as  fibrillary  contraction.  All  the  results  which 
follow  ligation  are  to  be  attributed  in  the  light  of  experiment  to  the 
sudden  anemia  which  is  thus  established.  The  removal  of  the 
ligature  and  the  return  of  the  blood  will  restore  the  nutrition  and  re- 
establish coordinate  contractions.  The  excised  heart  of  the  mammal 
may  be  again  made  to  beat  by  passing  warm  defibrinated  blood 
through  the  coronary  vessels  under  a  suitable  pressure. 

In  frogs  and  allied  animals  the  heart  is  nourished  by  blood  flow'- 
ing,  during  the  diastole,  from  the  interior  of  the  heart  into  a  system 
of  irregular  channels  which  penetrate  the  walls  in  all  directions. 
With  the  systole  the  blood  is  returned  to  the  cavities.  The  excised 
heart  of  the  mammal- — e.  g.,  the  cat — may  be  partially  nourished  in 
a  similar  manner  through  the  foramina  Thebesii.  If  the  w^arm 
defibrinated  blood  of  the  same  animal  be  introduced  into  the  ventricle 
under  a  pressure  of  about  75  mm.  of  blood,  the  heart  will  recommence 
and  continue  to  beat  for  a  period  varying  from  one  to  several  hours. 

The  Causation  of  the  Heart-beat. — The  beat  of  the  heart,  its 
frequency  and  regularity,  its  continuance  from  the  early  stages  of 
fetal  development  till  death,  has  long  been  an  interesting  subject 
for  physiologic  investigation.  Though  related  to  the  functional 
activities  of  the  body  at  large,  the  activity  of  the  heart  is  in  a  sense 
independent  of  them,  for  it  will  continue  for  a  variable  length  of  time 
after  they  have  ceased.  The  heart  of  the  frog  and  the  turtle  will 
continue  to  beat  under  appropriate  conditions  for  some  hours  after 
separation  of  all  its  anatomic  connections  and  removal  from  the  body. 
The  heart  of  the  dog  or  cat  will,  however,  beat  but  for  a  few  minutes. 
The  human  heart  would  in  all  probabihty  act  in  the  same  way. 

The  reason  for  the  longer  continuance  of  the  beat  of  the  excised 
heart  of  the  cold-blooded  animal  beyond  that  of  the  warm-blooded 
animal  lies  probably  in  the  difference  in  the  rate  of  their  respective 
metabolisms.  There  is  reason  to  believe  that  each  cell  of  the  heart- 
muscle,  in  common  with  other  tissue-cells,  during  life  stores  up  and 
holds  in  reserve  a  larger  quantity  of  nutritive  material  than  is  necessary 
for  its  immediate  needs.  When  separated  from  the  general  blood- 
supply,  the  cells  at  once  begin  to  utilize  this  reserved  material.  With 
its  exhaustion  the  irritabihty  declines  and  in  a  short  time  disappears. 
As  the  metabolism  is  far  more  rapid  in  the  warm-blooded  than  in  the 


THE  CIRCULATION  OF  THE  BLOOD.  295 

cold-blooded  animal,  it  is  probable  that  the  reserved  nutritive  material 
is  utilized  much  more  quickly  in  the  former  than  in  the  latter.  So 
long  as  it  lasts  in  either  class,  the  irritability  and  contractihty  persist. 
The  passage  of  defibrinated  oxygenated  blood  through  the  vessels  of 
the  excised  heart  of  the  dog  may  maintain  the  duration  of  the  irri- 
tabihty  for  a  period  of  from  one  to  six  hours. 

Whatever  the  immediate  or  exciting  cause  of  the  heart  contraction 
may  be,  the  fundamental  condition  for  its  manifestation  is  the  main- 
tenance of  the  irritability.  So  long  as  this  persists  at  the  normal 
level  the  heart-muscle  will  contract  in  response  to  the  appropriate 
stimulus. 

Nature  of  the  Stimulus. — As  the  heart  continues  to  beat  after 
removal  from  the  body,  it  is  evident  that  the  stimulus  does  not  origi- 
nate in  the  central  nerve  system  but  in  the  heart  itself.  Two  views 
have  been  held  as  to  its  origin  and  nature: 

1.  That  it  originates  in  the  nerve-cells  found  in  various  parts  of  the 

heart-muscle;  that  it  is  a  nerve  impulse  rhythmically  and  auto- 
matically discharged  by  these  cells  and  transmitted  by  their 
axons  to  the  heart-muscle  cells. 

2.  That  it  originates  in  the  muscle-cells  themselves;  that  it  is  chemic  in 

character  and  due  to  a  reaction  between  the  inorganic  salts  in  the 
muscle  cells  and  those  in  the  lymph  by  wliich  they  are  surrounded. 

According  to  the  first  view  the  stimulus  is  neurogenic,  according 
to  the  second  view  myogenic,  in  origin. 

The  presence  of  nerve-cells;  their  relation  to  the  muscle-cells;  the 
pronounced  rhythmic  activity  of  the  sinus  and  auricles  in  which  the 
nerve-cells  are  abundant ;  the  feeble  activity  of  the  apex,  in  which  they 
are  wanting, — these  and  other  facts  lend  support  to  the  view  that  the 
stimulus  originates  in  the  nerve-cells.  To  them  have  been  attributed 
the  power  of  automatic  activity. 

The  absence  of  nerve  cells  in  portions  of  the  heart-muscle,  which 
nevertheless  exhibit  rhythmic  contractions  for  quite  a  long  period 
of  time;  the  rhythmic  beat  of  the  embr}'omc  heart  before  the  migra- 
tion of  nerve-cells  to  its  walls  shows  that  the  stimulus  does  not  neces- 
sarily originate  in  nerve-cells.  Moreover,  Porter  has  conclusively 
shown  that  the  apex  of  the  dog's  heart,  which  is  generally  beheved 
to  be  totally  devoid  of  nerve-cells,  can  be  made  to  beat  for  hours  by 
feeding  it  through  its  nutrient  artery  with  warm  defibrinated  blood. 
Unless  it  be  assumed  that  the  heart-muscle  contracts  automatically, 
without  cause,  it  is  a  fair  assumption  that  the  exciting  cause  of  the 
contraction  arises  within  the  muscle-cells  themselves,  and  that  it  is 
in  all  probability  the  outcome  of  a  reaction  between  the  chemic  con- 
stituents of  the  blood  or  lymph  on  the  one  hand,  and  the  chemic 
constituents  of  the  muscle-cells  on  the  other.  Attempts  have  been 
made  to   isolate   these   constituents,    to   determine   not  only    their 


296  TEXT-BOOK  OF  PHYSIOLOGY. 

individual,  but  also  their  cooperative  action,  when  combined  in  pro- 
portions approximating  those  in  which  they  exist  in  the  blood. 

Action  of  Inorganic  Salts. — The  agents  known  to  be  direct- 
ly concerned  in  exciting  and  sustaining  the  heart-beat  are  sodium 
chlorid,  calcium  phosphate  or  chlorid,  and  potassium  chlorid.  Rin- 
ger's combination  of  these  salts  is  made  by  saturating  a  0.65  per  cent, 
solution  of  sodium  chlorid  with  calcium  phosphate,  and  then  adding 
to  each  100  c.c,  2  c.c.  of  a  i  per  cent,  solution  of  potassium  chlorid. 
A  frog's  heart  immersed  in  this  solution  will  continue  to  beat  for 
several  hours.  A  combination  of  the  chlorids  of  sodium,  calcium, 
and  potassium  is  equally  efificient  in  maintaining  the  heart-beat. 

The  collective  as  well  as  the  individual  actions  of  these  salts  have 
been  strikingly  brought  out  by  the  experiments  of  Professors  Howell 
and  Green,  from  whose  papers  the  following  statements  are  derived. 
In  these  experiments  strips  from  the  terminations  of  the  venae  cava^ 
and  from  the  ventricle  of  the  terrapin's  heart  were  employed.  The 
proportion  of  these  salts  most  favorable  to  the  contraction  of  the 
venae  cavas  strips  is  the  following:  viz.,  sodium  chlorid,  0.7  per  cent.; 
calcium  chlorid,  0.026  per  cent.;  potassium  chlorid,  0.03  per  cent.; 
for  the  contraction  of  the  ventricular  strips  a  larger  percentage  of  the 
calcium  is  required:  viz.,  0.04  to  0.05.  From  this  fact  it  is  inferred 
that  the  venae  cavas  region  is  more  sensitive  to  the  action  of  the  com- 
bined salts  than  the  ventricle.  With  the  latter  strength  of  the  solu- 
tion, the  ventricular  strips  may  contract  for  several  days.  In  the 
first  proportion  as  well  as  in  serum  the  ventricular  strips  do  not  con- 
tract, but  are  kept  in  good  condition  for  contraction  for  several  days. 
An  increase  in  the  quantity  of  the  calcium  chlorid  sufficient  to  raise 
the  percentage  to  0.04  or  0.05  wnll  after  a  brief  latent  period  give  rise 
to  rapid  and  energetic  contractions. 

The  action  of  the  individual  salts  is  also  best  shown  with  ven- 
tricular strips.  In  a  0.7  per  cent,  sodium  chlorid  solution  the  strip 
beats  rhythmically  and  energetically,  but  for  a  short  period  and 
with  gradually  diminishing  force,  until  it  entirely  ceases.  A  reason 
assigned  for  this  is  the  removal  of  other  salts  necessary  to  the  excita- 
tion of  the  contraction.  In  a  calcium  chlorid  solution — 0.9  per  cent. — 
i.  e.,  isotonic  with  the  sodium  chlorid — the  heart  strip  is  thrown  into 
strong  tone,  but  does  not  rhythmically  contract.  If,  however,  the 
strip  is  placed  in  normal  sahne,  and  calcium  chlorid  added  in  amounts 
equal  to  that  present  in  the  blood,  it  will  after  a  very  short  latent 
period  begin  to  contract  rapidly  and  energetically  and  for  a  longer 
tiine  than  when  in  sodium  chlorid  solution  alone.  The  contractions 
not  infrequently  occur  before  relaxation  is  completed,  so  that  the 
strip  passes  into  the  condition  of  contracture. 

In  potassium  chlorid  solutions  isotonic — 0.9  per  cent.^ — with 
sodium  chlorid  solution  the  heart  strip  also  fails  to  contract.     This 


THE  CIRCULATION  OF  THE  BLOOD.  297 

is  the  case  also  when  the  potassium  is  added  to  the  sodium  chlorid 
in  amount  practically  equal  to  that  found  in  the  blood. 

With  the  foregoing  combination  of  inorganic  salts  (Ringer's  or 
Howell  and  Green's)  in  which  the  heart  continues  to  contract  for 
many  hours,  it  is  believed  that  the  sodium  chlorid  maintains  the 
osmotic  pressure  between  the  heart  tissues  and  the  surrounding  fluid; 
that  the  calcium  salt  prevents  the  washing  out  of  the  calcium  salts, 
and  at  the  same  time  acts  as  a  stimulus  to  the  heart-cells;  that  the 
potassium  chlorid  acts  antagonistically  to  the  calcium.  From  these 
facts  it  may  be  inferred  that  the  stimulus  is  chemic  in  character 
and,  though  continuously  acting,  calls  forth  but  periodic  contractions. 

Th.  W.  Engelman  concludes,  as  a  result  of  long  continued  experi- 
mentation, that  a  stimulating  action  cannot  be  ascribed  to  the  above- 
mentioned  inorganic  salts  either  alone  or  in  combination  with  or- 
ganic matter  and  ox}-gen.  Blood  and  lymph  with  their  contained 
ingredients  merely  furnish  the  conditions  favorable  to  the  develop- 
ment of  the  excitation  process.  The  stimulus,  according  to  this 
experimenter,  arises  as  a  result  of  metabolic  processes  occurring 
within  the  cells  themselves  so  long  as  they  are  conditioned  by  the 
factors  just  mentioned. 

PROPERTIES  OF  THE  HEART-MUSCLE. 

1.  Irritability. — The  heart-muscle  in  common  with  other  muscles 

possesses  irritability  in  virtue  of  which  it  responds  by  a  change 
of  form  to  the  action  of  a  stimulus.  Whatever  the  stimulus, 
here,  as  elsewhere,  there  is  a  conversion  of  potential  into  kinetic 
energy — heat  and  mechanic  motion.  The  normal  physiologic 
stimulus  is  at  present  undetermined.  In  common  with  other 
forms  of  muscle  tissue,  the  heart  may  be  made  to  contract  by 
artificial  stimuli — e.  g.,  mechanic,  thermic,  chemic,  and  electric. 
The  irritabihty  depends  on  the  nutrition,  and  so  long  as  this  is 
maintained  the  muscle  will  respond  by  a  contraction  to  any  stim- 
ulus. The  irritability  is  most  marked  in  the  neighborhood  of  the 
venae  cavns  terminations.     It  is  least  marked  in  the  ventricles. 

2.  Conductivity. — The  heart-muscle  possesses  conductivity.     The 

excitation  process  and  the  subsequent  contraction  wave,  both  of 
which  take  their  rise  under  physiologic  conditions  near  the  venae 
cavae  terminations,  are  conducted  over  the  auricles,  thence  to  the 
ventricles  from  base  to  apex.  The  propagation  of  both  processes 
is  accomplished  by  muscle-tissue  alone,  independently  of  the 
nerve  systems.  The  conductivity,  however,  is  not  equally  well 
developed  in  every  part  of  the  heart.  This  is  especially  true  of 
the  tissue  at  the  auriculo- ventricular  junction.  At  this  point  the 
contraction  wave  is  delayed  for  an  appreciable  period,  a  condi- 
tion due  to  the  embryonic  character  of  the  muscle-tissue.     In  the 


TEXT-BOOK  OF  PHYSIOLOGY. 


frog's  heart  the  excitation  process  begins  in  the  sinus  venosus, 
from  which  it  passes  to  the  auricles,  thence  to  the  ventricles. 
The  excitation  process  as  well  as  the  contraction  wave  is  de- 
layed both  at  the  sinu-auricular  and  auriculo-ventricular  junc- 
tions. In  Fig.  125,  which  is  a  graphic  record  of  the  heart-beat, 
the  two  elevations  of  the  lever  on  the  up-stroke,  a  and  h,  repre- 
sent the  contraction  of  the  sinus  and  the  auricle  respectively, 

while  the  two  depressions  c  and  d  in- 
dicate the  delay  in  the  transmission  of 
the  contraction  wave  at  the  two  junc- 
tions. There  is  here  an  anatomic 
obstacle  to  the  conduction  of  the  con- 
traction. This  may  be  artificially  in- 
creased by  compressing  the  heart  be- 
tween the  auricles  and  ventricles  with 
a  clamp.  By  carefully  regulating  the 
pressure  it  is  possible  to  so  block  the 
wave  that  three  or  four  auricular  con- 
tractions may  occur  before  a  single  ventricular  contraction 
(Fig.  126).  A  similar  blocking  of  the  contraction  wave  in  the 
dog's  heart  has  been  accomplished  by  Erlanger,  by  compression  of 
the  auriculo-ventricular  groove  by  means  of  a  specially  devised 
hook-clamp.     When  the  structures — the  muscle-band  of  His — 


Fig.  125.— Record  of  the 
Contraction  of  the 
Frog's  Heart. 


Aur. 


Fig.  126.- 


Vent 


-Record  of  the  Auricular  and  Ventricular  Contractions  before 

AND    after   the    CLOSURE    OF    THE    ClAMP    AT    a. 


were  completely  compressed,  the  auricles  and  ventricles  beat  with 
an  independent  rhythm,  the  relation  of  the  auricle  to  ventricle 
being  as  3  to  i .  This  experiment  affords  a  possible  explanation 
of  the  altered  rhythm  of  the  auricles  and  ventricles  in  that 
pathologic  condition  known  as  Stokes- Adams  disease.  Erlanger 
has  demonstrated  from  a  study  of  the  cardiac  impulse,  the 
brachial  and  the  jugular  pulse,  that   the  cardiac  disturbances 


THE  CIRCULATION  OF  THE  BLOOD.  299 

are  due  to  a  diminution  in  conductivity  at  the  auriculo- ventricu- 
lar junction.  Since  the  ventricular  rhythm  was  at  times  in- 
dependent of  the  auricular,  the  blocking  must  have  been  com- 
plete. Thus  in  one  determination  the  rate  of  the  ventricular 
beats  was  27.6  per  minute,  while  the  rate  of  the  auricular  beats 
was  98  per  minute. 

Rhythmicity. — The  beat  of  the  heart  is  a  uniform  movement, 
occurring  at  regular  intervals.  Each  phase  of  each  beat  occupies  a 
regular  measure  of  time.  The  beat  is  therefore  rhythmic  in  char- 
acter. The  heart-muscle  as  a  whole  varies  in  rhythmic  power  in 
its  different  parts.  It  is  best  developed  in  the  frog  and  tortoise, 
in  the  sinus  venosus,  less  so  in  the  auricles,  least  in  the  ventricles. 
This  may  be  shown  by  division  of  the  tissue  between  sinus  and 
auricles  in  situ.  At  once  the  auriculo-ventricular  portion  ceases 
to  beat,  while  the  sinus  continues  contracting  as  usual.  In  a 
short  time  the  auricles  and  ventricles  begin  to  beat,  though  less 
rapidly  than  formerly.  Separation  of  the  auricle  from  the  ven- 
tricle is  again  followed  by  rest.  In  due  time  the  auricle  begins 
to  beat,  while  the  ventricle  remains  quiescent.  If  the  ventricle 
be  now  stimulated  in  a  rhythmic  manner,  it  may  resume  rhyth- 
mic activity.  These  facts  are  taken  as  an  indication  that  the 
rhythmic  power  is  developed  in  unequal  degree  in  the  three 
divisions  of  the  heart.  The  same  difference  in  the  rhythmicity 
of  the  auricles  and  ventricles  of  the  mammalian  heart  also 
exists,  though  perhaps  not  to  the  same  extent. 

Automatic ity. — The  heart-muscle  continuing  to  contract  rhythmi- 
cally, even  after  removal  from  the  body  and  without  the  aid  of 
any  external  stimulus  is  said  to  be  automatic  in  action.  This, 
however,  does  not  exclude  the  action  of  an  internal  stimulus. 

Tonicity. — The  heart-muscle,  Hke  the  vascular  muscle,  main- 
tains continuously  a  certain  degree  of  contraction,  termed  tone, 
upon  which  the  efficiency  of  the  heart  as  a  pumping  organ  is 
largely  dependent.  This  tone  may,  however,  be  increased  or 
decreased  by  the  action  of  various  external  agents.  Thus  the 
passage  of  dilute  solutions  of  various  drugs — e.  g.,  alkalies, 
digitalis — through  the  cavities  of  the  excised  heart  will  so  in- 
crease the  tone,  or  the  contractile  power,  that  complete  relaxa- 
tion is  prevented,  until  finally  the  heart  comes  to  a  standstill  in 
the  condition  of  systole.  The  passage  of  dilute  solutions  of 
lactic  acid,  muscarine,  etc.,  through  the  heart  will,  on  the  con- 
trary, so  decrease  the  tone  or  the  contractile  power  that  the 
normal  contraction  is  not  attained.  The  relaxation  therefore 
gradually  increases  until  the  heart  finally  comes  to  a  stand- 
still in  the  condition  of  diastole.  In  the  first  instance  the  tonicity 
is  said  to  be  increased;    in  the  second  instance,  decreased. 


300  TEXT-BOOK  OF  PHYSIOLOGY. 

The  Response  of  the  Heart  to  the  Action  of  a  Stimulus. — 

The  heart  of  the  frog  as  well  as  of  some  other  animals  may  be  brought 
to  a  standstill  by  the  ligation  of  the  tissues  between  the  sinus  veno- 
sus  and  the  auricle,  a  procedure  first  introduced  by  Stannius  and 
now  known  as  the  first  Stannius  ligature.  Under  such  circumstances 
the  heart  may  be  made  to  contract  by  stimulating  it  with  the  single 
induced  current.  With  each  passage  of  the  current  the  heart  con- 
tracts. Contrary  to  what  is  observed  in  other  muscles,  the  heart- 
muscle,  if  it  contracts  at  all,  at  once  reaches  its  maximal  value. 
Any  increase  in  the  strength  of  the  stimulus  above  the  threshold 
value  has  no  greater  effect  on  the  extent  or  force  of  the  contraction 
than  the  minimal  stimulus.  A  conclusion  which  may  be  drawn 
from  this  fact,  according  to  Engelman,  is  as  follows :  By  reason  of  the 
fact  that  the  heart  contracts  at  its  maximum  value  to  the  action 
of  any  strength  of  stimulus,  under  given  conditions,  there  is  alwa}"s 
ensured  a  complete  emptying  of  the  ventricular  contents  and  a  uni- 
form discharge  of  blood  into  the  arteries,  which  would  not  be  the 
case  if  the  extent  of  the  contraction  varied  with  the  strength  of  the 
stimulus;  and  there  are  reasons  for  believing  that  the  normal  stimu- 
lus for  the  contraction  varies  within  wide  limits  above  the  threshold 
value  both  in  normal  and  abnormal  conditions  of  the  heart.  The 
changes  in  the  extent  or  force  of  the  contraction  are  the  result,  not  of 
changes  in  the  intensity  of  the  stimulus,  but  of  changes  in  the  heart- 
muscle,  caused  by  variations  in  mechanical  resistances. 

The  periodicity  of  the  heart's  action  or  its  rhythm  may  also  be 
elucidated  by  the  foregoing  fact.  There  are  reasons  for  believing 
that  at  the  time  of  the  contraction  practically  all  of  the  available 
energy-holding  material  is  completely  utilized,  after  which  the  heart 
relaxes  and  remains  at  rest  in  the  diastohc  condition  for  a  given 
period;  and  before  a  second  excitation  wave  can  pass  from  the  si- 
nus over  the  heart  there  must  be  a  re-accumulation  of  energy-holding 
material.  This  is  accomplished  during  the  diastole.  By  virtue  of 
this  fact  the  heart  cannot  act  otherwise  than  in  a  periodic  or  rhyth- 
mic manner. 

Inasmuch  as  there  is  a  conversion  of  potential  into  kinetic  energy 
during  the  systole,  there  is  of  necessity  a  lowering  of  the  irritability, 
and  for  this  reason  the  heart  will  not  respond  to  the  action  of  a 
second  stimulus  during  the  systolic  period.  This  non-responsive- 
ness of  the  heart  may  be  shown  by  throwing  into  it  a  second 
stimulus  at  any  moment  during  the  systole.  Whatever  the  moment, 
the  extent  of  the  contraction  remains  the  same.  During  the 
systohc  period  the  heart  is  said,  therefore,  to  be  refractory  to  a  second 
stimulus;  and  if  the  stimulus  be  a  weak  one  it  may  continue  refrac- 
tory throughout  the  relaxation  and  even  possibly  during  a  portion  of 
the  diastole. 


THE  CIRCULATION  OF  THE  BLOOD.  301 

If,  however,  the  second  stimulus  be  of  average  strength  and 
thrown  into  the  heart  during  the  relaxation,  a  second  contraction 
or  extra  systole  is  developed  which  superposes  itself  on  the  first,  but 
the  height  of  the  contraction  is  no  greater  than  the  first  (Fig.  127). 
There  is,  therefore,  no  summation  of  effects  such  as  occurs  when 
skeletal  muscles  are  similarly  stimulated.  For- this  reason  a  tetanic 
condition  of  the  heart  cannot  arise.  With  the  relaxation  of  the  heart 
after  the  extra  systole  a  considerable  pause  in  the  heart's  action  oc- 
curs to  which  the  term  compensatory  pause  is  given,  which  is  as  long 
as  the  usual  relaxation  period  was  shortened  by  the  extra  systole 
plus  the  length  of  the  usual  pause.  The  duration  of  the  pause  must 
in  any  instance  be  sufficient  to  pemiit  of 
a  storage  of  energy- holding  compounds 
and  a  restoration  of  the  normal  period- 
icity or  rhythm.     The  explanation  that 

may  be  offered  for  the  development  of 1 

the  second  contraction  is  either  that  the    p^^    127— The  Extra  Con- 
energ}'-holding  material  was  not  wholly  traction  and  the  Com- 

destroved  or  that  during  the  short  time  pensatory  Pause.    The 

^•^'^  jir  .1  ^     ,•        1  break    in    the    horizontal 

which  elapsed  before  the  second  stimulus  ^ne  indicates  the  moment 

new  material  had  been  generated.  the  electric  current  passes 

If  a   series  of    successive   stimuli  be  through  the  heart, 

thrown   into  the   heart-muscle  the  effect 

will  vary  in  accordance  with  their  time  intervals.  Should  this  be 
less  than  about  three  seconds  there  will  be  a  gradual  increase  in  the 
height  for  some  half  dozen  contractions,  a  result  to  which  the  term 
"staircase"  has  been  given.  This  increase  in  the  height  of  the 
contraction  is  attributed  to  an  increase  in  the  irritability  and  con- 
tractility of  the  muscle  the  result  of  the  stimulation.  A  similar 
beneficial  effect  follows  successive  stimulation  of  skeletal  muscle. 

Though  many  of  the  experiments  relating  to  the  properties  of  the 
heart-muscle  have  been  made  on  the  hearts  of  frogs,  turtles,  and 
aUied  animals,  there  is  every  reason  for  believing  that  the  results  so 
obtained  hold  true,  with  minor  exceptions,  for  the  heart  of  the 
mammal. 

THE  NERVE  MECHANISM  OF  THE  HEART. 

By  this  term  is  meant  a  combination  of  nerves  and  nerve-centers 
which  cooperate  to  increase  or  decrease  either  the  rate  or  force — or 
both — of  the  heart's  contraction  in  accordance  with  the  needs  of  the 
system.  That  the  heart  is  normally  influenced  by  the  central  organs 
of  the  nerve  system  in  response  to  the  action  of  nerve  impulses  re- 
flected to  them  from  many  organs  of  the  body  is  a  matter  of  per- 
sonal experience;  that  it  is  abnormally  influenced  by  the  same  or 
other  organs  in  response  to  nerve  impulses  reflected  to  them  in  conse- 


302 


TEXT-BOOK  OF  PHYSIOLOGY. 


quence  of  pathologic  and  traumatic  processes  occurring  in  different 
regions  of  the  body,  and  that  both  heart  and  nerves  are  modified  in 
different  ways  by  the  action  of  drugs  introduced  into  the  body,  are 
matters  of  daily  chnical  experience. 


—3notio rial  Centers  I 
Ech'darating  (Blue) ) 
Depressing  (Red) 


Cardio  -Inhibitor  Centef 


Ganglion  Stdlatum 


Intra- Cardiac 
Nerue  Cells 


CardioGccelerator  Center 


Ya^usNerue 

[Inhibitor  (Red) 
j  Sensor  (Black) 


Sympathetic  Nerves     I 

fttreehra.tor&G'iiffmen.torj 


Fig.  127  A.     Diagram  of  the  nerve  mechanism  of  the  heart. 

The  nerves  comprising  this  mechanism  and  the  relation  they  bear 
one  to  another  are  represented  in  Fig.  127  A. 

It  was  stated  in  a  previous  paragraph,  page  294,  that  the  con- 
traction of  the  heart-muscle  is  independent  of  its  connection  with 


THE  CIRCULATION  OF  THE  BLOOD.  303 

the  central  organs  of  the  nerve  system,  and  that  it  will  continue  to 
contract  in  a  rhytlimic  manner  for  a  variable  length  of  time  even 
after  its  removal  from  the  body  of  the  animal,  the  length  of  time 
vandng  with  the  animal  and  the  conditions  to  which  it  is  subjected ; 
that  the  stimulus  is  myogenic  in  origin  and  chemic  in  character,  the 
result  of  a  reaction  between  the  inorganic  salts  in  the  muscle-cells 
and  those  in  the  lymph  by  which  they  are  surrounded.  It  has  also 
been  further  shown  that  even  in  the  living  animal  the  heart  will  con- 
tinue to  beat  and  fulfil  its  functions  after  division  of  all  nerves  in  con- 
nection with  it.  A  dog  thus  experimented  on  hved  for  eleven  months, 
and  be}ond  the  fact  of  becoming  fatigued  more  readily  upon  exertion 
than  formerly,  exhibited  no  striking  disturbance  of  his  functions. 
Nevertheless  groups  of  nerve-cells  are  present  in  certain  portions  of 
the  heart  in  all  classes  of  vertebrate  animals,  which  bear  an  anatomic 
and  physiologic  relation  to  the  heart-cells  on  the  one  hand,  and  to 
the  nerves  connecting  them  with  the  central  organs  of  the  nerve 
system  on  the  other  hand. 

Intra-cardiac  Nerve-cells. — In  the  frog  heart  a  group  of  nerve- 
cells  is  found  in  the  sinus  at  its  junction  with  the  auricle,  known  as 
the  crescent  or  ganghon  of  Remak;  a  second  group  is  found  at  the 
base  of  the  ventricle  on  its  anterior  aspect,  and  known  as  the  gan- 
glion of  Bidder;  a  third  group  is  found  in  the  auricular  septum, 
known  as  the  septal  ganglion,  or  v.  Bezold's  or  Ludwig's.  The 
majority  of  the  cells  are  situated  on  the  surface  of  the  heart  just 
beneath  the  pericardium.  From  the  cell-body  fine  non-medullated 
fibers  pass  into  the  substance  of  the  heart,  to  become  histologically 
and  physiologically  related  with  the  muscle-fiber.  These  nerve-cells 
were  formerly  regarded  as  the  sources  of  the  stimuli  for  the  heart's 
activities.  They  are  regarded  by  Gaskill  as  trophic  in  function, 
and  exerting  a  favorable  influence  on  the  nutrition  of  the  heart-muscle. 

In  the  dog  heart  the  nerve-cells  are  not  arranged  in  such  definite 
groups,  but  are  distributed  in  the  terminations  of  the  venae  cavse, 
pulmonary  veins,  the  walls  of  the  auricles,  and  in  the  neighborhood 
of  the  base  of  the  ventricles. 

Extra-cardiac  Nerves. — The  nerves  which  connect  the  heart 
with  the  central  nerve  system  are  two:  viz.,  the  vagus  or  pneumo- 
gastric,  and  the  sympathetic. 

The  Vagus. — Histologic  investigatioji  has  shown  that  the  vagus 
nerve-trunk  contains  meduUated  fibers  of  large  and  small  size.  Ex- 
periment has  shown  that  the  large  fibers  are  afferent,  the  small  fibers 
efferent  in  function.  The  large  afferent  fibers  arise  in  the  gangha 
situated  on  the  tmnk  of  the  nerve.  From  their  contained  nerve- 
cells  a  short-axon  process  proceeds  which  soon  divides  into  a  central 
and  a  peripheral  branch.  The  central  branch  passes  toward  and 
into  the  grav  matter  beneath  the  floor  of  the  fourth  ventricle  where 


304 


TEXT-BOOK  OF  PHYSIOLOGY. 


Viif/iis 


its  end-tufts  arborize  around  ncrvc-cclls;  the  peripheral  branch 
passes  toward  the  general  periphery  to  be  distributed  to  the  mucous 
membrane  of  the  lungs,  stomach,  intestine,  etc.  The  small  efferent 
fibers  are  the  peripherally  coursing  axons  of  nerve-cells  situated  in 
the  gray  matter  beneath  the  floor  of  the  fourth  ventricle  at  the  tip 
of  the  calamus  scriptorious.  The  exact  course  of  these  fibers  is  not 
definitely  known.  According  to  some  investigators,  they  leave  the 
medulla  by  way  of  the  spinal  accessory  nerve  and  enter  the  trunk  of 
the  vagus  through  the  internal  or  anastomotic  branch ;  according  to 
recent  investigations  made  by  Schaternikoff  and  Fried enthal,  they 
leave  the  medulla  along  the  path  by  which  the  afferent  fibers  enter 
and  never  become  associated  with  the  spinal  accessory  nerve  at 
its  origin. 

Below  the  origin  of  the  inferior  or  recurrent  laryngeal  nerves, 

branches  containing  the  efferent  fibers 
are  given  off,  which  pass  to  the  heart. 
The  terminal  branches  of  the  fibers  are 
not  distributed  directly  to  the  heart- 
muscle,  but  to  the  nerve-cells,  around 
the  bodies  of  which  they  end  in  basket- 
like formations.  The  fibers  in  the 
vagus  are  pre-ganglionic ;  those  of  the 
nerve-cells  post-ganglionic.  (See  Fig. 
128.) 

The  Sympathetic. — Histologic 
investigation  has  shown  that  the 
sympathetic  nerves  which  pass  to 
the  heart  are  non-meduUated.  Experi- 
ment has  shown  that  they  are  also 
efferent  in  function.  The  fibers  are 
peripherally  coursing  axons  of  nerve- 
cells  situated  in  the  ganglion  stellatum  and  inferior  cervical  ganglion. 
After  reaching  the  heart  they  may  terminate  directly  in  the  muscle- 
cell  or  indirectly  through  the  intervention  of  the  heart  nerve-cells. 
The  former  method  is  the  njore  probable.  The  nerve  cells  in  these 
ganglia  are  in  relation  with  small  medullated  nerve-fibers  which, 
emerging  from  the  cord  in  the  anterior  roots  of  the  second  and 
third  thoracic  nerves  pass  through  the  white  rami  communicantes, 
and  thence  to  the  ganglion  stellatum,  and  the  inferior  cervical  gan- 
glion, where  their  end  branches  arborize  around  the  nerve-cells.  The 
nucleus  of  origin  of  these  medullated  fibers  is  probably  in  the 
medulla  oblongata.  The  fibers  emerging  from  the  cord  are  pre- 
ganglionic,  those  emerging  from  the  ganglion,  post-ganglionic. 

In  the  frog  these  two  sets  of  nerve-fibers,  viz.,  the  efferent  vagus 
fibers  and  the  sympathetic  fibers,  pass  to  the  heart  in  the  common 


thetic  yeitron 


Cell 


Fig.  128.  —  Diagram  showing 
THE  Relation  of  the  Vagus 
TO  THE  Heart  Muscle-cell. 


THE  CIRCULATION  OF  THE  BLOOD. 


305 


sheath  of  the  vagus  nerve.  The  s}Tnpathetic  fibers  proper,  the  post- 
ganghonic  libers,  arise  from  nerve-cells  in  the  third  s}-mpathetic 
ganglion.  From  this  origin  they  ascend,  passing  successively  through 
the  second  sympathetic  ganglion,  the  annulus  of  Vieussens,  the  first 
sympathetic  ganghon,  to  the  ganglion  on  the  trunk  of  the  vagus,  at 
which  point  they  enter  the  sheath  of  this  nerve.  For  this  reason  the 
common  tnmk  which  descends  to  the  heart  is  generally  spoken  of  as 
the  va go-sympathetic  nerve.  The  pre-ganglionic  fibers  emerge  from 
the  cord  in  the  anterior  roots  of  the  third  spinal  nerve,  pass  through 
the  rami  communicantes  to  the  third  sympathetic  ganglion,  around 
the  cells  of  which  the  nerve-fibers  arborize. 

In  man  and  some  other  mammals  the  sympathetic  fibers  arising  in 
the  ganglion  stellatum  pass  direct  to  the  heart  without  any  anatomic 
connection  with  the  vagus  trunk. 

The  Physiologic  Action  of  the  Vagus  and  Sympathetic 
Nerves.— The  information  noAV  possessed  regarding  the  influence 
which  the  cen- 
tral nerve  sys- 
tem exerts  upon 
the  heart  has 
been  derived 
largely  from  ex- 
periments made 
on  the  nerves  of 
the  frog,  toad 
and   turtle.     To 

demonstrate  the  respective  actions  of  the  two  sets  of  fibers  they 
must  be  stimulated  or  divided  before  their  union  at  the  vagus 
ganghon. 

Stimulation  of  the  intra-cranial  roots  of  the  vagus  with  very  weak 
induced  currents  is  followed  by  a  gradual  diminution  in  the  rate  or 
rhythm  and  a  diminution  in  the  force  of  the  heart-beat.  If  the  in- 
duced currents  are  moderate  in  strength,  the  heart  will  at  once  come 
to  a  standstill  in  diastole.  Since  stimulation  of  the  nerve,  which  in 
all  probabihty  exaggerates  its  normal  function,  is  followed  by  a  period 
of  rest  or  inactivity,  the  vagus  is  said  to  have  a  retarchng  or  an  inhibi- 
tor influence  on  the  beat  of  the  heart. 

After  cessation  of  the  stimulation,  the  heart  resumes  its  activity. 
At  first  the  beat  usually  is  slow  and  feeble,  but  with  each  succeeding 
beat  both  rate  and  force  increase,  until  they  attain  or  exceed  that 
observed  prior  to  the  stimulation.  The  duration  of  the  inhibitory 
effect  varies  with  the  duration  of  the  stimulation.  Thus  during  and 
after  a  stimulation  of  thirty-eight  seconds  the  heart  of  the  toad  re- 
mained at  rest  for  292  seconds  (Gaskell). 


iMiMwmjmTOMvmTOmMW.wmvw.wwMvwwtfA'iMmmm ■ 

Fig.    129. I'RACING    SHOWIXG     THE     DIMINUTION  IN   THE 

Rate     of    the     Heaet-beat     following    Weak 
Tetanization  of  the  Vagus  Trunk. 


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TEXT-BOOK  OF  PHYSIOLOGY. 


Stimulation  of  the  sympathetic  fibers  prior  to  their  union  with 
the  vagus  is  followed  by  an  increase  in  the  rate  or  an  augmenta- 
tion in  the  force  of  the  heart-beat  or  both  at  the  same  time.  With 
the  cessation  of  the  stimulation  the  heart  returns  to  its  normal  con- 
dition. From  the  foregoing  facts  the  sympathetic  is  said  to  have  an 
accelerator  or  an  augmentor  influence  on  the  heart,  according  as  it 

accelerates  the  rate  or 
augments  the  force  of 
the  beat. 

Stimulation  of  the 
trunk  of  the  vagus  in 
the  frog  or  the  toad 
with  weak  tetanizing 
induced  electric  cur- 
rents is  followed  by 
increase    in     the 


mmummmmijmmiiiiHimmimimmiiiiiit 


Fig.  130. — Tracing  showing  Complete  Inhibition 
FOLLOWING  Strong  Tetanization  of  the 
Vagus  Trunk. 

an     mcrease    m 

rate  of  the  heart-beat  because  of  the  stimulation  of  the  accelerator 
fibers  which  apparently  respond  before  the  inhibitor  fibers;  stimu- 
lation with  somewhat  stronger  currents  is  followed  by  a  diminution 
in  the  rate  of  the  beat  because  of  the  greater  effect  on  the 
inhibitor  nerve-fibers  (Fig.  129).  Stimulation  with  strong  tetaniz- 
ing currents  is  followed  by  complete  inhibition,  and  for  a  short 
time  the  heart  remains  quiescent;  but  not  withstanding  the 
continued  stimulation,  the 
heart  commences  again  to  beat 
(Fig.  130).  Though  at  lirst 
the  beat  is  feeble,  it  soon  re- 
gains and  far  exceeds  its 
former  rate  and  force.  This 
may  be  attributed  to  a  fatigue 
of  the  terminals  of  the  in- 
hibitor fibers  or  to  an  over- 
powering action  of  the  aug- 
mentor fibers  arising  after  a 
rather  long  latent  period. 

The  foregoing  facts  are 
also  illustrated  in  Figs.  131  and 
132,  as  published  by  Gaskell. 

In  these  experiments  the  heart  was  suspended  and  clamped  in  the 
auriculo-ventricular  groove,  thus  permitting  both  auricle  and  ven- 
tricle to  be  attached  to  recording  levers. 

In  addition  to  the  changes  in  the  rate  and  force  of  the  heart 
caused  by  stimulation  of  the  inhibitor  and  the  augmentor  nerves, 
it  is  stated  by  Gaskell  that  there  is  also  during  the  inhibition  a 
decrease  in   the   conductivity  of  the  heart  at    both  the   sinu-auric- 


Fig.  131. — Tracing  showing  Diminished 
Amplitude  and  Slowing  of  the  Pul- 
sations OF  the  Auricle  and  Ventri- 
cle without  Complete  Stoppage 
during  Irritation  of  the  Vagus. — 
(From  Brunton,  after  Gaskell.) 


THE  CIRCULATION  OF  THE  BLOOD. 


307 


Aur. 
Vent. 

m 

C.8. 

'^'''^mmmmmmm 

ular  and  auriculo-ventricular  junctions,  and  an  increase  in  the 
conductivity  during  acceleration  of  the  beat.  The  decrease  in  con- 
ductivity may  be  so  pronounced  that  only  every  second  or  third 
contraction  of  the  auricle  will  be  followed  by  a  contraction  of  the  ven- 
tricle. In  other  instances  both  auricles  and  ventricles  remain  at 
rest  while  the  sinus  maintains  its  usual  rate. 

The  increase  in  conductivity  is  shown  by  first  artificially  block- 
ing the  contraction  wave  at  the  auriculo-ventricular  junction  with  the 
clamp,  until  only  every  second  or  third  auricular  contraction  is 
conducted  to  the  ventricle,  and  then  stimulating  the  sympathetic. 
At  once  the  auricular  contraction  forces  the  block,  and  passes  to 
the  ventricle,  calling  forth  a 
normal  contraction. 

In  the  mammal  the  same 
effects  follow  stimulation  of 
the  vagus  and  the  sympa- 
thetic as  in  the  frog.  If  the 
thorax  of  a  dog  is  opened 
and  artificial  respiration 
maintained,  the  heart  will 
continue  to  beat  in  a  practic- 
ally normal  manner  for  a 
long  time.  Stimulation  of 
the  vagus  with  induced  cur- 
rents of  moderate  strength 
will  be  followed  by  a  com- 
plete standstill  of  the  heart 
in  diastole,  during  which  the 
walls  are  relaxed  and  the 
heart-cavities  filled  with 
blood.  If  the  currents  are 
of  feeble  strength,  the  heart 
will  come  to  rest  gradually 
through  a  gradual  diminu- 
tion in  the  rate  and  force  of  the  contraction. 

Stimulation  of  the  sympathetic  may  be  followed  by  only  a  slight 
increase  in  the  rate,  especially  if  the  heart  action  is  normally 
very  rapid.  There  is,  however,  an  augmentation  of  the  force  and 
an  increase  in  conductivity  (Fig.  133). 

The  Cardio-inhibitor  Center. — In  the  dog,  and  probably  in 
many  other  mammals  also,  the  cardio-inhibitor  center,  in  the 
medulla,  exerts  a  more  or  less  constant  inhibitor  or  restraining  in- 
fluence on  the  heart's  activity.  This  is  indicated  by  the  fact  that  the 
rate  of  the  heart-beat  is  very  much  increased  by  simultaneous  divi- 
sion of  both  vasi.     For  this  and  other  reasons  it  is  believed  that  this 


Fig.  132. — Tracing  showing  the  Actions 
OF  THE  Vagus  on  the  Heart.  Aur., 
Auricular;  Vent.,  ventricular  tracing. 
The  part  between  perpendicular  lines 
indicates  period  of  vagus  stimulation. 
C.8  indicates  that  the  secondary  coil 
was  8  cm.  from  the  primary.  The  part 
of  tracing  to  the  left  shows  the  regular 
contractions  of  moderate  height  before 
stimulation.  During  stimulation,  and 
for  some  time  after,  the  beats  of  auricle 
and  ventricle  are  arrested.  After  they 
commence  again  they  are  single  at  first, 
but  soon  acquire  a  much  greater  ampli- 
tude than  before  the  application  of  the 
stimulus. — {From  Brunton,  ajter  Cas- 
kell.) 


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TEXT-BOOK  OF  PHYSIOLOGY. 


center  is  in  a  state  of  tonic  activity,  discharging  nerve  impulses  which 
exert  a  regulative  influence  on  the  cardiac  mechanism  in  accordance 
with  its  needs,  and  especially  in  reference  to  the  variable  resistances 
offered  to  the  flow  of  blood  which  the  heart  must  overcome.  The 
question  has  been  raised  as  to  whether  the  tonic  activity  of  the  center 
is  maintained  by  causes  within  itself,  the  result  of  an  interaction  of 
cell  substance  and  the  surrounding  lymph,  or  by  nerve  impulses 
reflected  to  it  through  aft'erent  or  sensor  nerves.  The  latter  suppo- 
sition is  supported  by  the  results  of  experimentation,  though  both 
factors  are  undoubtedly  of  importance. 

Stimulation  of  the  sensor  nerves  in  almost  any  region  of  the  body 
is  followed  by  a  slowing  of  the  heart's  action.  Thus  stimulation  of 
the  posterior  roots  of  the  spinal  nerves,  the  trunks  of  the  cranial 
sensor  nerves,  the  splanchnic  nerves,  the  pulmonary  branches  of  the 
vagus,  etc.,  give  rise  to  a  more  or  less  pronounced  inhibition.     As  a 


i:rc.J"JV:Acc.y 


Fig.  133. — Increase  in  the  Force  of  the  Ventricular  Contraction  (Curve  of 
Pressure  in  Right  Ventricle)  from  Stimulation  of  the  Sympathetic 
Fibers.     There  is  Little  or  no  Ch.^nge  in  Frequency. — {Franck.) 


rule,  stimulation  of  the  peripheral  terminations  of  these  nerves  is 
more  effective  than  stimulation  of  their  trunks,  hence  an  explanation 
is  at  hand  for  the  cardiac  inhibition  which  results  from  sudden  dis- 
tention of  the  stomach,  intestines,  or  lungs,  or  operative  procedures  in 
the  nose,  mouth,  and  larynx. 

The  Cardio-accelerator  Center. — The  exact  location  of  this 
center  is  not  as  yet  determined.  It  is  probably  in  the  medulla  ob- 
longata. The  experiments  of  Hunt  would  indicate  that  this  center  is 
also  in  a  condition  of  tonic  activity,  increasing  both  the  rate  and  force 
of  the  heart's  action;  that  it  antagonizes  the  action  of  the  inhibitory 
center  and  is  in  time  antagonized  by  it;  that  the  normal  rate  of  the 
heart  from  moment  to  moment  is  the  resultant  of  the  action  of  these 
two  opposing  forces.  These  experiments  also  support  the  view  that 
the  acceleration  of  the  heart  observed  during  stimulation  of  certain 
afferent  nerves  is  not  due  to  a  reflex  stimulation  of  the  accelerator 
center,  but  to  an  inhibition  of  the  cardio-inhibitor  center.     It  must 


THE  CIRCULATION  OF  THE  BLOOD.  309 

therefore  be  assumed  that  the  afferent  nerves  contain  two  sets  of 
nerve-fibers,  one  of  which  inhibits,  the  other  excites,  the  cardio- 
inhibitor  center. 

The  cardio-inhibitor  and  the  cardio-accelerator  centers  may  be 
increased  in  activity  also  by  nerve  impulses  descending  from  the 
cerebrum,  the  result  of  emotional  states;  thus  depressing  emotions 
according  to  their  intensity  may  so  increase  the  activity  of  the  cardio- 
inhibitor  center,  that  the  heart's  action  may  not  only  be  retarded  but 
even  completely  inhibited;  joyous  emotions,  as  the  contrary  may  so 
increase  the  activity  of  the  cardio-accelerator  center  that  the  heart's 
action  will  be  increased  in  both  its  rate  and  force. 

The  Depressor  Nerve. — The  vagus  trunk  also  contains  afferent 
fibers  stimulation  of  which  not  only  brings  about  a  reflex  inhibition 
of  the  heart,  but  also  a  dilatation  of  the  peripheral  arteries  and  a  fall 
of  blood-pressure  through  a  depressive  influence  on  the  vaso-motor 
centers.  To  this  nerve  the  term  depressor  has  been  given.  A  con- 
sideration of  the  physiologic  action  of  this  nerve  will  be  found  in  the 
section  devoted  to  the  nerve  mechanisms  concerned  in  the  mainte- 
nance of  the  blood-pressure. 


THE   VASCULAR  APPARATUS:   ITS  STRUCTURE  AND  FUNCTIONS. 

The  systemic  vascular  apparatus  consists  of  a  closed  system  of 
vessels  extending  from  the  left  ventricle  to  the  right  auricle,  and 
includes  the  arteries,  capillaries,  and  veins.  Though  serving  as  a 
whole  to  transmit  blood  from  the  one  side  of  the  heart  to  the  other, 
each  one  of  these  three  divisions  has  separate  but  related  functions, 
which  are  dependent  partly  on  differences  in  structure  and  physio- 
logic properties,  and  partly  on  their  relation  to  the  heart  and  its 
physiologic  activities. 

The  Structure,  Properties  and  Functions  of  the  Arteries. — 
The  arteries  serve  to  transmit  the  blood  ejected  from  the  heart  to 
the  capillaries;  that  this  may  be  accompHshed  they  divide  and  sub- 
divide and  ultimately  penetrate  each  and  every  area  of  the  body. 
Their  repeated  division  is  attended  by  a  diminution  in  size,  a  de- 
crease in  the  thickness  and  a  change  in  the  structure  of  their  walls. 

A  typical  artery  consists  of  three  coats:  an  internal,  the  tunica 
intima;  a  middle,  the  tunica  media;  an  external,  the  tunica  adven- 
titia. 

The  internal  coat  consists  of  a  structureless  elastic  basement 
membrane,  on  the  inner  surface  of  which  rests  a  layer  of  elongated 
spindle-shaped  endothehal  cefls.  The  middle  coat  consists  of 
several  layers  of  circularly  disposed,  non-striated  muscle  fibers, 
between  which  are  networks  of  elastic  fibers.  The  external  coat 
consists  of  bundles  of  connective    tissue   of    the   white  fibrous  and 


3IO 


TEXT-BOOK  OF  PHYSIOLOGY. 


yellow  elastic  varieties.  Between  the  external  and  middle  coats 
there  is  an  additional  elastic  membrane.  In  the  small  arteries 
there  is  but  a  single  layer  of  muscle-fibers.  In  the  large  arteries 
the  elastic  tissue  is  very  abundant,  exceeding  largely  in  amount  the 
muscle-tissue.  It  is  also  more  closely  and  compactly  arranged. 
The  external  coat  is  well  developed  in  the  large  arteries  (Figs. 
135  and  136). 

In  virtue  of  the  presence  in  their  walls  of  both  elastic  and  con- 
tractile elements,  the  arteries  possess  the  two  properties  of  elasticity 

and  contractility. 

The  elasticity  is  especially  well 
developed  in  the  large  arteries,  which 
are  capable,  therefore,  of  both  disten- 
tion and  elongation,  and,  when  the 
distending  force  is  withdrawn,  of  re- 
turning to  their  previous  condition. 
The  elasticity  permits  of  a  wide  vari- 
ation in  the  amount  of  blood  the 
arterial  system  can  hold  between 
its  minimum  and  maximum  disten- 
tion. Thus  the  capacity  of  the  aorta 
and  carotid  artery  of  the  rabbit  can 
be  increased  four  times  and  six  times 
respectively  by  raising  the  intra- 
arterial pressure  from  o  to  200  mm. 
of  mercur}\  The  elasticity  also  con- 
verts the  intermittent  movement  of 
the  blood  imparted  to  it  by  the  heart 
as  it  is  ejected  from  the  ventricle,  into 
a  remittent  movement  in  the  arteries 
and  finally  into  the  continuous  and 
equable  movement  observed  in  the 
capillaries.  This  is  accomplished  in 
the  following  manner:  With  each 
contraction  of  the  left  ventricle  more 
blood  is  ejected  into  the  aorta  than 
the  arteries  can  discharge  into  the  capillaries  and  veins  during  the 
time  of  the  contraction.  The  portion  not  so  discharged  exerts  a 
lateral  pressure  against  the  walls  of  the  arteries  which  at  once  dilate 
until  a  condition  of  equilibrium  is  established  between  the  pressure 
from  within  and  the  elastic  reaction  of  the  arterial  walls  from 
without.  With  the  cessation  of  the  contraction  the  elastic  walls 
recoil  and  propel  the  blood  toward  the  capillaries.  The  intermittent 
action  of  the  heart  is  thus  succeeded  by  the  continuously  reacting 
arterial  wall. 


Fig.  134. — Coats  or  a  Small 
Artery,  a.  Endothelium,  b. 
Internal  elastic  lamina,  c.  Cir- 
cular muscular  fibers  of  the 
middle  coat.  d.  The  outer 
coat. — {Landois  and  Stirling.) 


THE  CIRCULATION  OF  THE  BLOOD. 


311 


As  the  blood  advances  toward  the  periphery  of  the  arterial  system 
and  larger  amounts  pass  into  the  capillaries  both  the  distension  and 
the  elastic  recoil  diminish,  and  by  the  time  the  blood  reaches  the 
capillaries  its  intermittency  of  movement  has  been  so  far  obhter- 
ated  by  the  elastic  recoil  that  as  it  enters  the  capillaries  the  move- 
ment becomes  equable  and  continuous.  The  elasticity  thus  serves 
the  purpose  of  equalizing  the  movement  of  the  blood  throughout 
the  arterial  system. 

In  youth  the  arterial  walls  are  highly  distensible  and  elastic; 
in  advanced  years  they  are  frequently  relatively  rigid  and  inelastic; 
and  in  consequence  the  flow  of  blood  toward  and  into  the  capillaries 
approximates  in  its  characteristics  the  flow  of  a  fluid  through  a  rigid 
tube  under  the  intermittent  action  of  a  pump;  that  is,  the  intermit- 
tent movement  im- 
parted by  the  heart  is 
not  so  completely  con- 
verted into  a  continu-  '  . 
ous    movement,    and  i: 

hence  the  blood  flows 
through  the  capillaries      ^ 
durjing  the  systole       ^ 
with  greater  velocity, 
and  during  the  dias-      Fig. 
tole   with   less    veloc- 
ity, than   is    the  case 
when    the    vessel    is 
normally  elastic.    For 
these   and  other  rea- 
sons the    tissues    are 
not  so  well  nourished 
and  hence  their  nutrition  and  functional  activities  decline. 

The  contractility  permits  of  a  variation  in  the  amount  of  blood 
passing  into  a  given  capillary  area  in  a  unit  of  time.  Normally  each 
artery  has  a  certain  average  caliber  due  to  a  given  contraction  of  the 
muscle  coat.  Beyond  this  average  condition  the  artery  can  pass 
in  one  direction  or  the  other  by  either  a  relaxation  or  increased 
contraction  of  the  muscle  coat.  During  the  functional  activity  of 
any  organ  or  tissue  there  is  need  for  an  increase  in  the  amount  of 
blood  beyond  that  supplied  during  inactivity  or  rest.  This  is  ac- 
complished by  a  relaxation  of  the  muscle-fibers.  With  the  cessation 
of  activity  the  muscle-fibers  again  contract  and  reduce  the  amount  of 
blood  to  that  required  for  nutritive  purposes  only.  An  increased 
contraction  of  the  muscle-fibers  beyond  the  average  diminishes  the 
outflow  of  blood,  and  if  sufficiently  great  may  give  rise  to  anemia  and 
pallor.     The  contractile  elements  at  the   periphery  of   the  arterial 


135. — Transverse  Section  of  Part  of  the 
Wall  of  the  Posterior  Tibial  Artery  (Man). 
— (Schafer.)  a.  Endothelium  lining  the  vessel, 
appearing  thicker  than  natural  from  the  contrac- 
tion of  the  outer  coats,  b.  The  elastic  layer  of 
the  intima.  c.  Middle  coat  composed  of  muscle- 
fibers  and  elastic  tissue,  d.  Outer  coat  consisting 
chiefly  of  white  fibrous  tissue. — {Front  Yeo's  "Phys- 
iology. ") 


312 


TEXT-BOOK  OF  PHYSIOLOGY. 


system,  in  the  so-called  arteriole  region,  therefore  regulate  the  supply 
of  blood  to  the  tissues  in  accordance  with  their  functional  needs. 

The  Structure,  Properties  and  Functions  of  the  Capil- 
laries.— The  capillaries  are  small  vessels  that  connect  the  arteries 
with  the  veins.  Though  different  in  structure  from  a  small  artery 
or  vein,  there  is  no  sharp  boundary  between  them,  as  their 
structures  pass  imperceptibly  one  into  the  other.  A  true  capillary, 
however,  is  of  uniform  size  in  any  given  tissue  and  does  not  undergo 
any  noticeable  decrease  in  size  from  repeated  branchings.  The 
diameter  varies  in  different  tissues  from  0.0045  mm.  to  0.0075  mm., 
just  sufficiently  large  to  permit   the  easy  passage  of   a  single  red 

corpuscle.  The  length 
varies  from  0.5  mm.  to 
I  mm.  The  wall  of  the 
capillary  (Fig.  136)  is 
composed  of  a  single 
layer  of  nucleated  endo- 
thelial cells  v/ith  serrated 
edges  united  by  a  cement 
material.  Though 
extremely  short,  the 
capillaries  divide  and 
subdivide  a  number  of 
times,  forming  meshes 
or  networks,  the  close- 
ness and  general  ar- 
rangement of  which  vary 
in  different  localities. 

As  the  endothelial 
cells  are  living  structures 
and  characterized  by 
irritabihty,  contractility 
and  tonicity,  it  may  be 
assumed  that  the  capillary  w^all  as  a  whole  is  characterized  by  the 
same  properties.  Upon  the  possession  of  these  properties,  the  func- 
tions of  the  capillary  depend. 

The  junction  of  the  capillar}^  wall  is  to  permit  of  a  passage  of  the 
nutritive  materials  of  the  blood  into  the  surrounding  tissue  spaces 
and  of  waste  products  from  the  tissue  spaces  into  the  blood.  The 
structure  of  the  capillary  wall  is  well  adapted  for  this  purpose.  Com- 
posed as  it  is  of  but  a  single  layer  of  endothelial  cells,  the  diameter 
of  which  defies  accurate  measurement,  it  readily  permits,  under  cer- 
tain conditions,  of  the  necessary  exchange  of  materials  between  the 
blood  and  the  tissues.  The  forces  which  are  concerned  in  the  pas- 
sage of  materials  across  the  capillary  wall  are  embraced  under  the 


Fig.  136. — Capillaries.  The  Outlines  of  the 
Nucleated  Endothelial  Cells  with  the 
Cement  Blackened  by  the  Action  of  Sil- 
ver Nitrate. — {Landois  and  Stirling.) 


THE  CIRCULATION  OF  THE  BLOOD.  313 

terms  diffusion,  osmosis,  and  filtration.  As  a  result  of  the  interchange 
of  materials  the  tissues  are  provided  with  nourishment  and  relieved 
o  the  presence  of  waste  products.  The  blood  at  the  same  time 
changes  in  composition;  because  of  the  loss  of  oxygen  and  the  gain 
of  carbon  dioxid  it  changes  in  color  from  red  to  bluish  red. 

The  Structure,  Properties  and  Functions  of  the  Veins. — The 
veins  serve  to  collect  the  blood  from  the  capillary  areas  and  return 
it  to  the  right  side  of  the  heart.  As  they  emerge  from  the  capillary 
areas  the  veins,  which  in  these  regions  are  termed  venules,  are  quite 
small.  By  their  convergence  and  union  the  veins  gradually  increase 
in  size  in  passing  from  the  periphery  toward  the  heart.  Their 
walls  at  the  same  time  correspondingly  increase  in  thickness. 
The  veins  from  the  lower  extremities,  the  trunk,  and  abdominal 
organs  finally  terminate  in  the  inferior  vena  cava.  The  veins  from 
the  head  and  upper  extremities  terminate  in  the  superior  vena  cava. 
Both  venae  cavas  empty  into  the  right  auricle. 

A  typical  vein  consists  of  the  same  three  coats  as  ''^ 

the  artery:  viz.,  the  tunica  intima,  the  tunica  media, 
and  the  tunica  adventitia.  The  media,  however, 
does  not  possess  as  much  of  either  the  elastic  or 
muscle  tissues  as  the  artery,  but  a  larger  amount  of 
the  fibrous  tissue.  Hence  they  readily  collapse 
when  empty.  In  virtue  of  their  structure  the  veins 
also  possess  both  elasticity  and  contractility,  though 
in  a  far  less  degree  than  the  arteries.  These  Fig.  137.— Valves 
properties  come  into  play  and  are  of  value  in  °^  ^  \-£.i^. 
furthering  the  movement  of  the  blood  toward  the  valve,  i.  Free 
heart,  especially  after  a  temporary  obstruction.  edge    of    the 

Veins  are  distinguished  by  the  presence  of  valves  l^/soit)  '^' 
throughout  their  course.  These  are  arranged  in 
pairs  and  formed  by  a  reduplication  of  the  internal  coat,  strength- 
ened by  fibrous  tissue.  They  are  always  directed  toward  the  heart 
and  in  close  relation  to  the  walls  of  the  veins,  so  long  as  the  blood 
is  flowing  forward  (Fig.  137).  An  obstruction  to  the  flow  causes  the 
valves  to  turn  backward  until  they  meet  in  the  middle  line,  when  they 
act  as  a  barrier  to  regurgitation.  Under  these  circumstances  the 
elastic  tissue  permits  the  veins  to  distend  and  accommodate  the 
blood.  With  the  removal  of  the  obstruction  the  recoil  of  the  elas- 
tic tissue,  and  perhaps  the  contraction  of  the  muscle-tissue,  forces 
the  blood  quickly  onward. 

The  Stream-bed. — The  stream-bed,  the  path  along  which  the 
blood  flows,  varies  widely  in  its  total  sectional  area  in  different  parts 
of  its  course,  being  greatest  in  the  capillaries,  least  in  the  aorta  and 
venae  cavae.  In  passing  from  the  base  of  the  aorta  toward  the  capil- 
laries the  sectional  area  of  individual  arteries,  in  consequence  of 


314 


TEXT-BOOK  OF  PHYSIOLOGY. 


repeated  branching,  diminishes,  though  their  total  sectional  area 
increases  and  in  direct  proportion  to  their  distance  from  the  heart. 
In  the  capillary  system  the  sectional  area  of  an  individual  capillary  at- 
tains its  minimal  value,  though  the  total  sectional  area  attains  its  maxi- 
mal value.  Comparing  one  with  the  other,  it  has  been  estimated  that 
the  total  sectional  area  of  the  aortic  bed  is  to  the  total  sectional  area 
of  the  capillary  bed  as  i  is  to  600  or  800.  In  passing  from  the  capil- 
lary into  the  venous  system  the  sectional  area  of  individual  veins  in- 
creases, though  the  total  sectional  area  decreases  and  in  direct  pro- 
portion to  their  distance  from  the  capillaries. 

The  stream-bed  in  the   aorta  is  relatively  narrow,  but  widens 

Capillaries. 


Fig.  138. — Scheme  of  the  Circulatory  Apparatus. 

gradually  as  it  approaches  the  capillaries,  where  it  attains  its  maxi- 
mum width;  it  again  narrows  gradually  as  it  passes  into  the  veins, 
until  in  the  venae  cavae  it  becomes  almost  as  narrow  as  in  the  aorta.  As 
the  combined  sectional  areas  of  the  venae  cavae  are  greater  than  the  sec- 
tional area  of  the  aorta,  the  stream-bed  of  the  former  never  becomes 
as  narrow  as  that  of  the  latter.  These  facts  will  become  apparent 
if  the  vascular  apparatus  is  conceived  of  as  a  system  of  symmetri- 
cally branching  vessels  and  all  vessels  of  the  same  diameter  sym- 
metrically disposed  one  to  the  other  (Fig.  138).  The  gradual  increase 
in  the  width  of  the  stream-bed  which  results  from  this  repeated 
branching,  as  well  as  its  relative  width  in  the  arteries,  capillaries, 
and  veins,  is  graphically  shown  in  Fig.  139. 


BLOOD  PRESSURE. 

The  immediate  cause  of  the  movement  of  the  blood  from  the 
beginning  of  the  aorta  through  the  arteries,  the  capillaries,  and  the 
veins  to  the  right  side  of  the  heart,  is  a  difference  of  pressure  between 
these  two  points.  The  fact  that  the  blood  flows  from  the  aorta  to 
the  venae  cavae  indicates  that  there  is  a  higher  pressure  in  the  former 
than  in  the  latter.  The  same  holds  true  for  the  pulmonary  artery 
and  veins.  So  long  as  this  is  the  case,  the  blood  must  flow  from  the 
point  of  high  to  the  point  of  low  pressure. 


THE  CIRCULATION  OF  THE  BLOOD. 


315 


To  this  pressure  the  term  blood-pressure  is  given,  and  may  be 
defined  as  the  pressure  exerted  radially  or  laterally  by  the  mo\dng 
blood-stream  against  the  sides  of  the  vessels.  That  there  is  such  a 
pressure  within  the  arteries,  capillaries,  and  veins,  different  in 
amount  in  each  of  these  three  divisions  of  the  vascular  apparatus, 
is  evident  from  the  results  which  follow  division  of  an  artery 
or  a  vein  of  corresponding  size.  When  an  artery  is  divided,  the 
blood  spurts  from  the  opening  for  a  considerable  distance  and  with 
a  certain  velocity.  When  a  vein  is  divided,  the  blood  as  a  rule  merely 
wells  out  of  the  opening  with  but  shght  momentum.  These  results 
indicate  that  the  blood  in  the  arteries  stands  under  a  pressure  con- 


FiG.  139. — Diagram  Intended  to  Give  ax  Idea  of  the  Aggregate  Sectional 
Area  of  the  Different  Parts  of  the  A'ascttl-ar  System.  A.  Aorta.  C. 
Capillaries.  \'.  \'eins.  The  transverse  measurement  of  the  shaded  part  may  be 
taken  as  the  width  of  the  various  kinds  of  vessels,  supposing  them  fused  together. 

-{Yeo.) 

siderably  higher  than  that  of  the  atmosphere,  and  that  in  the  veins 
it  stands  under  a  pressure  perhaps  but  shghtly  above  that  of  the 
atmosphere.     Especially  true  is  this  of  the  larger  veins. 

The  same  facts  may  be  demonstrated  in  another  and  more  striking 
way.  A  dog  or  cat  is  anesthetized  and  securely  fastened  in  an  appro- 
priate holder.  The  carotid  arten,-  on  the  right  side  and  the  jugular 
vein  on  the  left  side  are  freely  exposed  and  clamped.  Into  the  artery 
there  is  inserted  on  the  distal  side  of  the  clamp  and  in  the  direction  of 
the  heart  a  cannula  to  which  is  connected  a  tall  glass  tube,  200  cm. 
high  and  of  about  4  mm.  internal  diameter.  Into  the  vein  there  is 
passed  on  the  proximal  side  of  the  clamp  and  in  the  direction  of  the 


3i6  TEXT-BOOK  OF  PHYSIOLOGY. 

capillaries  a  second  cannula,  to  which  is  connected  a  similar  tube, 
though  of  less  height.  If  the  two  clamps  are  removed  at  the  same 
time,  the  blood  will  mount  in  both  tubes  simultaneously.  In  the  arterial 
tube  the  blood  will  ascend  by  leaps  corresponding  to  the  heart-beats 
until  a  certain  height  is  reached,  when  the  column  becomes  stationary, 
being  kept  in  equilibrium  by  the  blood-pressure  within  the  vessel 
and  the  atmospheric  pressure  without.  Though  stationary  in  a 
general  sense,  the  blood-column  oscillates,  rising  and  falHng  with 
each  contraction  and  relaxation  of  the  heart.  Not  infrequently  larger 
excursions  of  the  column  are  seen  which  correspond  in  a  general  way 
to  the  respiratory  movement.  This  experiment  was  originally  per- 
formed on  the  horse,  by  the  Rev.  Stephen  Hales  (1732). 

In  the  venous  tube  the  blood  also  rises  to  a  certain  height,  after 
which  it  remains  quite  stationary,  as  the  effect  of  the  cardiac  con- 
traction is  not  propagated  under  normal  conditions  beyond  the 
arterial  system.  The  height  to  which  it  rises  is  but  slight  as  com- 
pared with  that  in  the  arterial  tube.  The  pressure  in  both  vessels  is 
thus  recorded  in  milhmeters  of  blood.  The  absolute  pressure  on 
any  given  unit  of  vessel  surface — e.  g.,  1  square  mm. — is  obtained 
by  multiplying  the  height  of  the  column,  expressed  in  millimeters, 
by  the  unit  of  surface,  and  then  determining  the  weight  of  this  mass 
of  blood.  Thus  if  the  height  of  the  column  of  blood  in  the  carotid 
artery  tube  is  2000  mm.,  then  the  pressure  on  i  sq.  mm.  is  2000  mm. 
of  blood.     The  weight  of  2000  c.mm.  of  blood  is  equal  to  2.1  grams. 

The  Arterial  Pressure. — For  accurate  and  long-continued  ob- 
servation the  arterial  blood-pressure  is  more  conveniently  studied 
by  means  of  a  U-shaped  tube  (a  manometer)  partially  filled  with 
mercui"y.  One  limb  of  the  manometer  is  connected  by  means  of  a 
tube  and  a  cannula  with  an  artery.  For  the  purpose  of  retarding 
coagulation  of  the  blood  and  for  preventing  the  escape  of  a  large 
volume  of  blood  from  the  vessels,  the  system  is  filled  with  a  solution 
of  carbonate  of  soda  of  sp.  gr.  1060  and  under  a  pressure  approxi- 
mately equal  to  that  in  the  vessel  of  the  animal  as  determined  in 
previous  experiments.  When  communication  is  established  between 
the  vessel  and  the  cannula,  the  mercurial  column  adjusts  itself  to  the 
pressure  in  the  artery.  It  at  once  begins  to  exhibit  the  same  cardiac 
and  respiratory  oscillations  and  undulations  as  did  the  column  of 
blood  in  the  previous  experiment. 

The  height  of  the  mercurial  column  kept  in  equiHbrium  by  the 
pressure  of  the  blood  within  and  the  pressure  of  air  without  the  vessel 
is  that  between  the  lower  level  of  the  mercury  in  the  proximal  and 
the  higher  level  in  the  distal  limb  of  the  manometer,  both  of  which 
can  be  read  off  on  a  scale  placed  between  the  two  limbs. 

The  height  of  the  mercury  as  well  as  its  oscillations  in  the  distal 
limb  may  be  recorded  by  placing  on  the  top  of  the  mercury  a  light 


THE  CIRCULATION  OF  THE  BLOOD. 


317 


float,  the  upper  end  of  which  carries  a  writing  point.  When  the 
latter  is  placed  in  contact  with  the  moving  blackened  surface  of  a 
recording  cylinder  or  kymograph,  the  height  and  the  oscillations  are 
recorded  in  the  form  of  a  tracing  similar  to  that  shown  in  Figs.  140 
and  141,  in  which  the  larger  curves  represent  the  respiratory,  the 
smaller  curves  the  cardiac  oscillations.     The  height  of  the  mercurial 


B  P  TRACING 


ABSCISSA 


Fig.  140. — Diagram  to  show  the  Rel.a.tiox  of  the  Mercurial  Manometer  to 
THE  Artery,  on  One  H.and,  and  to  the  Recording  Cylinder,  on  the  Other 
Hand,  when  Arr.anged  for  Recording  Blood-pressure. 

column  kept  in  equihbrium  at  any  particular  moment  is  determined 
by  measuring  the  distance  between  a  base-hne  or  abscissa,  which 
represents  the  position  of  the  mercury  at  atmospheric  pressure,  and 
any  given  point  on  the  trace  above,  and  multiplying  it  by  2,  for  the 
reason  that  the  mercury  sinks  in  the  proximal  hmb  as  high  at  it  rises 
in  the  distal  limb  of  the  manometer. 


Fig.  141. — Blood-pressure  Tr.acing. 


The  blood-pressure  as  revealed  by  the  tracing  may  be  resolved 
into  two  components:  viz.,  (i)  the  pressure  in  the  arteries  during  the 
period  of  the  cardiac  diastole,  which  is  termed  the  arterial  pres- 
sure; and  (2)  that  additional  pressure  occurring  at  the  time  of  the 


3i8  TEXT-BOOK  OF  PHYSIOLOGY. 

cardiac  systole,  which  is  termed  the  cardiac  pressure.  The  arte- 
rial pressure  is  represented  by  the  distance  between  the  base-line 
and  the  points  of  the  curve  corresponding  to  the  diastolic  rest;  the 
cardiac  pressure,  by  the  increase  of  distance  between  these  points 
and  the  apices  of  the  smaller  oscillations.  The  relation  of  these  two 
components  varies  in  different  animals  and  in  the  same  animal  at 
different  times.  If  the  arterial  pressure  is  low,  the  cardiac  increase 
may  be  considerable;  if  the  former  is  high,  the  latter  may  be  slight. 
The  relation,  however,  of  these  components  is  not  so  accurately  shown 
by  the  mercurial  manometer,  owing  to  the  inertia  of  the  mercury,  as 
by  one  of  the  various  forms  of  quickly  responsive  spring  manometers 
used  in  determining  the  rapid  variations  of  intra-cardiac  pressure. 
These  instruments  show  a  much  larger  rise  of  pressure  during  the 
systole,  often  amounting  to  as  much  as  one-third  or  one-fourth  of  the 
arterial  pressure. 

In-  a  series  of  experiments  it  will  be  found  that  the  arterial 
pressure,  though  rising  and  falhng  a  certain  number  of  milhmeters, 
yet  retains  a  fairly  constant  general  average,  the  result  of  an  adjust- 
ment between  the  number  of  heart-beats  per  minute  and  the  amount 
of  the  resistance  offered  to  the  escape  of  blood  into  the  capillaries  and 
veins.  In  a  tracing  in  which  the  respiratory  undulations  are  absent, 
the  arterial  pressure,  plus  one-half  of  the  cardiac  increase,  represents 
the  mean  arterial  pressure.  If  the  respiratory  undulations  are  pres- 
ent, as  is  generally  the  case,  the  mean  pressure  may  be  represented 
by  a  line  drawn  horizontally  across  the  tracing  midway  between  the 
apex  and  trough  of  the  undulation. 

Estimates  of  the  Arterial-Pressure. — By  means  of  the  kymo- 
graphic  methods  previously  mentioned  the  pressure  in  the  larger 
arteries  has  been  determined  for  all  classes  of  animals.  In  the  carotid 
artery  of  the  dog  it  ranges  from  140  to  160  mm.;  in  the  horse,  from 
160  to  170  mm.;  in  the  rabbit,  from  90  to  100  mm.  In  two  obser- 
vations made  on  human  beings  during  amputation  of  the  limb  the 
pressure  was  found  in  the  brachial  artery  of  one  patient  to  range  from 
no  to  120  mm.,  and  in  the  anterior  tibial  of  the  other  patient  from 
no  to  160  mm. 

The  Estimation  of  the  Arterial-Pressure  in  Man. — The  fore- 
going method  of  obtaining  the  blood-pressure  is  not  of  general 
apphcation  to  human  beings  for  obvious  reasons,  hence  special 
instruments  have  been  devised  by  means  of  which  the  pressure  may 
be  determined  at  least  approximately  without  any  surgical  procedure. 
Such  instruments  are  termed  sphygmomanometers.  One  of  the  best 
is  that  of  Mosso,  represented  in  Fig.  142.  It  consists  essentially  of 
rubber  capsules,  contained  within  metallic  tubes  and  into  which  two 
fingers  of  each  hand  are  inserted.  This  system  is  connected,  on  the 
one  hand,  with  a  pressure  apparatus,  and,  on  the  other,  with  a 


THE  CIRCULATION  OF  THE  BLOOD. 


319 


manometer  provided  with  a  scale.  A  float  and  writing-pen  record  the 
movements  of  the  mercurial  column  on  a  moving  blackened  surface. 
In  using  this  apparatus  the  pressure  is  raised  to  the  point  at  which 
the  mercurial  column  exhibits  the  greatest  oscillations. 

This  sphygmomanometer,  as  well  as  the  interpretation  of  the 
results  obtained  with  it,  are  based  on  the  theory  that  the  greatest 
oscillations  of  the  arterial  walls,  and  hence  the  greatest  oscillations 
of  the  mercurial  column,  take  place  when  the  external  pressure  is 
just  equal  to  the  mean  arterial  pressure,  the  latter  being  the  mean 
between  the  maximum  pressure  during  the  systole  and  the  minimum 
pressure  during  the  diastole  of  the  heart.  It  is  only  necessary,  there- 
fore, to  take  the  mean  of  the  readings  corresponding  to  the  excursions 


Fig.  142. — The  Sphygmomanometer  of  Mosso. 

of  the  mercurial  column  and  determine  from  them  the  mean  arterial 
pressure. 

It  has  been  experimentally  demonstrated,  however,  by  Howell 
and  Brush  that  this  interpretation  is  not  correct,  but  that  the  greatest 
oscillation  takes  place  when  the  external  pressure  justs  equals  the 
pressure  in  the  artery  at  the  end  of  the  cardiac  diastole.  These  ex- 
perimenters found  when  the  carotid  artery  of  one  side  w^as  connected 
with  a  minimum  valve  and  the  carotid  artery  of  the  opposite  side 
was  surrounded  by  a  plethysmograph  in  connection  with  a  manometer, 
that  the  diastolic  pressure  indicated  by  the  valve  just  equaled  the 
lowest  point  of  the  greatest  oscillation  indicated  by  the  manometer. 
Hence  it  is  to  be  inferred  that  the  greatest  oscillations  record  the 
diastolic  pressure. 


320 


TEXT-BOOK  OF  PHYSIOLOGY. 


An  excellent  sphygmomanometer,  especially  adapted  for  clinical 
use,  is  that  devised  by  Stanton  (Fig.  143  *). 

In  this  apparatus  the  systohc  pressure  is  determined  by  noting 
the  point  at  which  the  pulse  reappears  after  obhteration,  while  the 
diastohc  pressure  is  estimated  by  recording  the  point  at  which  the 
greatest  oscillations  occur  in  the  mercury  column  of  the  manometer. 
The  pressure  is  applied  to  the  arm  by  the  rubber  armlet  h,  which  is 
2,1  inches  wide.  This  is  the  widest  armlet  that  can  be  adjusted  to 
the  average-sized  arm  and    presents  distinct  advantages    over  the 


Fig.  143. — Stanton's  Sphygmomanometer. 


narrow  armlet  hitherto  employed.  This  armlet  is  prevented  from 
expanding  outward  by  a  cuff,  f,  of  double  thick  canvas  with  inserted 
strips  of  tin,  which  is  held  in  place  by  two  straps  which  completely 
encircle  the  cuff.  On  the  rigidity  of  this  depends  to  a  large  extent 
the  transmission  of  pulsation.  The  rubber  armlet  is  connected  by 
glass  with  a  stiff-walled  rubber  tube,  g,  which  in  turn  connects  with 
the  manometer.  The  manometer  is  perhaps  the  most  important  part 
of  the  apparatus.     It  is  constructed  entirely  of  metal  except  for  the 

*  The  following  description  of  this  apparatus  is  abstracted  from  the  Univ.  of 
Pa.  Medical  Bulletin,  Feb.,  1903. 


THE  CIRCULATION  OF  THE  BLOOD.  321 

glass  tube  containing  the  mercury  column.  The  chamber  c  com- 
municates by  means  of  a  metal  tube  with  the  glass  column  d,  which 
is  connected  by  a  screw-thread  at  3,  the  caliber  of  c  being  approx- 
imately 100  times  that  of  D.  The  cap  of  the  chamber,  which  screws 
on,  is  provided  with  a  metal  T  which  is  connected  at  2  with  the 
rubber  armlet  and  at  i  with  the  bulb,  used  as  an  air-pump.  At  A 
is  a  stop-cock  shutting  the  rubber  bulb  completely  from  the  rest 
of  the  apparatus,  while  at  b  is  a  screw-valve  which  allows  the  air 
to  escape  from  the  closed  system.  When  desired,  the  manometer 
can  be  made  portable  (without  removing  the  mercury)  by  screwing 
the  caps  i  and  2  into  either  end  of  the  T  at  i  and  2.  The  manometer 
is  then  tilted  away  from  the  glass  column  d  until  all  the  mercury 
has  run  into  the  chamber,  the  glass  is  then  unscrewed  and  cap  3 
screwed  in.  Before  removing  cap  3  the  manometer  must  always  be 
tilted,  else  the  mercury  will  be  lost. 

The  rubber  bulb  is  similar  to  those  found  on  atomizers. 

In  using  this  apparatus  the  pressure  is  raised  by  the  air-bulb 
forcing  air  into  the  closed  system — distending  the  rubber  armlet 
and  with  the  same  degree  of  force  displacing  the  mercury  in  c, 
driving  it  up  the  glass  column  d.  When  the  pulse  is  no  longer  felt, 
the  bulb  still  being  compressed,  the  arm  of  valve  A  is  turned 
until  it  is  at  right  angles  with  the  thumb  and  finger.  The  valve  b 
is  now  slowly  unscrewed  until  the  mercury  column  begins  to  fall. 
With  the  eye  on  the  scale  the  point  at  which  the  pulse  reappears  is 
mentally  noted  as  the  systohc  pressure.  Often  considerably  before 
the  reappearance  of  the  pulse  to  palpation,  a  pulsation  is  seen  in 
the  mercury  column.  As  the  column  slowly  falls  this  increases  up 
to  its  greatest  oscillation  and  then  diminishes.  The  lowest  point 
of  the  greatest  pulsation  is  noted  as  the  diastolic  pressure. 

From  experimental  data  and  from  theoretic  reasoning  it  is  certain 
that  the  pressure  in  the  carotid  and  femoral  arteries  is  less  than  in 
the  aorta  and  greater  than  in  the  small  peripheral  arteries.  In  other 
words,  there  is  a  fall  of  pressure  from  the  beginning  of  the  aorta  to 
the  arteriole  region.  The  fall  in  pressure,  however,  is  not  great  in 
the  larger  vessels  of  the  arterial  system.  It  is  only  in  the  smallest 
arteries,  before  their  passage  into  the  capillaries,  that  an  abrupt  fall 
in  pressure  takes  place. 

The  Capillary  Pressure. — The  small  size  of  the  capillaries  pre- 
cludes an  investigation  of  their  pressure  by  manometric  methods. 
It  may  be  stated,  however,  to  be  approximately  equal  to  the  pressure 
required  to  obliterate  their  lumen  and  to  whiten  the  skin.  The 
apparatus  of  v.  Kries  is  based  on  this  theory.  A  small  glass  plate, 
from  2.5  to  5  sq.  mm.,  is  fastened  to  the  under  surface  of  a  support 
of  suitable  size  carr\ang  a  small  scale  pan.  The  glass  plate  is  placed 
on  the  skin  near  the  root  of  a  finger-nail  and  the  scale  pan  gradually 


322 


TEXT-BOOK  OF  PHYSIOLOGY. 


weighted  until  the  vessels  are  obhterated,  as  shown  by  the  blanching 
of  the  skin.  From  results  obtained  with  this  apparatus  v.  Kries 
estimated  the  pressure  in  the  capillaries  of  the  hand  at  37  mm.  Hg, 
and  in  the  ear  at  20  mm. 

The  Venous  Pressure. — In  passing  from  the  capillaries  to  the 
heart  the  pressure  continues  to  fall.  The  increasing  size  of  the  veins 
permits  again  of  manometric  observations  in  different  regions.  In  the 
crural  vein  the  pressure  has  been  found  to  be  equal  to  14  mm.  Hg,  and 
in  the  brachial  vein  9  mm.  of  Hg.  In  the  jugular  and  subclavian  and 
other  vessels  near  the  heart  it  is  zero  or  even  negative;  that  is,  less  than 
atmospheric  pressure  to  the  extent  of  from  i  to  10  mm.  of  mercury. 

>.The  amount  and  relation  of  the  pressures  in  the  three  divisions  of 
the  systemic  vascular  apparatus  are  approximately  shown  in  Fig.  144. 


Fig.  144. — Diagram  showing  the  Relative  Height  of  the  Blood-pressure  in 
THE  Different  Regions  of  the  Vessels.  H.  Heart.  A.  Arteries.  C.  Capil- 
laries. V.  Large  veins.  0,  0,  being  the  zero  line  ( =atmospheric  pressure),  the 
pressure  is  indicated  by  the  height  of  the  curve.  The  numbers  on  the  left  give 
the  pressure  (approximately)  in  millimeters  of  mercury,  h.  Pressure  in  heart. 
a.  Arteriole  region  showing  sudden  fall  of  pressure,  c.  The  fall  of  pressure  in 
the  capillaries,     v.  The  negative  pressure  in  the  large  veins. — (Yeo.) 

The  Causes  of  the  Blood-pressure. — A  conception  of  the  blood- 
pressure  as  well  as  of  the  factors  which  cooperate  to  develop  it  in  the 
different  divisions  of  the  vascular  apparatus  will  be  more  readily  ob- 
tained if  the  phenomena  attending  the  flow  of  a  fluid  through  an  appar- 
atus similar  to  that  represented  in  Fig.  144  A,  be  first  understood. 

This  apparatus  consists  of  a  reservoir  or  pressure  vessel,  R,  provided 
with  a  horizontal  tube  with  rigid  walls,  the  diameter  of  which  varies  in  its 
three  divisions  a,  b,  c.  Into  the  horizontal  tube,  vertical  tubes  are  inserted 
at  equal  distances. 

If  the  reservoir  be  filled  with  fluid  the  latter  will  exert  a  downward 


THE  CIRCULATION  OF  THE  BLOOD. 


323 


pressure,  the  degree  of  which  will  depend  on  the  height  of  the  column 
and  may  be  represented  by  P.  Under  given  conditions  this  pressure  will 
act  as  a  driving  or  propelling  power.  If  the  stop-cock  at  0  be  opened  the 
fluid  will  be  driven  into  and  through  the  horizontal  tube  with  a  detinite 
velocity.  As  the  fluid  flows  through  the  horizontal  tube  it  meets  with  re- 
sistance due  to  friction  between  the  fluid  and  the  sides  of  the  tube  which 
progressively  diminishes  the  primary  propelling  power.  The  resistance 
offered  to  the  flow  of  the  fluid  gives  rise  to  a  lateral  pressure  against  the 
sides  of  the  tube  which  varies  in  the  three  divisions  as  shown  by  the  height 
to  which  the  fluid  rises  in  the  vertical  tubes.  In  the  division  c  the  pressure 
is  low,  for  the  reason  that  the  resistance  to  be  overcome  by  the  moving  fluid 
is  but  slight  in  amount;  in  the  division  b  the  pressure  is  higher  because  of  its 
narrower  caliber  and  its  greater  distance  from  the  outflow  orifice.  In  the 
division  a  the  pressure  is  highest  because  of  the  friction  offered  by  its  walls 
in  addition  to  that  offered  by  the  walls  of  b  and  c.  The  resistance  to  be 
overcome  by  the  moving  fluid  at  any  given  point  of  the  horizontal  tube  is 


Fig.    144  A. — A    Pressure    Vessel,  R,    with    out-flow     tube    a,  b,   c,   and 
manometers  inserted  at  different  points. 

represented  therefore  by  the  degree  of  the  lateral  pressure,  and,  conversely 
the  pressure  at  any  given  point  is  proportional  to  the  resistance  yet  to  be 
overcome.  (In  the  conduct  of  an  experiment,  the  propelling  power  should 
be  kept  constant  by  permitting  fluid  to  flow  into  the  reservoir  as  rapidly  as 
it  flows  out  of  the  horizontal  tube.) 

If  the  horizontal  tube  were  of  uniform  caliber  throughout,  the  fall  of 
pressure  would  be  directly  proportional  to  its  length;  but  as  it  is  not  uni- 
form but  unequal  in  diameter  in  its  three  divisions,  the  fall  of  pressure  from 
the  beginning  to  the  end  is  unequal  and  may  be  represented  by  the  irregular 
line  which  unites  the  upper  limits  of  the  fluid  in  the  vertical  tubes  and 
which,  if  continued,  would  meet  the  reservoir  at  the  point  y.  The  height 
of  the  fluid  at  this  point  indicates  approximately  the  amount  of  the  pro- 
pelling power  utilized  in  overcoming  the  resistance  offered  by  the  hori- 
zontal tube  to  the  flow  of  the  fluid  through  it.  The  remainder  of  the 
propelling  power,  represented  by  the  portion  of  the  column  between  x 


324  TEXT-BOOK  OF  PHYSIOLOGY. 

and  y,  indicates  approximately  the  amount  utilized  in  imparting  velocity 
to  the  fluid.  The  primary  propelling  power  is  thus  utilized  in  overcoming 
the  resistance  represented  by  the  pressure  and  in  imparting  velocity  to  the 
fluid.  The  pressure  in  the  horizontal  tube  is  therefore  the  resultant  of 
two  factors,  viz.,  the  propelling  power  of  the  fluid  in  the  reservoir  and  the 
resistance  offered  by  the  walls  of  the  tube. 

Variations  of  the  Pressure. — So  long  as  the  foregoing  factors  remain 
constant  the  pressure  remains  constant.  If  either  factor  changes  in  one 
direction  or  another  there  will  arise  a  change  in  the  relative  degree  of  pres- 
sure in  the  different  divisions  of  the  system.  Thus  if  the  diameter  of  the 
tube  h  is  increased  the  resistance  at  once  diminishes  and  there  will  be  a 
tendency  towards  an  equalization  of  pressure  in  all  parts  of  the  tube,  as 
indicated  by  the  dotted  line  t  t:  the  pressure  will  fall  in  a,  and  in  the  ad- 
joining part  of  b,  and  rise  in  c,  and  in  the  adjoining  part  of  b.  If  the 
diameter  of  the  tube  c  is  decreased  the  reverse  conditions  will  obtain.  For 
the  reason  that  the  walls  of  the  outflow  tube  are  rigid  any  increase  or  de- 
crease in  the  propelling  power  would  be  attended  by  a  proportional  increase 
or  decrease  of  the  pressure  in  a,  b,  and  c. 

The  phenomena  presented  by  the  flow  of  a  fluid  through  elastic  tubes 
are  somewhat  different  and  more  complicated  than  those  presented  by 
rigid  tubes,  and  they  are  still  further  complicated  when  the  propelling 
power  is  periodic  in  action  rather  than  constant.  Nevertheless  the  fore- 
going facts  serve  to  explain  in  a  general  way  certain  phenomena  presented 
by  the  circulatory  apparatus. 

In  correspondence  with  the  lavv^s  of  the  flow  of  fluids  through 
both  rigid  and  elastic  tubes,  the  flow  of  blood  through  the  blood- 
vessels under  the  driving-power  of  the  heart  encounters  friction. 
This  is  to  be  sought  for  not  between  the  blood  and  the  inner  surface 
of  the  vessel,  but  rather  in  the  cohesion  of  the  particles  of  blood. 
This  it  is  which  offers  resistance  to  the  onward  movement  of  the 
blood  and  which  must  be  overcome  if  the  circulation  is  to  be 
maintained.  Close  to  the  inner  surface  of  the  vessel  there  is  a 
layer  of  blood  which  is  motionless,  known  as  the  still  layer,  caused 
by  an  adhesion  between  the  blood  and  the  vessel.  Between  this 
layer  and  the  axis  of  the  stream  there  is  an  infinite  number  of 
layers.  The  cohesion  between  these  layers  gradually  diminishes 
in  passing  from  the  periphery  to  the  center  of  the  stream.  The 
larger  the  stream,  the  less  is  the  cohesion,  and  the  reverse.  In  the 
large  arteries  the  still  layer  is  small  in  amount  as  compared  with  the 
total  volume  of  blood  passing  through  them ;  hence  the  axial  cohesion 
is  readily  overcome  and  the  friction  is  but  slight.  In  the  smallest 
arteries  and  capillaries  the  ratio  changes  and  the  friction  rapidly 
increases  and  to  a  considerable  extent.  In  the  veins  the  ratio  again 
changes,  approximating  that  in  the  arteries.  As  the  veins  pass  from 
the  periphery  toward  the  heart  and  their  individual  sectional  areas 
increase  there  is  again  a  diminution  of  the  friction. 


THE  CIRCULATION  OF  THE  BLOOD.  325 

As  a  consequence  of  the  friction  throughout  the  entire  vascular 
apparatus  the  blood  experiences  a  resistance  to  its  onward  move- 
ment which,  working  backward,  causes  the  blood  to  exert  a  lateral 
or  radial  pressure  against  the  walls  of  the  vessels.  The  high  pres- 
sure in  the  aorta  is  the  result  of  the  total  resistance  of  the  vascular 
system,  and  the  pressure  at  any  point  of  the  system  represents  the 
resistance  yet  to  be  overcome.  The  larger  part  of  the  pressure  in 
the  arterial  system,  however,  is  due  to  the  resistance  offered  by  the 
smallest  arteries  and  capillaries,  and  when  peripheral  resistance  is 
alluded  to  as  a  factor  in  the  production  or  in  the  variation  of  blood- 
pressure,  the  resistance  of  these  vessels  is  mainly  understood. 

The  primar}^  factor  in  the  production  of  the  pressure  is  the  pump- 
ing action  of  the  heart.  Should  there  be  any  cessation  in  its  activity, 
the  elastic  walls  of  the  arteries  would  recoil  and  force  the  blood  into 
the  veins.  There  would  be  coincidently  a  fall  of  pressure  equal  to 
that  of  the  atmosphere.  Even  under  normal  circumstances  this 
condition  is  approximated  during  the  diastole.  The  recoil  of  the 
arterial  wall  by  which  the  forward  movement  of  the  blood  is  main- 
tained is  attended  by  a  fall  in  pressure.  But  before  this  reaches  any 
considerable  extent,  the  heart  again  contracts  and  forces  its  contained 
volume  of  blood  into  the  arteries. 

That  this  may  be  accomplished  it  is  essential  that  the  cardiac 
energy  be  sufficient  not  only  to  drive  a  portion  of  the  blood  through 
the  capillaries  into  the  veins,  but  to  oppose  the  recoihng  arteries,  and 
to  distend  them  to  their  previous  extent,  so  that  the  incoming  volume 
of  blood  may  be  accommodated.  This  at  once  reestablishes  the 
pressure  at  its  former  level.  The  alternate  rise  and  fall  of  the  pres- 
sure is  represented  by  the  oscillations  of  the  mercurial  column  or  by 
the  small  elevations  and  depressions  on  the  kymographic  tracing. 

During  the  contraction  of  the  heart  the  kinetic  energy  is  trans- 
formed into  potential  energ}%  represented  by  the  tense  distended  walls 
of  the  arteries.  With  the  relaxation  of  the  heart  and  the  closure  of 
the  semilunar  valves  the  potential  energy  of  the  arteries  is  again  trans- 
formed into  kinetic  energy,  represented  by  the  moving  blood.  The 
arter}^  thus  continues  the  work  of  the  heart  during  its  period  of  in- 
activity. The  rapidity  with  which  the  cardiac  contractions  succeed 
each  other  prevents  the  pressure  from  sinking  below  a  certain  average 
level. 

The  uniform  level  of  the  arterial  pressure  depends  on  the  fact  that 
though  more  blood  enters  the  arteries  during  the  systole  than  escapes 
into  the  capillaries  and  veins,  as  shown  by  the  rise  of  the  mercurial 
column  in  the  manometer,  this  is  compensated  for  by  a  continued 
escape  during  the  diastole  as  shown  by  the  fall  of  the  mercurial  col- 
umn. So  long  as  the  inflow  of  blood  is  equaled  by  the  outflow,  there 
is  a  balancing  of  opposing  forces  and  the  pressure  is  maintained  at  a 
uniform  level. 


326 


TEXT-BOOK  OF  PHYSIOLOGY. 


VARIATIONS  IN  THE  BLOOD  PRESSURE, 

A.  In  the  Arterial  Pressure. — It  is  evident  from  the  preced- 
ing statements  that  the  arterial  blood-pressure  as  a  whole  may  be 
increased  by: 

1.  An  increase  in  the  rate  or  force  of  the  heart's  contraction. 

2.  An  increase  in  the  peripheral  resistance. 

3.  An  increase  in  the  general  volume  of  the  arterial  blood. 
And  that  it  may  be  decreased  by : 

1.  A  decrease  in  the  rate  and  force  of  the  heart's  contraction. 

2.  A  decrease  in  the  peripheral  resistance. 

3.  A  decrease  in  the  general  volume  of  blood. 

If  when  the  arterial  pressure  is  in  a  condition  of  equilibrium  the 
heart  ejects  into  the  arteries  in  a  given  period  of  time  an  increased 
quantity  of  blood  as  a  result  of  an  increased  rate  of  contraction,  there 
will  be  an  accumulation  of  blood  temporarily  in  the  arteries  (the 

peripheral  resistance  remain- 
ing the  same),  for  the  press- 
ure is  only  sufficient  to  force 
into  the  capillaries  a  given 
volume. 

The  same  result  could  be 
brought  about  by  an  increase 
in  the  force  or  power  of  the 
contraction,  the  frequency  re- 
maining the  same.  An  in- 
crease in  the  volume  of  blood 
ejected  at  each  contraction 
will  necessarily  lead  to  an  ac- 
cumulation. With  the  accu- 
mulation there  goes  an  in- 
creased distention  of  the  artery  and  a  corresponding  increase  of  press- 
ure. In  a  short  time,  therefore,  the  increased  pressure  will  force  out 
of  the  arteries  at  a  higher  rate  of  speed  this  excess  of  blood  until 
the  outflow  again  equals  the  inflow.  This  restores  the  equilibrium 
but  establishes  the  mean  pressure  at  a  higher  level. 

If  the  peripheral  resistance  is  increased  by  a  contraction  of  the 
muscular  walls  of  the  arterioles,  the  frequency  and  force  of  the 
heart  remaining  the  same,  there  will  also  be  an  accumulation  of 
blood  in  the  arteries  until  their  increased  distention  and  consequent 
rise  of  pressure  become  sufficient  to  increase  the  rapidity  of  outflow 
until  it  counterbalances  the  inflow. 

The  converse  of  these  statements  also  holds  true.  If  when  the 
general  arterial  pressure  is  in  a  condition  of  equilibrium  the  heart 


Fig.  145. — Fall  of  Blood-pressure  from 

1         Arrest  of  the  Heart's  Action  due 

TO  Stimulation  of  the  Vagus  begun 

AT  a  and  Stopped  at  b. — {Landois  and 

Stirling.) 


THE  CIRCULATION  OF  THE  BLOOD.  327 

ejects  into  the  arteries  in  a  given  period  of  time  a  decreased  quantity 
of  blood,  either  as  a  result  of  a  decrease  in  the  rate  or  power  or  both, 
there  will  soon  be  a  diminution  of  the  arterial  distention  and  a  con- 
sequent fall  in  pressure  (Fig.  146).  This  continues  until  the  outflow 
no  longer  exceeds  the  inflow.  Equihbrium  will  again  be  established, 
but  the  pressure  will  be  at  a  lower  level. 

If  the  peripheral  resistance  is  diminished  by  a  dilatation  of  the 
arterioles,  the  heart's  contractions  remaining  the  same,  the  existing 
pressure  soon  diminishes.  The  outflow  of  blood  at  once  lessens  in 
rapidity  and  so  continues  until  it  counterbalances  the  inflow.  The 
equilibrium  is  again  restored,  but  the  pressure  is  at  a  lower  level. 

B.  In  Capillary  Pressure. — The  pressure  in  the  capillaries, 
though  for  the  most  part  possessing  a  permanent  value,  is  sub- 
ject to  variations  in  accordance  with  variations  in  the  pressure  in 
either  the  arterial  or  venous  systems  or  both.  The  marked  difference 
in  the  pressure  in  the  large  arteries  and  the  capillaries  is  partly  due  to 


Fig.  146. — Fall  of  Blood-pressure  from  Diminution  of  the  Peripheral  Resist- 
ance, THE  Result  of  a  Dilatation  of  the  Arterioles,  brought  about  by 
Stimulation  of  the  Central  End  of  the  Depressor  Is-erve.  Stimulation 
begun  at  a,  and  stopped  at  b. — {Landois  and  Stirling.) 

the  resistance  offered  by  the  narrow  arterioles.  If  the  latter  dilate 
in  any  given  area,  the  capillary  pressure  increases  because  of  the 
propagation  into  them  of  the  arterial  pressure.  The  reverse  condition 
would  decrease  the  pressure.  On  the  other  hand,  any  interference 
with  the  outflow  from  any  given  area,  due  to  venous  compression, 
would  Hkewise  increase  the  pressure ;  any  factor  which  would,  on  the 
contrary,  favor  the  outflow  would  decrease  the  pressure.  Indepen- 
dent of  any  change  in  the  arteriole  resistance,  it  is  evident  that  a  rise 
in  arterial  pressure  alone  would  increase  the  capillary  pressure.  If 
both  arterial  and  venous  pressures  rise,  the  capillary  pressure  increases ; 
if  both  fall,  it  decreases. 

C.  In  Venous  Pressure. — Independent  of  any  change  in  the 
venous  pressure  in  a  given  area  from  local  or  temporarily  acting 
causes, — e.  g.,  aspiration  of  the  thorax  or  heart,  muscle  contrac- 
tions, change  of  position,  etc., — the  general  venous  pressure  will  be 
increased  by  a  decrease  in  the  value  of  those  factors  which  produce 


328  "        TEXT-BOOK  OF  PHYSIOLOGY. 

the  difference  of  pressure  between  the  arteries  and  veins.  An  in- 
crease in  the  value  of  these  factors  would  necessarily  decrease  the 
pressure. 

THE  VELOCITY  OF  THE  BLOOD. 

From  the  number  of  heart-beats  per  minute,  72,  and  the  amount 
of  blood  discharged  from  the  left  ventricle  at  each  beat,  180  c.c,  it  is 
evident  that  the  blood  must  be  flowing  through  the  vascular  appa- 
ratus with  a  certain  velocity,  for  during  the  minute  the  entire 
volume  of  blood,  5769  grams,  must  have  passed  twice  through  the 
heart.  Direct  observation  of  the  escape  of  blood  from  the  central 
end  of  a  divided  artery,  and  from  the  peripheral  end  of  a  divided 
vein,  as  well  as  of  the  flow  through  the  capillaries  as  seen  with  the 
microscope,  shows  that  the  velocity  of  the  flow  varies  in  different 
parts  of  the  vascular  apparatus.  In  the  arteries,  moreover,  the  flow 
is  not  quite  uniform,  but  experiences  alternate  acceleration  and 
retardation  with  each  heart-beat.  In  the  capillaries  and  veins  the 
flow  is  continuous  and  uniform,  as  the  conditions  of  the  arterial  walls 
are  such  as  to  completely  overcome  the  intermittency. 

If  the  systemic  vascular  apparatus  be  conceived  of  as  a  system 
of  tubes  which  have  symmetrically  divided  and  subdivided,  and  have 
again  united  and  reunited  in  a  corresponding  manner,  it  is  clear  that 
the  total  sectional  area  will  steadily  increase  from  the  beginning  to 
the  middle  of  the  system,  and  then  as  steadily  decrease  from  the  middle 
to  the  end  of  the  system.  In  such  a  system  the  same  volume  of  blood 
must  pass  through  any  given  section  in  a  unit  of  time  if  the  balance 
of  the  circulation  is  to  be  maintained.  As  the  velocity  of  a  fluid  is 
inversely  as  the  sectional  area  of  the  tubes  through  which  it  flows,  it 
foUows  that  the  initial  mean  velocity  of  the  blood  in  the  aorta  will 
steadily  decrease  as  it  flows  into  the  steadily  enlarging  blood-path 
until  it  reaches  a  minimal  value  in  the  middle  of  the  capillary  system ; 
and  that  it  will  again  steadily  increase  as  it  flows  into  the  narrowing 
blood-path  until  it  reaches  the  heart.  The  initial  mean  velocity  of 
the  blood  in  the  aorta  wiU  not  be  attained  in  the  venae  cavae,  for  the 
reason  that  the  total  sectional  area  of  the  latter  is  somewhat  greater 
than  that  of  the  former.  The  same  facts  hold  true  for  the  pulmonic 
vascular  system. 

The  Velocity  in  the  Aorta. — From  the  well-known  fact  that  the 
velocity  with  which  a  fluid  is  flowing  through  a  tube  may  be  deter- 
mined by  dividing  its  sectional  area  into  the  quantity  discharged 
in  a  unit  of  time,  attempts  have  been  made  to  determine  the  mean 
velocity  of  the  blood  at  the  beginning  of  the  aorta.  If  it  be  assumed 
that  the  volume  discharged  at  each  contraction  is  180  c.c,  as 
stated  by  Vierordt,  and  the  number  of  heart-beats  per  minute  at  72, 


THE  CIRCULATION  OF  THE  BLOOD. 


329 


the  total  volume  discharged  per  minute  would  be  12,960  c.c,  or  215 
c.c.  per  second.  The  sectional  area  of  the  aorta  at  its  origin  is  6.15 
sq.  cm.  On  the  principle  above  stated,  these  two  factors  would 
show  a   velocity   of    350  mm.  per  ^^ 

second.     An  objection  to  this  esti-  ? 

mate  is  that  the  amount  of  blood 
discharged — /.  e.,  the  contraction 
volume — is  much  larger  than  recent 
investigations  warrant.  Different 
observers  have  estimated  that  in 
man  the  contraction  volume  is  con- 
siderably less,  probably  not  more 
than  So  c.c. 


Fig.  147. — Volkmann's  Hemodromometer 
C,  C.  Arterial  cannulas. 


Fig.  148. — LuDwiG  and  Dogiel's 
Rheometer.  X,  Y.  Axis  of 
rotation.  A,  B.  Glass  bulbs. 
h,  k.  Cannulas  inserted  in  the 
divided  artery,  e,  e^,  rotates 
on  g,  f.     c,  d.  Tubes. 


The  Velocity  in  the  Arteries. — The  mean  velocity  of  the  blood 
in  the  larger  and  more  superficially  lying  arteries  has  been  determined 


330 


TEXT-BOOK  OF  PHYSIOLOGY. 


by  Volkmann  with  the  hemodromometer,  by  Ludwig  and  Dogiel 
with  the  stromuhr,  and  by  other  investigators  with  different  forms  of 
apparatus. 

Since  neither  the  blood  nor  any  particle  placed  in  it  can  be  seen 
through  the  walls  of  the  artery,  it  occurred  to  Volkmann  to  inter- 
calate along  the  course  of  a  vessel  a  U-shaped  glass  tube  about  one 
meter  in  length  with  a  lumen  the  diameter  of  that  of  the  selected 
vessel,  into  and  through  which  the  blood  could  be  made  to  flow. 
The  mechanic  construction  of  the  apparatus  is  such  (Fig.  147)  that 
the  blood  can  be  made  to  flow  directly  into  the  distal  portion  of  the 
artery  across  the  base  or  indirectly  by  way  of  the  glass  tube.  Pre- 
vious to  the  intercalation  of 
the  tube  it  is  filled  with 
serum  or  normal  saline 
solution.  With  the  turn- 
ing of  the  cocks  at  B  the 
blood  enters  the  glass  tube 
and  drives  the  serum  ahead 
of  it  into  the  arterial  system. 
From  the  difference  in  time 
between  the  moment  the 
blood  enters  and  the 
moment  it  leaves  the  tube 
and  from  the  length  of  the 
tube  the  velocity  is  deter- 
mined. 

The  stromuhr  or  rheo- 
meterof  Ludwig  (Fig.  148) 
is  constructed  on  the  same 
principle,  but  instead  of  the 
glass  tube  having  the  same 
diameter  it  is  considerably 
enlarged  on  its  two  sides. 
The  bulbs  are  fastened 
to  a  metalhc  disk  which  rotates  around  an  axis  in  the  metalhc 
base  which  carries  the  tubes  to  be  inserted  into  the  arteries. 
With  this  device  it  is  possible  to  place  either  bulb  in  connection 
with  the  proximal  end  of  the  artery.  Previous  to  the  experiment  the 
proximal  bulb  is  filled  with  oil,  the  distal  bulb  with  serum  or  normal 
sahne.  On  removing  the  clips  on  the  artery  the  blood  flows  into 
the  proximal  bulb  and  drives  the  oil  into  the  distal  bulb.  As  soon  as 
'  the  former  is  filled  with  blood  the  bulbs  are  reversed  and  the  same 
relative  conditions  are  attained.  This  is  repeated  a  number  of 
times.  Knowing  the  capacity  of  the  bulbs,  and  the  number  of 
times  they  are  filled  in  a  given  period,  the  total  quantity  of  blood 


Fig.  149. — The  Hemodromograph  of  Chau- 
VEAU  AND  LoRTET.  A,  B.  Tube  inserted 
in  artery.  C.  Lateral  tube  connected  with 
a  manometer.  h.  Index  moving  in  a 
caoutchouc  membrane,  a.     G.  Handle. 


THE  CIRCULATION  OF  THE  BLOOD. 


331 


discharged  is  obtained.  This  divided  by  the  sectional  area  of  the 
artery  gives  the  velocity.  The  following  values  have  thus  been 
obtained:  For  the  carotid  of  the  dog,  205  to  357  mm.  per  second; 
for  the  carotid  of  the  horse,  306  mm. ;  for  the  metatarsal  artery 
of  the  horse,  56  mm.  (Volkmann).  For  the  carotid  of  rabbits,  94  to 
226  mm.;  for  the  carotid  of  the  dog,  349  to  733  mm.  (Dogiel). 
The  variations  in  the  velocity  of  the  blood  in  the  arteries  during 
the  different  phases  of  the  cardiac  cycle  have  been  determined  by 
Chauveau  and  Lortet  with  the  hemodromograph  (Fig.  149).  This 
consists  of  a  metaUic  tube  carrying  a  graduated  disk.  At  one  point 
the  tube  is  perforated  but  covered  with  a  rubber  band  through  which 
passes  an  index.  When  the  tube  is  inserted  into  the  divided  ends  of 
an  artery,  the  current  of  blood  strikes  the  short  arm  of  the  index  and 
gives  to  the  outer  long  arm  a  movement  in  the  opposite  direction. 


;; — -...^^^ 

^^^>\  r 

1                                   --''''' 

1                            ,,-- 

^\  \ ' 

1                              ^^' 

^^x  \( 

1 

'\  \ 

\    /''' 

.  \/ 

/; 

\    A 

/: 

\ ''   \ 
V      \ 

/    ; 

i\       \ 

/        1 

„/ 

I       X.              •»->. 

o-^                                          > 

/'         ^-v^           ~~~"'-~^ 

0-^'                                  '^ 

/                 ^.^^            '    t- 0 

— 0        ■                                                                           ^ 

1                      '^"~~ — 

Arteries. 


Fig.  150. 


Capillaries. 
-,  Blood-pressure.     ,  Velocity.     — o- 


Veins. 
-o,  Sectional  area. 


The  extent  of  the  excursion  indicates  the  velocity.  The  apparatus  is 
first  graduated  with  currents  of  water  of  known  velocity.  With  this 
instrument  Chauveau  found  that  in  the  horse  the  velocity  during  the 
systole  was  520  mm.  per  second,  at  the  beginning  of  the  diastole  220 
mm.  per  second,  and  during  the  pause  150  mm.  per  second. 

The  Velocity  in  the  Capillaries. — The  rate  of  flow  in  the  capil- 
lary vessels  can  not  be  experimentally  determined.  It  has  been  esti- 
mated by  Vierordt  at  0.5  mm.  per  second  in  his  own  retinal  capillaries; 
by  Weber  at  0.8  mm.  In  frogs  the  velocity  can  be  fairly  well  deter- 
mined by  observing  the  time  required  for  a  corpuscle  to  pass  over  one 
or  more  divisions  of  an  ocular  micrometer.  Weber  calculated  in  this 
way  that  the  velocity  is  0.5  mm.  per  second. 

As  the  velocity  varies  inversely  with  the  sectional  area,  it  becomes 
possible  to  approximately  determine  the  relation  of  the  sectional  area 


332  TEXT-BOOK  OF  PHYSIOLOGY. 

of  the  capillary  system  to  that  of  the  aorta  from  the  above-mentioned 
velocities.  If  it  be  assumed  that  the  velocity  in  the  aorta  averages 
300  mm.  and  in  the  capillaries  0.5  mm.  per  second,  then  the  sectional 
area  of  the  capillaries  is  to  that  of  the  aorta  as  600  to  i. 

The  Velocity  in  the  Veins. — In  the  venous  system  the  velocity 
increases  in  proportion  as  the  sectional  area  decreases.  In  the  jugu- 
lar vein  Volkmann  found  the  velocity  225  mm.  per  second,  which 
was  about  one-half  that  in  the  aorta  of  the  same  animal.  The  reason 
for  the  slow  rate  of  movement  in  the  jugular  vein  is  to  be  found  in  the 
fact  that  the  sectional  area  of  the  combined  venae  cavae  is  about  twice 
that  of  the  aorta;  hence  the  relation  of  the  sectional  area  of  the  cap- 
illary system  to  the  sectional  area  of  the  venae  cavae  is  about  300  to  i. 

The  blood-pressure,  the  velocity  of  the  blood,  the  sectional  area 
of  the  vascular  apparatus,  and  their  relation  one  to  the  other  are 
shown  in  Fig.  150. 

THE  PULSE. 

The  pulse  may  be  defined  as  a  periodic  expansion  and  recoil  of 
the  arterial  system.  The  expansion  is  caused  by  the  discharge  from 
the  heart  into  the  arteries  of  a  volume  of  blood  during  the  systole;  the 
recoil  is  due  to  the  elastic  reaction  of  the  arterial  walls  on  the  blood, 
driving  it  forward  into  and  through  the  capillaries,  during  the  dias- 
tole. 

At  the  close  of  the  cardiac  diastole  the  arterial  system  is  full  of 
blood  and  considerably  distended.  During  the  occurrence  of  the 
succeeding  systole  a  definite  volume  of  blood  is  again  discharged 
into  the  aorta.  The  incoming  volume  of  blood  is  now  accomodated 
by  the  discharge  of  a  portion  of  the  general  blood  volume  into  the 
capillaries  and  by  the  expansion  of  the  arteries.  The  expansion 
naturally  begins  at  the  root  of  the  aorta  and  at  the  beginning  of  the 
systole.  As  the  blood  continues  to  be  discharged  from  the  heart, 
adjoining  segments  of  the  aorta  and  its  branches  expand  in  quick 
succession,  and  by  the  time  the  systole  is  completed  the  expansion 
has  traveled  over  the  entire  arterial  system  as  far  as  the  capillaries. 
With  the  cessation  of  the  systole,  the  recoil  of  the  arterial  walls  at 
once  occurs. 

This  expansion  movement  which  thus  passes  from  the  beginning 
to  the  end  of  the  arterial  system  in  the  form  of  a  wave  is  known  as  the 
pulse-wave  or  pulse.  Coincident  with  the  expansion  and  recoil  of  the 
arterial  system  there  is  a  slight  alternate  increase  and  decrease  of 
the  general  blood-pressure,  as  shown  by  the  small  curves  on  a  blood- 
pressure  tracing,  and  for  this  reason  the  expansion  and  recoil  is 
termed  the  pressure  pulse. 

The  pulse-wave  which  thus  spreads  itself  over  the  entire  arterial 
system  with  each  systole  of  the  heart  can  be  perceived  in  certain 


THE  CIRCULATION  OF  THE  BLOOD.  333 

localities  by  the  eye,  by  the  sense  of  touch,  and  investigated  with 
various  forms  of  apparatus  or  instrumental  means.  The  pulse- 
wave,  or  at  least  the  elevation  of  the  soft  tissues  overlying  it,  can  be 
seen  in  the  radial  artery,  where  it  passes  across  the  wTist- joint,  in  the 
carotid  artery,  in  the  temporal  SLTtery,  in  the  arteries  of  the  retina  under 
certain  conditions,  with  the  ophthalmoscope.  If  the  ends  of  the  fingers 
are  firmly  placed  over  the  radial  artery,  not  only  the  increase  and  de- 
crease of  pressure,  but  also  many  of  the  peculiarities  of  pulse-wave,  may 
be  perceived.  Without  much  difficulty  it  may  be  perceived  that  the 
expansion  takes  place  quickly,  the  recoil  relatively  slowly;  that  the 
waves  succeed  one  another  with  a  certain  frequency,  corresponding 
to  the  heart-beat;  that  the  pulsations  are  rhythmic  in  character,  etc. 
Inasmuch  as  the  individuality  of  the  pulse-wave  varies  at  different 
periods  of  life  and  under  different  physiologic  and  pathologic  condi- 
tions, various  terms  more  or  less  expressive  have  been  suggested  for 
its  var}dng  pecuharities.  Thus  the  pulse  is  said  to  be  jrequent  or 
infrequent  according  as  it  exceeds  or  falls  short  of  a  certain  average 
number — 72  per  minute;  quick  or  slow,  according  to  the  suddenness 
with  which  the  expansion  takes  place  or  strikes  the  fingers;  hard  or 
soft,  tense  or  easily  compressible,  according  to  the  resistance  which 
the  vessel  offers  to  its  compression  by  the  fingers;  large,  full,  or  small, 
according  to  the  volume  of  blood  ejected  into  the  aorta,  or,  in  other 
words,  the  degree  of  fullness  of  the  arterial  system. 

Frequency  of  the  Pulse. — As  the  pulse  or  the  arterial  expansion 
is  the  direct  result  of  the  heart's  action,  its  frequency  must,  under 
physiologic  conditions,  coincide  with  that  of  the  heart.  All  condi- 
tions which  modify  the  rate  of  the  heart  will  modify  at  the  same  time 
the  rate  of  the  pulse.     (See  page  285). 

The  Velocity  of  Propagation  of  the  Pulse-wave. — The  propa- 
gation of  the  pulse- wave  from  its  origin  at  the  root  of  the  aorta  to  any 
given  point  of  the  arterial  system  occupies  an  appreciable  period  of 
time.  The  difference  in  time  between  the  systole  and  the  appearance 
of  the  pulse- w^ave  at  the  dorsal  artery  of  the  foot  can  be  appreciated 
by  the  sense  of  touch.  The  absolute  time  occupied  by  the  wave  in 
reaching  this  point  was  determined  by  Czermak  to  be  0.193  second. 
The  rate  at  which  the  wave  is  propagated  over  the  vessels  of  the  lower 
extremity  has  been  estimated  by  the  same  observer  at  11. 16  meters 
per  second,  and  for  the  upper  extremities  at  but  6.7  meters  per  second. 
Other  experimenters  have  obtained  for  the  lower  extremities  some- 
what different  results,  varying  from  6.5  to  11  meters  per  second. 
Weber's  original  estimate  was  from  7.92  to  9.24  meters  per  second. 
The  slower  rate  of  movement  in  the  vessels  of  the  upper  extremities 
has  been  attributed  to  a  greater  distensibility  of  their  walls,  a  condi- 
tion unfavorable  to  rapid  propagation.  For  this  reason  a  low  arterial 
tension  will  occasion  a  delay  in  the  appearance  of  the  pulse-wave  in 


334 


TEXT-BOOK  OF  PHYSIOLOGY. 


any  portion  of  the  body;  a  high  arterial  tension  will  of  course  have  the 
opposite  effect.     The  difference  in  the  speed  of  the  pulse-wave  and 
the  blood-current  shows  that  they  are  not  identical  and  must  not  be 
confounded  with  each  other. 
■^The  Sphygmograph. — The  alternate  expansion  and  recoil  of  an 


Fig.  151. — Von  Frey's  Sphygmograph.  G.  S.  Metal  framework.  P.  Button 
attached  to  spring.  F.  Vertical  rod.  U.  Clock-work  which  turns  the  recording 
cylinder.     VI.  Time  marker. 


artery  caused  by  the  temporary  increase  and  decrease  of  pressure 
following  each  heart-beat  can  be  graphically  recorded  on  a  travehng 
surface  by  means  of  a  special  apparatus,  the  sphygmograph  or  pulse- 
writer.  The  tracing  obtained  in  the  form  of  a  curve  is  termed  the 
pulse-curve  or  the  sphygmogram.     Different  forms  of  this  apparatus 

have  been  devised  by  Marey,  Dudgeon, 
V.  Frey,  and  many  others.  The  in- 
strument of  V.  Frey  is  shown  in  Fig. 
151.  This  consists  first  of  a  metal 
framework  by  which  the  apparatus  is 
fastened  to  the  arm  and  support  given 
to  the  lever,  recording  surface,  etc. 
The  essential  part  is  the  spring  carry- 
ing a  button  which  is  placed  over  the 
artery,  usually  the  radial,  before  it 
crosses  the  wrist-joint.  A  vertical  rod  transmits  the  movement  of 
the  spring  to  the  recording  lever;  the  movements  of  the  latter  are 
recorded  on  a  small  cylinder  inclined  slightly  so  that  the  upstroke 
may  be  vertical.     A    small   electro-magnet    serves    to  -record    the 


Fig.  152. — The  Pulse  Curve  or 
Sphygmogram. 


TPIE  CIRCULATION  OF  THE  BLOOD.  335 

time  relations  of  the  changes  in  the  blood  pressure.  An  average 
tracing  taken  from  the  radial  artery  is  shown  in  Fig.  152.  This, 
however,  is  not  a  tracing  of  the  pulse-wave,  but  rather  a  record 
of  the  changes  in  pressure,  their  succession  and  time  relations, 
which  follow  each  beat  of  the  heart.  The  artery  usually  selected  for 
obtaining  a  sphygmogram  is  the  radial.  This  artery  lies  quite 
superficially,  covered  only  by  connective  tissue  and  skin  and  sup- 
ported by  the  flat  surface  of  the  radial  bone,  conditions  most  favor- 
able to  technical  investigation. 

The  sphygmogram  or  pulse-curve  may  be  divided  into  two  portions : 
viz.,  a  line  of  ascent  from  a  to  b,  and  a  line  of  descent  from  b  to  d  (Fig. 
152).  In  normal  tracings  the  former  is  almost  vertical  and  caused  by 
the  sudden  expansion  of  the  artery  immediately  following  the 
ventricular  contraction;  the  latter  is  in  general  obhque,  due  to  the 
recoil  of  the  arterial  walls,  occupies  a  longer  period  of  time,  and  is 
marked  by  several  elevations  and  depressions,  both  of  which  indicate 
that  the  restoration  to  equihbrium  is  neither  immediate  nor  uncom- 
phcated.  One  of  these  elevations  is  quite  constant  and  known  as 
the  dicrotic  wave,  c;  the  depression  or  notch  just  preceding  it  is 
known  as  the  dicrotic  notch.  Pre-  and  post-dicrotic  waves  are  not 
infrequently  present.     The  summit   is  generally  sharp  and  pointed. 

The  vertical  direction  of  the  hne  of  ascent  is  taken  as  an  indica- 
tion that  the  arterial  walls  expand  readily,  that  the  blood  is  dis- 
charged quickly,  and  that  the  ventricular  action  is  not  impeded. 
An  oblique  direction  of  the  hne  of  ascent  is  an  indication  that  the 
reverse  conditions  obtain.  The  height  varies  inversely  as  the 
arterial  pressure,  other  things  being  equal;  being  high  with  a  low 
pressure,  and  low  with  a  high  pressure. 

The  dicrotic  elevation  shows  that  a  second  expansion  wave  is  de- 
veloped which  interrupts  temporarily  the  recoil  of  the  arterial  walls. 
The  origin  of  this  second  expansion  has  been  the  subject  of  much 
investigation,  and  at  present  it  may  be  said  that  the  question  is  not 
fully  decided.  It  is  asserted  by  some  investigators  that  it  is  central 
in  origin,  beginning  at  the  base  of  the  aorta  and  passing  to  the  pe- 
riphery; by  others,  that  it  is  peripheral  in  origin,  beginning  near  the 
capillary  region  and  reflected  to  the  heart.  The  former  view  is  the 
one  more  generally  accepted.  According  to  it,  the  expansion  is  the 
result  of  the  sudden  closure  of  the  aortic  valves,  a  backward  surge  of 
the  blood-column  against  them.  The  sudden  arrest  of  the  blood 
and  its  accumulations  again  expands  the  aorta. 

The  dicrotic  notch  is  therefore  taken  as  the  moment  at  which  the 
ventricular  systole  ceases  and  the  aortic  valves  close.  From  this  fact 
it  is  evident  that  immediately  after  the  first  expansion  the  pressure 
begins  to  fall,  even  though  the  ventricular  systole  continues,  owing 
to  the  discharge  of  blood  from  the  arterial  into  the  capillary  and 


33^ 


TEXT-BOOK  OF  PHYSIOLOGY. 


venous  systems.  The  height  of  the  dicrotic  wave  or  the  depth  of 
the  dicrotic  notch  is  favored  by  low  arterial  tension  and  highly  elastic 
arteries.  Both  features  are  diminished  by  the  reverse  conditions. 
The  apex  is  sometimes  rounded  and  even  fiat,  indicative  of  a  great 
diminution  in  arterial  elasticity.  The  sphygmogram  not  infrequently 
varies  considerably  from  the  normal  type  in  different  pathologic 
conditions  of  the  circulatory  apparatus.  A  consideration  of  these 
variations  does  not  fall  within  the  scope  of  this  work. 

The  Volume  Pulse. — If  an  individual  artery  expands  with  each 
systole  and  recoils  with  each  diastole  of  the  heart,  the  same  is  true  of 
all  arteries,  and  as  a  result  the  volume  of  any  organ  or  part  of  the  body 
must  undergo  similar  changes.  To  such  alternate  changes  in  volume 
the  term  volume  pulse  is  given.  The  extent  to  which  an  organ  will 
increase  in  volume  will  depend  to  some  extent  on  its  elasticity.  The 
reason  for  the  increase  in  volume  is  the  resistance  offered  to  the  flow 


Fig.  153. — Mosso's  Plethysmogeaph.  G.  Glass  vessel  for  holding  a  limb.  F. 
Flask  for  varying  the  water-pressure  in  G.  T.  Recording  apparatus. — {Landois 
and  Stirling.) 


of  blood  into  and  through  the  capillaries;  the  decrease  in  volume  to 
the  overcoming  of  the  resistance  through  the  arterial  recoil. 

The  variations  in  volume  may  be  recorded  by  enclosing  the  organ 
in  a  rigid  glass  or  metal  vessel,  which  at  one  point  is  in  com- 
munication with  a  recording  apparatus,  e.  g.,  a  tambour  with  a 
lever  or  mercurial  manometer  with  float  and  pen.  The  space  be- 
tween the  organ  and  vessel  is  filled  with  normal  saline,  air,  or  oil. 
Such  an  apparatus  is  known  as  a  plethysmograph.  A  well-known 
form  of  plethysmograph  is  that  of  Mosso  (Fig.  153).  Many  forms 
of  this  apparatus  have  been  devised  in  accordance  with  the  character 
of  the  organ — spleen,  kidney,  etc. — to  be  investigated,  though  the 
principle  underlying  them  is  essentially  the  same.  In  addition  to 
changes  in  volume  due  to  the  heart's  action,  most  organs  undergo 
additional  changes  in  volume  from  respiratory  and  vaso-motor  causes. 


THE  CIRCULATION  OF  THE  BLOOD. 


337 


THE  CAPILLARY  CIRCULATION. 

In  certain  regions  of  the  body  of  many  animals  it  is  possible,  on 
account  of  the  delicacy  and  transparency  of  the  tissues,  to  observe 
not  only  the  flow  of  blood  through  the  smaller  arteries,  capillaries, 
and  veins,  but  many  of  the  phenomena  connected  with  it,  to  which 
reference  has  already  been  made.  The  structures  usually  selected 
for  the  observation  of  these  phenomena  are  the  interdigital  mem- 
branes, the  tongue,  the  lung,  the  bladder,  and  the  mesentery  of  the 
frog.  Though  any  one  of  these  structures  will  afford  an  admirable 
view  of  the  blood-flow,  the 
mesentery  for  many  reasons 
is  the  most  satisfactory.  For 
a  comparison  of  the  phenom- 
ena observed  in  the  cold- 
blooded animals  with  those 
in  the  warm-blooded  animals 
the  omentum  of  the  guinea-pig 
may  be  employed.  If  the  frog 
is  the  subject  of  experiment,  it 
should  be  slightly  curarized 
and  the  brain  destroyed  by 
pithing.  The  animal  is  then 
placed  on  a  small  board  cap- 
able of  adjustment  to  the  stage 
of  the  microscope.  The  abdo- 
men is  then  opened  along  the 
side  and  a  loop  of  intestine 
withdrawn  and  placed  around 
a  cork  ring  which  surrounds 
an  opening  in  the  side  of  the 
frog  board.  The  loop  of  the 
intestine  should  be  so  placed 
that  it  will  lie  between  the  ob- 
server and  the  body  of  the  frog.  The  mesentery  thus  exposed  must 
be  kept  moist  with  normal  saline  solution. 

When  examined  with  low  powers  of  the  microscope,  arteries,  veins, 
and  capillaries  will  be  found  occupying  the  field  of  vision.  Their 
general  arrangement,  their  size  and  connections,  can  be  readily  deter- 
mined. After  a  few  preliminary  adjustments  a  region  will  be  found 
in  which  the  blood  is  flowing  in  opposite  directions.  The  vessel 
apparently  carrying  blood  away  from  the  observer  is  an  artery;  the 
vessel  apparently  carrying  blood  toward  the  observer  is  a  vein;  the 
smallest  vessels  are  capiflaries.  The  blood  in  the  artery  is  of  a 
brighter  color  than  the  blood  in  the  vein;  the  blood  in  the  capilla- 
ries is  almost  colorless.     The  arterial  blood-stream  not  infrequently 


Fig.  154. — The  Vessels  of  the  Frog's 
Web. — a.  Trunk  of  vein,  and  {b,  b)  its 
tributaries  passing  across  the  capillary 
network.  The  dark  spots  are  pigment 
cells. — {Yeo's  "Physiology. ") 


338 


TEXT-BOOK  OF  PHYSIOLOGY. 


shows  remittancy,  an  alternate  acceleration  and  retardation,  corre- 
sponding to  each  heart-beat;  the  capillary  and  venous  streams  are 
uniform  and  continuous.  The  relative  velocities  in  the  three  sets 
of  vessels  are  indicated  by  the  movement  of  the  red  corpuscles.  In 
the  arteries  they  pass  before  the  eye  so  rapidly  that  they  can  not  be 
distinguished;  in  the  capillaries  they  pass  so  slowly  that  both  form 
and  structure  may  be  determined;  in  the  veins,  though  again  mov- 
ing rapidly,  they  can  often  be  distinguished. 

The  relative  positions  of  the  red  and  white  corpuscles  in  the 
blood-stream  are  also  apparent;  the  former  occupy  the  central,  the 
latter  the  peripheral  portion,  at  the  same  time  adhering  to  the 
sides  of  the  vessel.  Between  the  axial  portion  of  the  stream  occu- 
pied by  the  red  corpuscles  and  the  wall  of  the  vessel  there  is  a  clear 

still  layer  of  plasma,  the  re- 
y^  suit  of  an  adhesion  of  the 

plasma  to  the  wall.  It  is 
this  feature  which  gives  rise 
to  the  friction  between  suc- 
cessive layers  of  the  blood- 
stream, the  resistance  to  the 
blood-flow,  and  the  devel- 
opment of  blood-pressure. 
The  relative  breadth  of  the 
still  layer  and  amount  of 
friction  are  greater  in  small 
than  in  large  vessels. 

The  volume  of  blood 
passing  into  any  given  cap- 
illary area  is  determined 
by  the  degree  of  contraction 
of  the  arterioles.  Thus  on 
the  apphcation  of  warm  sa- 
line solution,  which  relaxes  the  arterioles,  there  is  a  large  increase 
in  the  inflow  of  blood;  vessels  previously  invisible  suddenly  come 
into  view  as  the  blood  with  its  corpuscles  passes  into  them.  On  the 
apphcation  of  cold  water,  which  contracts  the  arterioles  and  dimin- 
ishes the  inflow,  many  of  the  smaller  vessels  entirely  disappear  from 
view.  The  alternate  contraction  and  relaxation  of  the  arterioles 
will  therefore  determine  the  quantity  of  blood  flowing  into  and 
through  the  capillary  system. 

Migration  of  the  White  Corpuscles. — ^A  phenomenon  fre- 
quently observed  in  the  capillary  vessels  of  the  mesentery  or  of 
the  bladder  of  the  frog  is  the  passage  of  the  white  corpuscles 
through  the  walls  into  the  surrounding  lymph-spaces.  To  this  pro- 
cess the  term  migration  or  diapedesis  is  given.      After  the  tissues 


Fig.  155. — Small  Vessel  of  a  Frog's  Mesen- 
tery SHOWING  Diapedesis.  w,  w.  Vas- 
cular walls,  a,  a.  Poiseuille's  space,  r,  r. 
Red  corpuscles.  /,  /.  Colorless  corpuscles 
adhering  to  the  wall,  and  c,  c,  in  various 
stages  of  extrusion.  /,  /.  Extruded  cor- 
puscles.— {Landois  and  Stirling.) 


THE  CIRCULATION  OF  THE  BLOOD.  339 

have  been  exposed  to  the  air  for  some  time  or  subjected  to  an 
irritant,  the  vessels  dilate  and  become  distended  with  blood.  In 
a  short  time  the  blood- stream  slows,  and  finally  comes  to  rest. 
The  condition  of  stasis  is  then  estabhshed.  During  the  develop- 
ment of  this  condition  the  white  corpuscles  accumulate  in  large 
numbers  along  the  inner  surface  of  the  vessels  and  soon  begin  to 
pass  through  the  vessel- walls.  This  they  do  by  protruding  a  portion 
of  their  substance  and  inserting  it  into  and  through  the  vessel- 
wall.  This  once  accomphshed,  the  remainder  of  the  cell  in  due 
time  follows  until  it  has  entirely  passed  out  into  the  tissue-space. 
The  opening  in  the  cell-wall  now  closes.  The  successive  steps  in 
this  process  are  shown  in  Fig.  155.  As  this  migration  occurs  mainly 
after  the  circulation  has  ceased  or  when  the  tissues  present  the 
phenomena  of  approaching  inflammation,  it  is  difficult  to  state  in 
how  far  it  is  strictly  a  physiologic  process. 

The  Venous  Circulation. — The  blood,  having  passed  through 
the  capillary  vessels,  is  gathered  up  by  the  veins  and  conveyed  to  the 
right  side  of  the  heart.  As  the  veins  converge  and  unite  to  form 
larger  and  larger  trunks  the  sectional  area  gradually  diminishes,  and 
hence  the  velocity  of  the  blood-flow  increases,  though  it  never  attains 
the  velocity,  even  in  the  vense  cavae,  that  it  had  in  the  aorta,  for  the 
reason  that  the  sectional  area  of  the  venae  cavse  is  considerably  larger 
than  that  of  the  aorta.  The  pressure  also  is  very  low  in  the  larger 
veins  because  the  friction  still  to  be  overcome  is  relatively  very  slight. 

The  capacity  of  the  venous  system  is  considerably  greater  than 
that  of  the  arterial  system,  as  there  are  usually  two  and  even  three 
veins  accompanying  each  artery.  This,  taken  in  connection  with  its 
greater  distensibihty,  makes  of  the  venous  system  a  reservoir  in  which 
blood  can  be  stored.  On  this  reservoir  the  arterial  system  can  call 
for  that  amount  of  blood  necessary  for  the  maintenance  of  its  normal 
volume  and  pressure,  and  into  it  any  excess  can  be  discharged. 
The  relative  amounts  contained  in  the  two  systems  are  regulated  by 
the  nervous  system.  The  movement  of  the  blood  through  the  veins 
is  accomplished  by  the  cooperation  of  several  forces,  reference  to 
which  will  be  made  in  a  following  paragraph. 


THE  PULMONIC  VASCULAR  APPARATUS. 

The  pulmonic  vascular  apparatus  consists  of  a  closed  system 
of  vessels  extending  from  the  right  ventricle  to  the  left  auricle,  and 
includes  the  pulmonary  artery,  capillaries,  and  pulmonary  veins.  In 
its  anatomic  structure  and  physiologic  properties  it  closely  resembles, 
if  it  is  not  identical  with,  the  systemic  apparatus. 

The  stream-bed  widens  from  the  beginning  of  the  pulmonary 
artery  to  the  middle  of  the  capillary  system;  it  again  narrows  from 
this  point  to  the  terminations  of  the  pulmonary  veins. 


340  TEXT-BOOK  OF  PHYSIOLOGY. 

The  movement  of  the  blood  from  the  beginning  to  the  end  of  the 
system  is  due  to  a  difference  of  pressure  between  these  two  points, 
the  result  of  the  friction  between  the  blood  and  the  vascular  walls. 
From  the  difference  in  the  extent  of  the  pulmonic  and  systemic 
systems  it  is  evident  that,  other  things  being  equal,  the  friction  is  less, 
and  therefore  also  the  pressure  is  less  in  the  former  than  in  the  latter. 
This  view  is  supported  by  the  difference  in  the  thickness  of  the  walls  of 
the  right  and  left  sides  of  the  heart.  The  pressure  in  the  pulmonary 
artery  of  the  dog  was  shown  by  Beutner  to  be  about  one-third  that 
in  the  aorta;  by  Bradford  and  Dean  to  be  one-fifth.  The  velocity 
of  the  blood-stream  in  each  of  the  three  divisions  of  the  system  can 
not  wtU  be  determined.  The  time  occupied  by  a  particle  of  blood 
in  passing  from  the  right  to  the  left  ventricle  has  been  estimated  at 
one-fourth  the  time  required  to  pass  from  the  left  to  the  right  ven- 
tricle. Assuming  the  latter  to  be  thirty  seconds,  the  former  would  be 
seven  and  one-half  seconds. 

The  capillary  vessels  are  spread  out  in  a  very  elaborate  manner 
just  beneath  the  inner  surface  of  the  pulmonary  air-cells,  and  form 
by  their  close  relation  to  it,  a  mechanism  for  the  excretion  of  carbon 
dioxid  and  the  absorption  of  oxygen.  The  extent  of  the  capillary 
surface  is  very  great.  It  has  been  estimated  at  200  square  meters. 
The  amount  of  blood  flowing  through  this  system  hourly  and  exposed 
to  the  respiratory  surface  is  about  800  liters.  The  reason  for  the 
existence  of  the  pulmonary  circulation  is  the  renewal  of  the  oxygen 
volume  in  the  blood  and  the  ehmination  of  the  carbon  dioxid;  for 
the  accomplishment  of  both  objects  ample  provision  is  here  made. 
The  flow  of  blood  through  the  cardio-pulmonary  vessels  is  subject 
to  variation  during  both  inspiration  and  expiration  in  consequence 
of  their  relation  to  the  respiratory  apparatus.  The  mechanism  by 
which  these  variations  are  produced  will  be  considered  in  the  chapter 
devoted  to  Respiration. 


FORCES  CONCERNED  IN  THE  CIRCULATION  OF  THE  BLOOD. 

The  Contraction  of  the  Heart. — The  primary  forces  which  keep 
the  blood  flowing  from  the  beginning  of  the  aorta  to  the  right 
side  of  the  heart  and  from  the  beginning  of  the  pulmonary  artery 
to  the  left  side  are  the  contractions  of  the  left  and  right  ventricles 
respectively.  This  is  evident  from  the  fact  that  each  ventricle 
at  each  contraction  not  only  overcomes  the  pressure  in  the  aorta 
and  pulmonary  artery,  the  sum  of  all  resistances,  but  imparts  a 
given  velocity  to  the  blood.  Since  the  pressure  continuously 
falls  from  the  beginning  to  the  end  of  each  system,  it  follows  that 
the  blood  must  flow  from  the  point  of  high  to  the  point  of  low 
pressure      During  the  interval  of  the  heart's  activity  the  walls  of 


THE  CIRCULATION  OF  THE  BLOOD.  341 

the  arteries,  to  which  the  heart's  energy  was  largely  transferred, 
now  take  up  and  continue  the  work  of  the  heart,  and  by  recoiling 
drive  the  blood  forward  and  into  the  venous  system.  Though 
the  heart's  energy  is  probably  sufficient  to  drive  the  blood  into 
the  opposite  side  of  the  heart,  it  is  supplemented  by  other 
forces — e.  g.  : 

2.  Muscle  Contraction. — As  a  result  of  the  relation  which  the  veins 

bear  to  the  muscles  in  all  parts  of  the  body  it  is  clear  that  with 
each  contraction  and  relaxation  of  the  muscles  there  will  be  exerted 
an  intermittent  pressure  on  the  veins.  With  each  contraction 
the  blood  on  the  proximal  side  will  at  once  be  driven  forward 
with  increased  velocity,  while  that  on  the  distal  side  will  be  re- 
tarded, will  accumulate  and  distend  the  veins,  owing  to  the 
closure  of  the  valves ;  with  the  relaxation  of  the  muscle  the  elastic 
and  contractile  tissues  in  the  walls  of  the  veins  will  come  into 
play  and  force  the  blood  forward. 

3.  Thoracic  Aspiration. — The  inspiratory  movement  aids  the  flow 

of  blood  through  the  venae  cavae  and  their  tributaries.  With 
each  inspiration  the  pressure  within  the  thorax  but  outside  the 
lungs  undergoes  a  diminution  more  or  less  pronounced  in  ac- 
cordance with  the  extent  of  the  movement.  As  a  result,  the 
blood  in  the  large  veins,  now  subjected  to  a  pressure  greater  than 
that  in  the  thorax,  flows  more  rapidly  toward  the  heart.  With 
each  expiration  the  reverse  obtains. 

4.  Action  of  the  Valves. — It  is  quite  probable  that  gravity  opposes 

to  some  extent  the  flow  of  blood  through  the  veins  below  the  level 
of  the  heart.  This  opposition  to  the  upward  flow  is  largely  pre- 
vented by  the  valves,  for  each  retardation  is  immediately  checked 
by  their  closure  and  support  given  to  the  column  of  blood. 
The  influence  of  gravity  is  shown  when  the  relation  of  the  arm  to 
the  heart  is  changed.  Thus,  if  the  arm  be  allowed  to  hang  pas- 
sively by  the  side  of  the  body,  the  veins,  especially  on  the  back  of 
the  hand,  will  become  distended  with  blood.  If  now  the  arm  be 
raised,  the  blood  will  flow  rapidly  toward  the  heart,  as  shown  by 
the  rapid  emptying  of  the  veins. 
Work  Done  by  the  Heart. — The  work  which  the  left  ventricle 

performs  at  each  contraction  when  it  discharges  its  contained  volume 

of  blood  into  the  aorta  is : 

1.  To  overcome  the  total  resistance  of  the  systemic  vascular  appa- 

ratus expressed  in  terms  of  aortic  pressure ;  and — 

2.  To  impart  velocity  to  the  blood. 

The  pressure  in  the  aorta  is  not  absolutely  determined,  though 
for  many  reasons  it  may  be  assumed  to  be  about  250  mm.  Hg, 
or  its  equivalent,  a  column  of  blood  3.21  meters  in  height.  As  the 
heart  discharges  188  grams,  the  work  done  may  be  calculated  by 


342  TEXT-BOOK  OF  PHYSIOLOGY. 

multiplying  the  weight  by  the  height :  viz.,  0.188  X  3.2  =  0.6016  kilo- 
grammeter. 

The  velocity  of  the  blood  in  the  aorta  has  been  approximately 
estimated  at  0.5  meter  per  second.  The  work  done  in  imparting  this 
velocity  to  188  grams  is  estimated  by  squaring  the  velocity  and  dividing 
by  the  accelerating  force  of  gravity  {^~J^Yi)  ^^^  multiplying  the 
quotient  by  0.188.  The  quotient  of  the  first  two  values  represents 
the  distance  a  body  would  have  to  fall  to  acquire  this  velocity: 
viz.,  0.0127  meter.  The  work  done  is  therefore  0.188  X  0.0127,  or 
0.0023  kilogrammeter. 

The  entire  work  of  the  left  ventricle  is  the  sum  of  these  two 
amounts,  or  0.604  kilogrammeter.  Assuming  that  the  heart  beats  72 
times  per  minute,  the  work  done  daily  would  be  0.604  X  72  + 
60  X  24,  or  62.622  kilogrammeters.  The  right  ventricle  approxi- 
mately performs  about  one-third  of  this  amount  of  work  in  over- 
coming the  resistance  offered  by  the  pulmonary  system  and  in 
imparting  velocity  to  its  contained  volume  of  blood.  The  work  of 
the  entire  heart  would  therefore  be  for  the  twenty-four  hours  about 
83.496  kilogrammeters. 

THE  NERVE  MECHANISM  OF  THE  VASCULAR  APPARATUS. 

The  middle  coat  of  the  arteries,  and  especially  of  those  in  the 
peripheral  region  of  the  arterial  system,  consists  of  a  well-defined 
layer  of  non-striated  muscle-fibers  arranged  in  a  circular  direction  or 
at  right  angles  to  the  long  axis  of  the  vessel.  In  the  physiologic  con- 
dition these  fibers  are  in  a  state  of  continuous  contraction,  more  or 
less  pronounced,  and  give  to  the  arteries  a  certain  average  caliber 
which  permits  a  definite  volume  of  blood  to  flow  through  them  in  a 
given  unit  of  time. 

The  cause  of  this  tonic  contraction  is  not  definitely  known.  It 
has  been  attributed  to  the  action  of  local  nerve-ganglia,  to  the  pres- 
sure of  blood  from  within,  to  the  influence  of  organic  substances  in  the 
blood,  the  products  of  gland  activity:  e.  g.,  adrenalin  or  epinephrin. 

This  tonic  contraction  of  the  vascular  muscle  is  subject  to  increase 
or  decrease  in  accordance  with  the  action  of  various  agents.  In- 
creased contraction  will  result  in  a  decrease  of  the  caliber  and  a 
reduction  in  the  outflow  of  blood.  Decrease  of  the  contraction  or 
relaxation  will  result  in  an  increase  both  of  the  caliber  and  outflow 
of  blood.  The  small  arteries  thus  determine  the  volume  of  blood 
passing  to  any  given  area  or  organ  in  accordance  with  its  functional 
activities. 

The  Vaso-motor  Nerves. — The  activities  of  the  vascular  muscle 
are  regulated  by  the  central  nerve  system  through  the  intermedia- 
tion of  nerve-fibers,  termed  vaso-motor  nerves.     Of  these  there  are 


THE  CIRCULATION  OF  THE  BLOOD.  343 

two  kinds,  one  which  increases  or  augments  the  contraction,  the 
vaso-constrictors  or  vaso-augmentors ;  another  which  decreases  or 
inhibits  the  contraction,  the  vaso-dilatators  or  vaso-inhihitors. 

The  vaso-motor  nerves  of  both  classes,  unhke  the  ordinary  motor 
nerves,  do  not  pass  directly  to  the  muscle-fiber,  but  indirectly  by  way 
of  the  ganglia  of  the  sympathetic  nerve  system.  In  these  ganglia 
the  vaso-motor  nerves,  which  come  from  the  central  nerve  system, 
terminate,  breaking  up  into  tufts,  which  arborize  around  the  nerve- 
cells.  From  the  cells  new  nerve-fibers  arise  which  then  pass  without 
interruption  to  their  final  destination. 

The  nerve-fibers  which  emerge  from  the  central  nerve  system 
are  extremely  fine  in  caliber  and  medullated;  those  which  emerge 
from  the  sympathetic  ganglia  are  equally  fine,  but  non-medullated. 
The  former  are  termed  pre- ganglionic,  the  latter  post- ganglionic 
fibers.  The  ganglion  in  which  the  pre-ganglionic  fibers  end  is  not 
necessarily  found  in  the  pre-vertebral  or  lateral  chain;  it  may  be 
found  in  the  collateral  or  even  in  the  peripheral  group. 

The  vaso-constrictor  nerves  take  their  origin  from  nerve-cells 
located  in  the  anterior  horns  and  lateral  gray  matter  of  the  spinal 
cord.  They  emerge  from  the  cord  in  company  with  the  fibers  which 
compose  the  anterior  roots  of  the  spinal  nerves  from  the  second 
thoracic  to  the  second  or  third  lumbar  nerves  inclusive.  A  short 
distance  from  the  cord  they  leave  the  anterior  roots  as  the  white  rami 
communicantes  and  enter  the  pre-vertebral  or  lateral  sympathetic 
ganglia.  From  the  results  of  many  observations  and  experiments 
it  is  probable  that  the  great  majority  of  the  vaso-constrictor  nerves 
terminate  in  these  ganglia;  that  is  to  say,  it  is  here  that  the  pre- 
ganglionic fibers  arborize  around  the  contained  nerve-cells.  From 
the  nerve-cells  new  fibers  arise,  the  post-ganglionic,  which  pass  to  the 
blood-vessels  of  the  head,  to  the  upper  and  lower  extremities,  and  to 
the  thoracic  and  abdominal  viscera. 

The  vaso-constrictors  for  the  head  emerge  from  the  spinal  cord 
in  the  first  four  thoracic  nerves,  thence  pass  successively  into  and 
through  the  ganglion  stellatum  (the  first  thoracic),  the  annulus  of 
Vieussens,  the  inferior  cervical  ganglion,  the  sympathetic  cord  to 
the  superior  cervical  ganglion,  around  the  cells  of  which  they 
arborize.  From  this  ganglion  the  new  fibers  follow  the  carotid 
artery  and  its  branches  to  their  terminations. 

The  vaso-constrictors  for  the  fore-limbs  emerge  from  the  cord  in 
the  roots  of  the  fourth  to  the  tenth  thoracic  nerves  inclusive.  Through 
the  white  rami  they  pass  into  the  sympathetic  chain,  after  which  they 
take  an  upward  direction  and  terminate  around  the  cells  of  the  gan- 
glion stellatum.  From  this  ganglion  the  new  fibers  enter,  by  way  of 
the  gray  rami  communicantes,  the  trunks  of  the  cervical  nerves  which 
unite  to  form  the  brachial  plexus  and  by  this  route  pass  to  the  blood- 
vessels. 


344  TEXT-BOOK  OF  PHYSIOLOGY. 

The  vaso-constrictors  for  the  hind-Hmbs  emerge  from  the  cord 
in  the  roots  of  the  eleventh  dorsal  to  the  second  or  third  lumbar  nerves 
inclusive.  They  then  pass  through  the  white  rami  to  the  lower 
lumbar  and  upper  sacral  ganglia.  Thence  by  way  of  the  gray 
rami  they  pass  into  the  nerve-trunks  which  unite  to  form  the  sacral 
nerves  and  by  this  route  pass  to  the  blood-vessels. 

The  vaso-constrictors  for  the  viscera  of  the  abdominal  cavity  pass 
by  way  of  the  splanchnic  nerves  directly  into  the  collateral  gangha, 
the  semilunar,  the  superior  mesenteric,  the  inferior  mesenteric,  and 
the  sacral.  From  these  ganglia  an  elaborate  network  of  non-medul- 
lated  fibers  passes  to  the  blood-vessels  of  the  stomach,  intestines,  and 
other  viscera.  The  great  splanchnic  nerve  is  one  of  the  most  im- 
portant vaso-constrictor  trunks  of  the  body,  on  account  of  the  large 
vascular  area  it  controls. 

The  existence,  course,  distribution,  and  functions  of  the  vaso- 
constrictor nerves  have  been  determined  by  a  variety  of  methods, 
physiologic  and  anatomic.  Stimulation  of  the  nerve-trunks  under 
appropriate  conditions  gives  rise  to  a  contraction,  division  to  a  dilata- 
tion of  the  blood-vessels.  The  physiologic  continuity  of  the  pre- 
ganglionic fibers  with  the  nerve-cells  of  the  sympathetic  ganglia 
has  been  shown  by  the  intra- vascular  injection  or  the  local  applica- 
tion of  nicotin.  This  agent,  as  shown  by  Langley,  has  a  selective 
action  on  the  arborizations  of  the  pre-ganglionic  fibers,  and  when 
given  in  sufficient  doses  suspends  their  conductivity;  hence  stimu- 
lation of  the  pre-ganglionic  fibers  is  without  effect,  though  stimulation 
of  the  post-ganghonic  fibers  is  followed  by  the  usual  contraction. 

The  following  will  serve  as  illustrations  of  the  functions  of  vaso- 
constrictor nerves.  Division  of  the  great  splanchnic  is  followed  by 
a  marked  dilatation  of  the  blood-vessels  of  the  intestinal  tract ;  stimu- 
lation of  the  peripheral  end  by  their  contraction.  Division  of  the 
cervical  cord  of  the  sympathetic  is  followed  by  dilatation  of  the  blood- 
vessels of  the  side  of  the  head;  stimulation  of  the  peripheral  end  by 
their  contraction. 

The  vaso-dilatator  nerves  have  their  origin  for  the  most  part  in 
nerve-cells  situated  in  the  region  of  the  spinal  cord  included  between 
the  origins  of  the  second  dorsal  to  the  second  lumbar  nerves  inclusive, 
though  they  are  not  confined  to  this  region.  Some  vaso-dilatator 
fibers  have  their  origin  in  the  medulla  oblongata,  others  in  the  sacral 
region  of  the  spinal  cord. 

The  general  course  of  the  dilatator  fibers  for  the  intestinal  tract 
is  the  same  as  that  of  the  vaso-constrictors,  though  instead  of  becom- 
ing related  to  the  nerve-cells  in  the  pre-vertebral  ganglia,  they  pass 
by  way  of  the  splanchnics  to  the  collateral  ganglia,  the  semilunar, 
the  superior  and  inferior  mesenteric,  and  perhaps  to  peripheral 
ganglia  in  or  near  the  blood-vessels  themselves. 


THE  CIRCULATION  OF  THE  BLOOD.  345 

The  vaso-dilatators  for  the  hmbs  are  found  in  the  common  nerve- 
trunks  associated  with  the  usual  motor  and  sensor  fibers,  though  the 
exact  route  by  which  they  pass  from  the  spinal  cord  to  the  peripheral 
nerves  has  not  in  all  cases  been  determined.  Their  cell  stations  have 
not  been  definitely  located.  The  vaso-dilatator  nerves  for  the  blood- 
vessels of  the  submaxillary  gland  arise  in  the  medulla,  pass  outward 
in  the  trunk  of  the  facial  nerve,  and  reach  the  gland  by  way  of  the 
chorda  tympani  branch.  Their  cell  station  is  in  the.  ganghon  near 
the  hilum  of  the  gland.  The  vaso-dilatator  nerves  for  the  blood- 
vessels of  the  corpora  cavernosa  of  the  penis,  the  nervl  erigenles, 
have  their  origin  in  the  sacral  region  of  the  spinal  cord;  and  emerge 
in  the  roots  of  the  second  and  third  sacral  nerves.  Their  cell  station 
is  in  the  ganglion  near  the  organ. 

The  existence,  course,  distribution,  and  functions  of  the  vaso- 
dilatator fibers  have  been  determined  by  the  same  methods  employed 
in  the  investigation  of  the  vaso-constrictors.  Thus  division  and 
stimulation  of  the  peripheral  end  of  the  chorda  tympani  nerve  are 
at  once  followed  by  an  active  dilatation  of  the  blood-vessels  of  the 
submaxillary  gland.  The  inflow  of  blood  is  so  great  that  the  gland 
becomes  bright  red  in  color.  Its  tissues  being  unable  to  appropriate 
all  the  oxygen,  the  blood  emerges  in  the  veins  almost  arterial  in  char- 
acter. Stimulation  of  the  peripheral  ends  of  the  divided  nervi 
erigentes  is  followed  by  similar  effects  in  the  blood-vessels  of  the 
corpora  cavernosa.  Slow  stimulation,  once  per  second,  of  the  periph- 
eral end  of  a  divided  sciatic  nerve  is  followed  by  dilatation  of  the 
blood-vessels  of  the  leg. 

From  these  and  many  other  facts  of  a  similar  character  it  is  highly 
probable  that  the  blood-vessels  of  each  organ  are  under  the  control 
of  two  antagonistic  classes  of  nerve-fibers,  one  augmenting  the 
degree  of  their  contraction,  the  vaso-constrictors,  the  other  diminish- 
ing it  through  inhibition,  the  vaso-inhibitors.  Through  the  coopera- 
tive antagonism  of  these  two  classes  of  nerves  the  cahber  of  the  blood- 
vessels and  thereby  the  volume  of  the  blood  is  accurately  adapted  to 
the  needs  of  each  organ  both  during  rest  and  during  activity.  It 
is  also  to  the  alternate  activity  of  these  nerves  that  the  variations 
occurring  from  time  to  time  in  the  volume  of  organs  are  to  be  at- 
tributed. 

The  vaso-constrictors  and  the  vaso-dilatators  differ  somewhat  in 
their  physiologic  properties,  as  shown  by  the  results  of  experiment. 
Thus,  when  a  mixed  nerve,  i.  e.,  one  containing  both  classes  of 
fibers, — e.  g.,  the  sciatic, — is  stimulated  with  frequently  repeated 
induced  currents,  the  constrictor  effect  is  the  more  pronounced,  the 
dilatator  effect  being  wanting  or  prevented;  when  stimulated  with 
slowly  repeated  induced  currents,  the  dilatator  effect  is  the  more 


346 


TEXT-BOOK  OF  PHYSIOLOGY. 


pronounced.    These  different  effects  are  strikingly  shown  in  Fig.  156, 
A  and  B. 

In  the  experiment  of  which  these  tracings  are  the  result  the  leg 
of  a  cat  was  enclosed  in  a  plethysmograph  and  the  variations  in  volume 
due  to  dilatation  or  contraction  of  the  vessels,  following  stimulation 
of  the  sciatic  nerve,  were  recorded  by  means  of  a  tambour  and  lever 
on  a  slowly  revolving  cylinder.  In  A  the  fall  of  the  curve  indicates 
a  diminution  of  volume,  from  contraction  of  blood-vessels  following 
a  rate  of  stimulation  of  the  sciatic  nerve  of  16  per  second  for  fifteen 
seconds.  In  B  the  rise  of  the  curve  indicates  an  increase  in  volume 
from  dilatation  of  the  vessels  following  a  rate  of  stimulation  of  i  per 
second  for  fifteen  seconds  (Bowditch  and  Warren).  With  different 
rates  of  stimulation  somewhat  different  results  are  obtained. 

After  division  of  a  mixed  nerve  the  vaso-constrictors  degenerate 
and  lose  their  influence  over  the  blood-vessel  in  from  four  to  five 

days,  the  vaso- dilatators  in 
from  seven  to  ten  days,  as 
shown  by  the  response  to 
electrical  stimulation. 


^mm^ 

1 

i 

/%Krtl 

Fig.  156. — Plethysmogeams  of  the  Hind-leg  of  the  Cat  following  Stimulation 
OF  the  Sciatic  Nerve.  In  A  the  rate  of  stimulation  was  sixteen  per  second,  in 
B  one  per  second  for  fifteen  seconds. 


When  a  nerve  is  cooled,  the  vaso-constrictors  lose  their  irritability 
before  the  vaso-dilatators. 

Vaso-motor  Centers. — The  nerve-cells  throughout  the  spinal 
cord  from  which  the  vaso-motor  nerves  take  their  origin  may  be 
regarded  as  nerve-centers  which  through  their  related  nerve-fibers 
exert  from  time  to  time  either  a  constrictor  or  a  dilatator  influence 
over  the  blood-vessels.  Though  both  the  vaso-constrictor  and  vaso- 
dilatator centers  are  in  a  state  of  continuous  activity,  the  former 
decidedly  preponderates,  as  shown  by  the  maintenance  of  a  tonic 
contraction  of  the  blood-vessels.  The  activity  of  both  centers  may 
be  increased  or  decreased,  augmented  or  inhibited,  by  nerve  impulses 
reflected  to  them  from  the  periphery  through  afferent  nerves  or 


THE  CIRCULATION  OF  THE  BLOOD.  347 

through  nerve-fibers  descending  the  cord  from  higher  levels  of  the 
nervous  system.  Experiment  has  shown  that  when  a  definite  region 
of  the  medulla  oblongata  is  punctured  or  in  anywise  destroyed  there 
is  an  immediate  dilatation  of  the  blood-vessels  throughout  the  body 
and  a  fall  of  blood-pressure  below  one-half  or  one-third  of  the  normal 
value.  This  region  has  a  width  of  one  and  a  half  millimeters  and 
extends  longitudinally  for  a  distance  of  four  or  five  miUimeters, 
terminating  at  a  point  four  milhmeters  above  the  tip  of  the  calamus 
scriptorius. 

A  transection  of  the  medulla  above  the  upper  Hmit  of  this  area  is 
without  effect  on  the  blood-pressure.  A  similar  section  below  it, 
however,  is  at  once  followed  by  vascular  dilatation,  a  loss  of  vascular 
tone,  and  a  general  fall  of  blood-pressure.  Subsequent  stimulation 
of  the  peripheral  end  of  the  divided  medulla,  the  animal  being  curar- 
ized  and  artificial  respiration  maintained,  will  give  rise  to  a  marked 
contraction  of  the  blood-vessels  and  a  rise  of  blood-pressure  up  to 
and  far  beyond  the  normal  value. 

If  the  experimental  lesion  is  limited  to  the  area  mentioned  in  the 
foregoing  paragraph,  the  vascular  dilatation  passes  away  after  a 
time,  the  blood-vessels  regain  their  normal  tone,  and  the  pressure 
again  rises.  These  facts  indicate  that  the  area  is  to  be  regarded  as 
the  general  vaso-motor  {constrictor)  center  which  maintains  the  tonus 
of  the  blood-vessels  through  its  dominating  influence  over  the  vaso- 
motor centers  in  the  cord,  the  latter  acting  in  a  subsidiary  manner  to 
the  former.  The  nerve-fibers  which  transmit  the  regulative  nerve 
impulses  from  the  general  to  the  subsidiary  centers  are  found  in  the 
lateral  columns  of  the  cord.  There  is  no  evidence  for  the  existence 
of  a  general  vaso-dilatator  center  in  the  medulla.  Since  the  blood- 
vessels maintain  a  more  or  less  constant  tone,  it  is  assumed  that  the 
vaso-motor  centers  are  in  a  state  of  continuous  activity.  In  how  far, 
however,  this  activity  is  the  result  of  chemic  changes  between  the 
cells  and  the  surrounding  lymph  and  blood,  or  the  result  of  con- 
stantly arriving  nerve  impulses  reflected  from  the  periphery  or  from 
higher  regions  of  the  nervous  system,  is  not  readily  determinable. 
Both  factors  are  probably  involved. 

Direct  Stimulation  of  the  Vaso-motor  Centers. — The  general 
vaso-motor  (constrictor)  center  at  least  is  markedly  influenced  by 
the  quantity  and  quality  of  blood  and  lymph  circulating  around  and 
through  it.  If  the  blood-supply  to  the  medulla  and  associated 
structures  be  diminished  by  compression  of  the  carotid  arteries,  the 
activity  of  the  center  is  at  once  increased,  as  shown  by  increased  vas- 
cular contraction  and  a  rise  of  pressure.  Restoration  of  the  blood- 
supply  is  followed  by  a  return  of  the  center  to  its  normal  degree  of 
activity.  Increased  blood-supply,  as  in  cerebral  hyperemia,  is  at- 
tended by  a  fall  in  blood-pressure  indicating  a  decrease  in  the  activity 


34S  TEXT-BOOK  OF  PHYSIOLOGY. 

of  the  center.  A  diminution  in  the  percentage  of  oxygen  or  an  in- 
crease in  the  percentage  of  COj  in  the  blood  will  increase  the  activity 
of  the  center.  In  asphyxia  especially,  the  center  is  extremely  excit- 
able, as  shown  by  a  rise  of  the  arterial  tension.  The  subsidiary 
centers  in  the  spinal  cord  are  influenced  by  corresponding  conditions. 

Reflex  Stimulation  of  the  Vaso-motor  Centers. — The  results 
of  experiment  make  it  certain  that  the  degree  of  vascular  contraction 
maintained  by  the  cooperative  antagonism  of  the  vaso-motor  centers 
can  be  increased  or  decreased  by  nerve  impulses  reflected  to  the  cord 
and  medulla  from  the  periphery  or  from  the  brain.  The  effect  may 
be  general,  or  local  and  confined  to  the  area  from  which  the  im- 
pulses arise.     The  following  experiments  may  be  cited  as  illustrations : 

Stimulation  of  the  central  end  of  a  divided  posterior  root  of  a 
spinal  nerve  gives  rise  to  increased  vascular  contraction,  as  shown 
by  the  rise  of  blood-pressure.  The  same  holds  true  for  any  sensory 
nerve.  Stimulation  of  the  central  end  of  the  divided  sciatic  will  give 
rise  to  opposite  results,  according  to  the  strength  of  the  stimulus, 
weak  stimuli  producing  dilatation,  strong  stimuli  producing  contrac- 
tion of  the  vessels.  Stimulation  of  the  central  end  of  the  divided 
vagus  gives  rise  to  dilatation  of  the  vessels  of  the  lips,  cheeks,  and  nasal 
and  palatal  mucous  membranes.  Stimulation  of  the  auricular  nerve 
in  the  rabbit  will,  according  to  the  strength  of  the  currents,  give  rise 
to  either  contraction  or  dilatation.  Stimulation  of  the  tongue  is 
followed  by  dilatation  of  the  vessels  of  the  submaxiUary  gland. 

In  explanation  of  these  different  results  it  has  been  assumed  that 
in  the  afferent  nerves  there  are  two  classes  of  fibers,  one  which  in- 
creases, the  other  decreases  the  activity  of  the  vaso-constrictor  centers. 
The  first  is  termed  pressor,  the  second  depressor.  Inasmuch  as  the 
vascular  dilatation  is  often  greater  than  the  dilatation  which  follows 
division  of  the  vaso-motor  fibers  themselves,  it  has  been  assumed 
by  some  that  the  general  vascular  tonus,  as  well  as  its  variations  from 
time  to  time,  is  the  resultant  of  the  simultaneous  activity  and  varia- 
tions in  activity  of  both  vaso-constrictor  and  vaso-dilatator  centers; 
that  in  the  afferent  nerves  there  are  fibers  which  when  stimulated 
augment,  for  example,  the  vaso-constrictor  center  and  inhibit  the 
vaso-dilatator  centers,  or  the  reverse.  The  result,  either  contraction 
or  dilatation,  which  follows  stimulation  of  their  peripheral  termina- 
tions will  depend  on  the  character  of  the  physiologic  stimulus. 

In  a  similar  manner  the  vaso-motor  centers  are  influenced  by 
emotional  states,  fear  causing  a  contraction,  shame  a  dilatation,  of 
the  vessels  of  the  face  and  neck.  It  is  probable  that  these  effects  are 
due  to  the  transmission  of  nerve  impulses  from  the  cortex  to  the  vaso- 
motor centers  in  the  medulla. 

The  Depressor  Nerve.— In  the  rabbit  there  is  a  small  nerve 
formed  by  the  union  of  a  branch  from  the  trunk  of  the  vagus  with  a 


THE  CIRCULATION  OF  THE  BLOOD.  349 

branch  from  the  superior  laryngeal.  The  peripheral  distribution  of 
this  nerve  is  over  the  wall  of  the  ventricle.  The  same  nerve  is  found 
in  many  other  animals.  In  some,  as  the  dog,  it  is  bound  up  in  the 
vago-sympathetic.  In  man  it  is  also  present,  though  shortly  after 
its  origin  it  enters  the  trunk  of  the  vagus.  Division  of  this  nerve  is 
without  effect  either  on  the  heart  or  the  vessels.  Stimulation  of  the 
peripheral  end  has  neither  an  accelerator  nor  an  inhibitor  action 
on  the  heart.  Stimulation  of  the  central  end  is  followed  by  a  fall  in 
blood-pressure,  frequently  to  a  level  below  one-half  the  normal  value ; 
at  the  same  time  there  is  a  diminution,  brought  about  reflexly,  in  the 
rate  of  the  heart-beat.  The  fall  in  pressure,  however,  is  not  due  to 
this  cause,  for  it  occurs  equally  well  after  division  of  all  the  cardiac 
nerves.  For  this  reason  the  nerve  was  termed  the  depressor  nerve. 
On  exposure  of  the  abdominal  cavity,  it  is  observed  during 
stimulation  of  the  depressor  that  there  is  a  notable  dilatation  of  the 
intestinal  vessels.  From  this  fact  it  was  assumed  that  the  action  of 
the  depressor  nerve  was  to  lower  the  general  pressure  through  reflex 
dilatation  of  these  vessels.  It  has  been  shown  by  Porter  and  Beyer 
that  if  the  splanchnics  are  divided  and  the  peripheral  end  stimulated 
so  as  to  maintain  the  tonus  of  the  intestinal  vessels,  and  hence  the 
general  pressure,  stimulation  of  the  depressor  nerve  will  nevertheless 
be  followed  by  a  fall  of  the  blood-pressure  almost  as  great  as  when 
the  splanchnics  are  intact.  From  this  it  is  evident  that  the  depressor 
nerve  is  related  to  centers  which  influence  the  vascular  apparatus  in  its 
entirety.  It  has  been  supposed  that  through  it  the  heart  can  protect 
itself  from  injurious  results  of  an  excessive  rise  of  arterial  pressure. 


CHAPTER    XIII. 
RESPIRATION. 

Respiration  is  a  process  by  which  oxygen  is  introduced  into,  and 
carbon  dioxid  removed  from,  the  body.  The  assimilation  of  the 
former  and  the  evolution  of  the  latter  take  place  in  the  tissues  as  a 
part  of  the  general  process  of  nutrition.  Without  a  constant  supply 
of  oxygen  and  an  equally  constant  removal  of  the  carbon  dioxid, 
those  chemic  changes  which  underhe  and  condition  all  life  phenom- 
ena could  not  be  maintained. 

The  general  process  of  respiration  may  be  considered  under  the 
following  headings,  viz. : 

1.  The  anatomy  and  general   arrangement  .of  the  respirator}'  ap- 

paratus. 

2.  The  mechanic  movements  of  the  thorax  by  which  an  interchange 

of  atmospheric  and  intra-pulmonary  air  is  accomphshed. 

3.  The  chemistry  of  respiration,  the  changes  in  composition  ex- 

perienced by  the  air,  blood,  and  tissues. 

4.  The  nerve  mechanism  by  which  the  respiratory  movements  are 

maintained. 

THE  RESPIRATORY  APPARATUS. 

The  respiratory  apparatus  consists  essentially  of: 

1.  The  lungs  and  the  air-passages  leading  into  them:  viz.,  the  nasal 

chambers,  mouth,  pharynx,  larynx,  and  trachea. 

2.  The  thorax  and  its  associated  structures. 

The  nasal  chambers  are  the  natural  entrances  for  the  inspired 
air.  Their  comphcated  structure  slightly  retards  the  movement  of 
the  air,  in  consequence  of  which  its  temperature  and  moisture  are 
adjusted  to  the  physiologic  conditions  of  the  lower  respiratory  pas- 
sages. The  mouth,  though  frequently  serving  as  an  entrance  for  air, 
is  not  primarily  a  respiratory  passage.  Both  the  nasal  chambers 
and  the  mouth  communicate  posteriorly  with  the  pharynx,  in  which 
the  respiratory  and  the  deglutitory  passages  cross  each  other,  the 
former  leading  directly  into  the  \a.rynx. 

The  larynx  is  a  comphcated  mechanism  serving  the  widely 
different  though  related  functions  of  respiration  and  phonation.  It 
consists  of  a  framework  of  cartilages,  articulating  one  with  another, 
united  by  hgaments  and  moved  by  muscles;  it  is  covered  externally 
with  fibrous  tissue  and  hned  with  mucous  membrane.     The  superior 

350 


RESPIRATION. 


351 


opening  of  the  larynx,  the  glottis,  is  triangular  in  shape,  the  base 
being  directed  upward  and  forward,  the  apex  downward  and  back- 
ward.    The  inchnation  of  the  glottic  opening  is  almost  vertical. 
Kt  The  cavity  of  the  larynx  is  partially  subdivided  by  the  inter- 
position of  the  vocal  bands  into  a  superior  and  an  inferior  portion. 


Fig.  157. — Trachea  and  Bronchial  Tubes,  i,  2.  Larynx.  3,  3.  Trachea.  4. 
Bifurcation  of  trachea.  5.  Right  bronchus.  6.  Left  bronchus.  7.  Bronchial 
division  to  upper  lobe  of  right  lung.  8.  Division  to  middle  lobe.  9.  Division 
to  lower  lobe.  10.  Division  to  upper  lobe  of  left  lung.  11.  Division  to  lower 
lobe.  12,  12,  12,  12.  Ultimate  ramifications  of  bronchi.  13,  13,  13,  13. 
Lungs,  represented  in  contour.  14,  14.  Summit  of  lungs.  15,  15.  Base  of 
lungs. — {Sappey.) 


The  opening,  bounded  by  the  vocal  bands,  is  also  triangular  in  shape, 
though  in  this  case  the  base  is  directed  backward,  the  apex  forward. 
(See  chapter  on  Voice  and  Speech.) 

The  introduction  of  the  vocal  bands  narrows  at  this  level  the  air- 
passage  and  to  some  extent  interferes  with  the  free  entrance  of  air. 


352 


TEXT-BOOK  OF  PHYSIOLOGY. 


According  to  the  investigations  of  Semon,  the  area  of  the  air-passage 
above  and  below  the  phonatory  apparatus  is  about  200  sq.  mm.; 
while  the  area  bounded  by  the  vocal  apparatus  is  but  155  sq.  mm. 
during  quiet  respiration. 

The  trachea  is  a  tube,  some  12  centimeters  in  length,  from  one- 
half  to  three-fourths  of  a  centimeter  in  breadth,  extending  from  the 
lower  border  of  the  larynx  to  a  point  opposite  the  fifth  dorsal  verte- 
bra. It  consists  of  an  external  fibrous  and  an  internal  mucous 
membrane,  between  which  is  a  series  of  superposed  C-shaped  arches 
or  rings  of  elastic  cartilage,  some  18  or  20  in  number.  Between  the 
fibrous  and  mucous  coats  posteriorly,  and  occupying  the  space  be- 
tween and  attached  to  the  free  ends  of  the  cartilages,  there  is  a  layer 
of  transversely  arranged  non-striated  muscle-fibers,  known  as  the 
tracheal  muscle.     The  contraction  of  this  muscle  would  approximate 

the  ends  of  the  arches  and  so  di- 
minish the  caliber  of  the  tube. 
The  surface  of  the  mucous  mem- 
brane is  covered  by  a  layer  of 
stratified  columnar  ciHated  epi- 
thelium (Fig.  158).  In  the  sub- 
mucous tissue  there  are  a  number 
of  glands  the  ducts  of  which  open 
on  the  free  surface. 

Opposite  the  fifth  dorsal  ver- 
tebra the  trachea  divides  into  a 
right  and  a  left  bronchus.  Each 
bronchus  again  subdivides  into 
two  or  three  branches,  which 
penetrate  the  corresponding  lung. 
The  lungs  in  the  physiologic 
condition,  occupy  the  greater  part 
of  the  cavity  of  the  thorax.  They  are  separated  from  each  other 
by  the  contents  of  the  mediastinal  space:  viz.,  the  heart,  the  large 
blood-vessels,  the  esophagus,  etc.  Each  lung  is  somewhat  pyramidal 
in  shape  with  the  apex  directed  upward.  The  outer  surface  is  con- 
vex and  corresponds  to  the  general  conformation  of  the  thorax. 
The  inner  surface  is  concave  and  accommodates  the  contents  of  the 
mediastinal  space.  At  about  the  middle  of  the  inner  surface  there 
enter  the  lung,  the  bronchi,  and  blood-vessels.  The  under  surface 
of  the  lung  is  concave  and  rests  on  the  diaphragm.  The  posterior 
border  is  convex;  the  anterior  border  is  thin. 

A  histologic  analysis  of  the  lung  shows  it  to  consist  of  the  branches 
of  the  bronchi,  their  subdivisions  and  ultimate  terminations,  blood- 
vessels, lymphatics  and  nerves,  imbedded  in  a  stroma  of  fibrous  and 
elastic  tissue.  The  anatomic  relations  which  these  structures  bear 
one  to  another  is  as  follows:  — 


Fig.  158. — Transverse  Section  of 
THE  Trachea  of  a  Kitten. — ■ 
{Stirliftg.) 


RESPIRATION. 


153 


Bronchiole. .. 


Infundibulum 


Fig.  159. — Scheme  or  a  Bronchiole  Terminat- 
ing IN  Alveolar  Passages,  those  Leading 
into  Infundibula  beset  with  Air-cells. — 
{Landois  and  Stirling.) 


Within  the  substance  of  the  lung  the  bronchi  divide  and  subdivide, 
giving  origin  to  a  large 
number  of  smaller 
branches,  the  bronchial 
tubes,  which  penetrate 
the  lung  in  all  directions. 
With  this  repeated  sub- 
division the  tubes  become 
narrower,  their  walls  thin- 
ner, their  structure  sim- 
pler. In  passing  from 
the  larger  to  the  smaller 
tubes  the  cartilaginous 
arches  become  shorter 
and  thinner,  and  finally 
are  represented  by  small 
angular  and  irregularly 
disposed  plates.  In  the 
smallest  tubes  the  carti- 
lage entirely  disappears. 
With  the  diminution  of 
the  caliber  of  the  tube 
and   a   decrease    in    the 

thickness  of  its  walls,  there  appears  a  layer  of  non-striated  muscle- 
fibers  between  the  mucous  and  submucous 
tissues,  which  completely  surrounds  the 
tube  and  becomes  especially  well  developed 
in  those  tubes  devoid  of  cartilage.  The 
fibrous  and  mucous  coats  at  the  same  time 
diminish  in  thickness. 

When  the  bronchial  tube  has  been  re- 
duced to  the  diameter  of  about  one  milli- 
meter, it  is  known  as  a  bronchiole  or  a  ter- 
minal bronchus.  From  the  sides  of  the 
terminal  bronchus  and  from  its  final  ter- 
mination there  is  given  off  a  series  of  short 
branches  which  soon  expand  to  form  lobules 
or  alveoh  (Fig.  159).  The  cavity  of  the 
alveolus  is  termed  the  infundibulum. 
From  the  inner  surface  of  the  alveolus  and 
of  the  passageway  leading  into  it,  there 
project  thin  partitions  which  subdivide  the 
outer  portion  of  the  general  cavity  or  in- 
fundibulum into  small  spaces  the  so-called 
air-sacs  or  air-cells  (Fig.  160).  The  wall  of  the  alveolus  is  extremely 
23 


Fig.  160. — Single  Lobule 
OF  Human  Lung.  a. 
Alveolar  passage,  b. 
Cavity  of  lobule  or  in- 
fundibulum. c.  Pul- 
monary sacs. — {Dal- 
ton.) 


354 


TEXT-BOOK  OF  PHYSIOLOGY. 


thin  and  consists  of  fibro-elastic  tissue,  supporting  a  very  elaborate 
capillary  network  of  blood-vessels.     The  bronchial  system  as  far  as 

the  alveolar  passages 
is  lined  by  ciliated  epi- 
thelium.  The  air- 
sacs  are  lined  by  flat 
epithehal  plates  of  ir- 
regular shape,  termed 
the  respiratory  epithe- 
lium (Fig.  i6i).  The 
alveoli  are  united  one 
to  another  by  fibro- 
elastic  tissue. 

In  consequence  of 
the  presence  of  the 
elastic  tissue,  the 
lungs  are  distensible 
and  elastic.  After  re- 
moval from  the  body 
the  elastic  tissue  at 
once  recoils,  forcing 
out  a  portion  of  the 
contained  air.  The 
Under  pressure,  how- 
These  proper- 


FlG, 


i6i. — Section  of  Silvered  Lung  of  Kitten, 
INCLUDING  Portions  of  Infundibulum  and 
Air-sac.  a.  Small  polyhedral  epithelial  cells 
covering  the  wall  of  the  infundibulum.  b.  Fibro- 
elastic  framework,  c.  Large  flattened  epithelial 
plates  lining  air-sac,  among  which  lie  small  groups 
of  small  cells  {d). — {Pier sol.) 


condition  of  the  lung  is  now  one  of  collapse 

ever,  the  lung  can  be  readily  distended  or  inflated. 

ties    endure    for   a    long    period 

after  death,  if  not  indefinitely,  if 

the  lungs  are  properly  preserved. 

The  capacity  of  the  lungs  can  be 

made  to  vary  within  rather  wide 

limits  in  virtue  of  the  presence  of 

the  elastic  tissue. 

The  Pulmonary  Blood-ves- 
sels.— The  pulmonary  artery 
which  conducts  the  venous  blood 
from  the  heart  to  the  lungs  di- 
vides beneath  the  arch  of  the 
aorta  into  a  right  and  a  left 
branch.  Each  branch  with  its 
subdivisions  enters  the  lung  at 
the  hilum  in  company  with  the 
larger  divisions  of  the  bronchi. 
Within  the  lung  the  arteries  di- 
vide and  subdivide  in  a  manner  corresponding  to  that  of  the  bron- 
chial tubes,  which  they  follow  to  their  ultimate  terminations.     As 


Fig.  162. — The  Relation  of  the  Pul- 
monary Artery,  PA,  and  the  Pul- 
monary Vein,  PV,  to  the  Lobules, 
A  A.     B.  The  Bronchiole. 


RESPIRATION. 


355 


the  pulmonary  lobules  are  approached,  a  small  arterial  branch 
plunges  into  the  wall  of  the  lobule  (Fig.  162),  in  which  it  forms 
an  elaborate  capillary  network  which  surrounds  and  embraces  the 
air-sacs  on  all  sides.  As  this  network  is  to  subserve  the  respir- 
atory exchange  of  gases  it  lies  nearer  the  inner  than  the  outer  surface 
of  the  lobule  and  in  close  relation  to  the  respiratory  epithelium. 
The  air  and  blood  are  thus  brought  into  intimate  relationship,  being 
separated  only  by  the  respiratory  epithelium  and  the  wall  of  the  capil- 


FiG.  163. — Bronchi  and  Lungs,  Posterior  View.  1,1.  Summit  of  lungs.  2, 2.  Base 
of  lungs.  3.  Trachea.  4.  Right  bronchus.  5.  Division  to  upper  lobe  of  lung. 
6.  Division  to  lower  lobe.  7.  Left  bronchus.  8.  Di\ision  to  upper  lobe.  g. 
Division  to  lower  lobe.  10.  Left  branch  of  pulmonar}' artery.  11.  Right  branch. 
12.  Left  auricle  of  heart.  13.  Left  superior  pulmonary  vein.  14.  Left  inferior 
pulmonary  vein.  15.  Right  superior  pulmonary  vein.  16.  Right  inferior  pul- 
monary vein.  17.  Inferior  vena  cava.  18.  Left  ventricle  of  heart  10.  Ri.'ht 
ventricle. — (Sappey.) 


lary  vessel.  The  blood  emerging  from  the  capillary  ^•essels  is  con- 
ducted by  a  corresponding  converging  system  of  vessels,  the  pulmon- 
ary veins,  out  of  the  lungs  and  into  the  left  auricle  of  the  heart.  The 
main  function  of  the  pulmonary  apparatus  and  the  pulmonar}^  divi- 
sion of  the  circulatory  apparatus  is  to  afford  a  ready  means  for  the 
exhalation  of  the  carbon  dioxid  and  the  absorption  of  oxygen.  In 
consequence  of  this  exchange  of  gases  the  blood  changes  in  color  from 
dark  bluish-red  to  scarlet  red.  The  relations  of  the  heart  and  its 
vessels  to  the  lungs  and  bronchial  tubes  are  shown  in  Fig.  163. 


356 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  Thorax. — The  thorax,  in  which  the  respiratory  organs  and 
their  associated  structures  are  lodged,  is  conic  in  shape,  though 
somewhat  compressed  from  before  backward.  Its  apex  is  directed 
upward,  its  base  downward.  The  walls  of  the  thorax  are  composed, 
first,  of  a  bony  framework  or  skeleton  and,  second,  of  muscles  and 
fascia.  The  bony  framework  is  formed  posteriorly  by  the  thoracic 
vertebrae  and  the  posterior  extremities  of  the  ribs,  laterally  by  the 
ribs,  and  anteriorly  by  the  costal  cartilages  and  the  sternum.     The 

superior  opening,  through  which 
pass  the  trachea,  esophagus,  and 
blood-vessels,  is  oval  in  outhne 
and  measures  from  side  to  side 
about  12.5  cm.,  and  from  before 
backward  about  6.25  cm.  The 
interior  opening  is  of  large  size, 
but  irregular  in  its  boundaries 
from  the  upward  inclination  of 
the  ribs  and  the  downward  pro- 
jection of  the  sternum. 

The  ribs  which  form  a  large 
part  of  the  thoracic  walls  consti- 
tute a  series  of  bony  arches  at- 
tached posteriorly  to  the  vertebrae 
and  anteriorly  to  the  sternum 
through  the  intermediation  of 
their  cartilages.  The  last  two 
form  an  exception.  The  ribs  are 
somewhat  twisted  upon  them- 
selves and  pursue  an  obhque 
direction  from  above  downward 
and  forward.  As  a  result  the 
anterior  extremity  lies  at  a  lower 
level  than  the  posterior.  The 
costal  cartilages  are  directed 
upward  and  forward,  with  the 
exception  of  the  upper  three,  which  are  almost  horizontal.  The 
general  arrangement  and  appearance  of  the  thorax  are  shown  in 
Fig.   164. 

The  costo-vertebral  and  costo-chondral  and  the  chondro-sternal 
articulations  are  diarthrodial  in  character  and  endow  the  thoracic 
walls  with  a  considerable  degree  of  mobility.  The  costo-vertebral 
joints  are  two  in  number,  the  first  being  formed  by  the  beveled  head 
of  the  rib  and  the  bodies  of  two  adjoining  vertebrae;  the  second,  by 
the  tubercle  of  the  rib  and  the  transverse  process.     The  costo-chon- 


FiG.  164. — Thorax,  Anterior  View. 
I.  Manubrium  sterni.  2.  Gladio- 
lus. 3.  Ensiform  cartilage  of 
xiphoid  appendix.  4.  Circumference 
of  apex  of  thorax.  5.  Circum- 
ference of  base.  6.  First  rib.  7. 
Second  rib.  8,  8.  Third,  fourth, 
fifth,  sixth,  and  seventh  ribs.  9. 
Eighth,  ninth,  and  tenth  ribs.  10. 
Eleventh  and  twelfth  ribs.  11,  11. 
Costal  cartilages. 


RESPIRATION. 


00/ 


dral  and  the  chondro-sternal  articulations,  as  their  names  imply, 
are  formed  by  the  ribs,  cartilages,  and  sternum. 

The  muscles  which  complete  the  formation  of  the  thoracic  walls 
are  as  follows:  the  diaphragm,  the  intercostales  externi  and  interni, 
the  levatores  costarum,  the  triangularis  sterni,  and  the  infra-cos- 
tales. 

The  diaphragm  is  the  musculo-membranous  sheet  which  closes 
the  inferior  open- 
ing of  the  thorax 
and  completely 
separates  its  cav- 
ity from  that  of 
the  abdomen.  It 
consists  of  two 
muscles  which 
arise  from  the 
bodies  of  the  first 
three  or  four 
lumbar  vertebras 
and  neighboring 
fascia,  from  the 
border  of  the  six 
lower  ribs,  and 
from  the  ensi- 
form  cartilage 
(Fig.  165).  From 
this  extensive 
origin  the  mus- 
cle fibers  pass 
centrally  to  be 
inserted  into  a 
common  tendon. 
As  the  direction 
of  the  fibers   is 

from  below  upward  and  inward,  the  diaphragm  is  somewhat  dome- 
shaped.  Its  inferior  border  is  for  a  short  distance  in  contact  with 
the  sides  of  the  thorax. 

The  intercostales  externi,  eleven  in  number  on  each  side,  occupy 
the  spaces  between  the  ribs  to  which  they  are  attached  from  the 
tubercle  to  the  anterior  extremity  (Fig.  166  and  167).  Their  fibers, 
which  are  arranged  in  parallel  bundles,  are  directed  from  above 
downward  and  from  behind  forward.  The  point  of  attachment, 
therefore,  of  any  given  bundle  of  fibers  to  the  rib  above,  lies  nearer 
the  vertebral  column,  nearer  the  fulcrum,  than  the  point  of  attach- 
ment below. 


Fig.  165. — Diaphragm,  Inferior  Aspect,  i.  Anterior  and 
middle  leaflet  of  central  tendon.  2.  Right  leaflet.  3. 
Left  leaflet.  4.  Right  crus.  5.  Left  crus.  6,  6.  In- 
tervals for  phrenic  nerves.  7.  Muscular  fibers,  from 
which  the  Hgamenta  arcuata  originate.  8.  Muscular  fi- 
bers that  arise  from  the  inner  surface  of  the  six  lower  ribs. 
9.  Fibers  that  arise  from  ensiform  cartilage.  10.  Open- 
ing for  inferior  vena  cava.  11.  Opening  for  esophagus. 
12.  Aortic    opening.     13,    13.  Upper    portion    of    trans- 


versaUs  abdominis,  turned  upward  and  outward. 
Anterior    leaflet    of    transversalis    aponeurosis.     15, 
Quadratus    lumborum.     16,    16.  Psoas    magnus. 
Third  lumbar  vertebra. 


14. 

15- 
17- 


358 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  intercostales  interni,  eleven  in  number,  occupy  the  spaces 
between  and  are  attached  to  the  ribs  from  the  tubercle  to  the  anterior 
extremity  of  the  cartilages.  Their  fibers,  which  are  also  arranged 
in  parallel  bundles,  are  directed  from  above  downward  and  back- 
ward (Fig.  1 66  and  167). 

The  levatores  costarum  are  twelve  in  number  on  either  side.     They 

arise  from  the  tips  of 
>2^    -  the    transverse    pro- 

cesses of  the  last  cervi- 
cal and  the  thoracic 
vertebrae  with  the  ex- 
ception of  the  last. 
From  the  point  of 
origin  the  fibers  pass 
downward  and  out- 
ward in  a  diverging 
manner  to  be  inserted 
into  the  ribs  between 
the  tubercle  and  the 
angle.  Their  action, 
as  their  name  implies, 
is  to  elevate  the  pos- 
terior portion  of  the 
ribs  (Fig.  167). 

The  triangularis 
sterni  arises  from  the 
side  of  the  posterior 
surface  of  the  lower 
third  of  the  sternum 
and  is  inserted  by 
fleshy  shps  into  the 
cartilages  of  the  ribs 
from  the  second  to 
the  sixth. 

From  the  fact  that 
the  inferior  opening 
of  the  thorax  as  well 
as  the  intercostal 
spaces  are  completely 
closed  by  the  foregoing 
muscles,  and  from  the 
further  fact ,  that  the 
superior  is  closed  by 
fascia  except  at  those  points  through  which  pass  the  trachea,  blood- 
vessels and  esophagus,  the  cavity  of  the  thorax  is  absolutely  air-tight. 


Fig.  166. — Showing  the  Situation,  the  Points  of 
Attachment,  and  Direction  of  the  Inter- 
costal Muscles,  i.  The  intercostales  externi. 
2.  The  intercostales  interni.  3.  The  intercarti- 
laginei. — (Deaver.) 


RESPIRATION. 


359 


The  Pleurae. — Each  lung  is  surrounded  by  a  closed  invaginated 
serous  sac,  the  pleura,  of  which  the  inner  portion  is  reflected  over 
and  is  closely  adherent  to  the  surface  of  the  entire  lung  as  far  as  its 
root;  the  outer  portion  is  reflected  over  the  inner  wall  of  the  thorax, 
the  superior  surface  of  the  diaphragm,  and  the  viscera  of  the 
mediastinum.  Under  normal  conditions  these  two  layers  of  the 
pleura,  the  visceral  and  parietal,  are  in  contact,  or  at  most  separated 
only  by  a  thin  capillary  layer  of  lymph.  The  presence  of  this  fluid 
prevents  friction  as  the  two  surfaces  play  against  each  other  in 
consequence  of  the  movements  of  the  lungs. 


THE  MECHANIC  MOVEMENTS  OF  THE  THORAX. 

The  blood  receives  oxygen  from,  and  yields  carbon  dioxid  to,  the 
alveoh  of  the  lungs,  as  it  flows  through  the  pulmonary  capillaries. 
That  this  exchange  of  gases  may  continue,  it  is  of  primary  impor- 
tance that  the  air  within  the  alveoh  be  removed  as  rapidly  as  it  is 
vitiated.  This  is  accomphshed  by  an  alternate  increase  and  decrease 
in  the  capacity  of  the  thorax,  accom- 
panied by  corresponding  changes  in 
the  capacity  of  the  lungs.  During 
the  former  there  is  an  inflow  of 
atmospheric  air  (inspiration),  during 
the  latter  an  outflow  of  intrapul- 
monary  air  (expiration).  The  con- 
tinuous recurrence  of  these  two 
movements  brings  about  that  de- 
gree of  pulmonary  ventilation  nec- 
essary to  the  normal  exchange  of 
gases  between  the  blood  and  the  air. 
The  two  movements  together  con- 
stitute a  respiratory  act  or  cycle. 

In  the  course  of  the  respiratory 
cycles  the  thorax  presents  alter- 
nately a  short  period  of  rest — viz., 
between  the  end  of  an  expiration 
and  the  beginning  of  an  inspiration 
— and  a  relatively  long  period  of 
activity,  including  both  inspiration 

and  expiration.  The  former  may  be  regarded  as  the  static,  the 
latter  as  the  dynamic  condition  of  the  thorax.  In  the  static  condi- 
tion, the  thorax  and  its  contained  and  associated  organs  sustain  a 
definite   relation   one  to  another;  in  the  dynamic  conditions  these 


Fig.  167. — View  from  behind  of 
Four  Dorsal  Vertebrae  and 
Three  Attached  Ribs,  show- 
ing THE  Attachment  of  the 
Elevator  Muscles  of  the  Ribs 
and  the  Intercostals.  I 
Long  and  short  elevators.  2. 
External  intercostal.  3.  Internal 
intercostal. — {A  llefi  Thomson.) 


36o  TEXT-BOOK  OF  PHYSIOLOGY. 

relations  undergo  a  change  the  extent  of    which   is  proportional 
to  the  extent  of  the  movements.* 


THE  STATIC  CONDITION. 

Relation  of  the  Thoracic  Organs. — Intra-pulmonary  Pres- 
sure: Intra-thoracic  Pressure. — In  the  static  condition  of  the 
thorax  the  lungs,  by  virtue  of  their  distensibility,  completely  till 
all  parts  of  the  thoracic  cavity  not  occupied  by  the  heart  and 
great  blood-vessels  (Fig.  i68).  This  condition  is  maintained  by  the 
pressure  of  the  air  within  the  lungs,  the  intra- puhnonary  pressure,  which 
with  the  respiratory  passages  open,  is  that  of  the  atmosphere,  760 
mm.  Hg.  This  relation  persists  so  long  as  the  thoracic  cavity  remains 
air-tight.  If  the  skin  and  muscles  covering  an  intercostal  space  be  re- 
moved the  lung  can  be  seen  in  close  contact  with  the  parietal  layer  of 
the  pleura  gliding  by  with  each  inspiration  and  expiration.  If,  how- 
ever, an  opening  be  now  made  in  the  pleura  sufficient  to  admit  air,  the 
lung  immediately  collapses  and  a  pleural  cavity  is  estabhshed  (pneu- 
mothorax). The  pressure  of  air  within  and  without  the  lung  counter- 
balancing, at  the  moment  the  opening  is  made,  the  elastic  tissue  at 
once  recoils  and  forces  a  large  part  of  the  air  out  of  the  lung.  This 
is  a  proof  that  in  the  normal  condition,  the  lungs,  distended  by  at- 
mospheric pressure  from  within,  are  in  a  state  of  elastic  tension  and 
ever  endeavoring  to  pull  the  visceral  layer  of  the  pleura  away  from 
the  parietal  layer.  That  they  do  not  succeed  in  doing  so  is  due  to  the 
fact  that  the  atmospheric  pressure  from  without  is  prevented  from 
acting  on  the  lung  by  the  firm  unyielding  walls  of  the  thorax. 

Intra-thoracic  Pressure. — As  a  result  of  the  elastic  tension  of 
the  lungs  a  fractional  part  of  the  intra-pulmonary  pressure,  760  mm. 
Hg,  is  counterbalanced  or  opposed,  so  that  the  heart  and  great  vessels 
and  other  intra-thoracic  viscera  are  subjected  to  a  pressure  somewhat 
less  than  that  of  the  atmosphere;  the  amount  of  this  pressure  will  be 
that  of  the  atmosphere  less  that  exerted  by  the  elastic  tissue  of  the 
lung  in  the  opposite  direction,  expressed  in  terms  of  millimeters  of 
mercury.  In  the  thorax,  but  outside  the  lungs,  there  then  prevails 
a  pressure,  intra-thoracic  pressure,  negative  to  the  pressure  inside  the 
lungs. 

The  amount  of  this  intra-thoracic  pressure  can  be  approximately 

*  It  is  a  matter  of  dispute  as  to  whether  or  not  there  is  an  absolute  cessation  of 
movement  of  the  thoracic  walls  at  the  end  of  expiration.  A  graphic  record  of  the 
movement  shows  that  if  there  is  no  absolute  cessation,  the  movement  is  so  slight  that, 
for  the  purposes  here  intended,  a  pause  may  be  admitted.  With  this  admission  it 
is,  however,  recognized  that  the  forces,  both  elastic  and  muscular,  which  are  always 
acting  on  the  thoracic  walls,  though  in  opposite  directions,  have  not  ceased  to  act, 
but  have  become  so  nearly  equal  that  for  a  brief  period  they  are  practically  in  a  con- 
dition of  equilibrium,  during  which  the  thoracic  walls  are  stationary. 


RESPIRATION. 


!6i 


determined  in  several  ways.  Thus,  if  shortly  after  death  a  mer- 
curial manometer  be  inserted  air-tight  into  the  trachea  of  a  human 
being  and  the  thorax  opened,  the  lungs  will  recoil  and  compress  their 
contained  air.  The  mercurial  manometer  will  at  once  show  an 
excess  of  pressure  in  the  trachea  of  about  6  mm.  This  was  taken 
by  Bonders  as  a  measure  of  the  force  with  which  the  lungs  endeavor 
to  recoil.  The  intra-thoracic  pressure  would  be,  therefore,  atmos- 
pheric pressure,  760  mm,,  less  6  mm.,  or  754  mm.  Hg.  Another 
method  is  to  insert  a  rubber  catheter  through  a  small  opening  in  an 
intercostal  space  into  the  thoracic  cavity.     The  air  which  enters 


Fig.  168. — Section  of  Thorax  with  the  Lungs,  Heart,  and  Principal  Vessels 
5.  Catheter  introduced  into  the  pleural  space  and  connected  with  a  manometer. — 
{After  Moral  and  Doyen.) 


through  the  open  extremities  of  the  catheter  and  leads  to  a  collapse 
of  the  lungs  may  be  subsequently  aspirated,  when  the  lung  returns  to 
its  normal  position.  The  catheter  is  then  placed  in  connection  with 
a  '  water  manometer.  On  establishing  a  communication  between 
them,  by  the  turning  of  a  stopcock,  the  water  will  rise  in  the  proximal 
and  fall  in  the  distal  hmb  of  the  manometer,  indicating  a  pressure 
in  the  thorax  negative  to  that  in  the  lung.  The  difference  in  the 
level  of  the  water  in  the  two  limbs  of  the  manometer,  expressed 
in  milUmeters  of  mercury,  would  also  represent  the  force  with  which 
the  elastic  tissue  strives  to  recoil,   the  extent  to  which  it  opposes 


362  TEXT-BOOK  OF  PHYSIOLOGY. 

the  atmospheric  pressure.  This  subtracted  from  the  atmospheric 
pressure  would  give  the  intra-thoracic  pressure.  In  the  Hving  dog 
this  latter  is  less  than  the  former,  to  the  extent  of  from  3-5  to  5.5  mm. 
For  the  same  reason  the  superior  surface  of  the  diaphragm  also  ex- 
periences a  pressure  less  than  that  of  the  atmosphere.  Owing  to  the 
soft  and  yielding  character  of  the  abdominal  walls  the  atmospheric 
pressure  is  transmitted  through  the  abdominal  organs  to  the  inferior 
surface  of  the  diaphragm.  The  pressure  being  greater  from  below 
than  above,  the  diaphragm  is  forced  upward  until  it  assumes  the  dome- 
hke  appearance  it  usually  presents.  (These  relations  are  shown  in 
Fig.  168.) 

The  cause  of  the  negativity  of  the  intra-thoracic  pressure  is  con- 
nected with  the  change  in  the  relation  of  the  lungs  to  the  thorax 
attending  the  first  inspiration.  Previous  to  birth  the  walls  of  the 
alveoli  and  bronchioles  are  collapsed  and  in  apposition.  The  larger 
bronchial  tubes  in  all  probability  contain  fluid.  The  lungs  therefore 
are  devoid  of  air  (atelectatic),  and,  having  a  specific  gravity  greater 
than  water,  readily  sink  when  placed  in  this  fluid.  The  capacity 
of  the  thorax  does  not  exceed  the  volume  of  the  lungs.  With  the  first 
inspiration,  however,  the  thoracic  walls  take  a  new  position.  The 
air  at  once  rushes  into  the  lungs  and  distends  them.  But  as  the 
capacity  of  the  thorax  even  at  the  end  of  the  expiration  is  now  greater 
than  the  volume  whicli  the  lungs  could  assume  without  consider- 
able distention,  there  at  once  arises  the  elastic  recoil  in  the  opposite 
direction,  the  condition  which  gives  rise  to  the  negativity  of  the 
pressure  in  the  thoracic  cavity.  It  is  also  probable  that  as  the  child 
develops,  the  thorax  grows  more  rapidly  than  the  lungs,  giving 
rise  to  a  condition  which  would  increase  and  accentuate  the  elastic 
tension  and  thus  increase  the  negativity  of  the  intra-thoracic  pressure. 

THE  DYNAMIC   CONDITION. 

In  the  dynamic  condition  as  previously  stated  these  relations 
and  pressures  change.  Thus  the  diaphragm  descends,  the  ribs 
ascend,  the  sternum  advances  and  the  lungs  expand.  The  intra- 
pulmonary  pressure  varies  during  both  inspiration  and  expiration. 
With  the  enlargement  of  the  thorax  through  muscle  activity,  there 
goes  a  corresponding  increase  in  the  size  and  capacity  of  the  lungs 
in  consequence  of  the  expansion  and  pressure  of  the  air  in  the 
pulmonary  alveoli.  With  the  expansion  of  the  air  its  pressure  falls; 
but  though  it  is  now  less  than  atmospheric,  it  is  yet  much  greater 
than  the  opposing  force  of  the  lung  tissue.  As  a  result  of  the  fall 
of  intra-pulmonary  pressure  there  is  a  rapid  inflow  of  air,  which 
continues  until  atmospheric  pressure  is  restored;  that  is,  at  the 
end  of  the  inspiration.  With  the  diminution  of  the  thorax,  through 
the  recoil  of  the  elastic  tissue  of  the  thoracic  and  abdominal  walls, 
there  goes  a  corresponding  decrease  of  lung  capacity,  in  consequence 


RESPIRATION. 


563 


Insp 


Intra-pulmonary  pressure. 


Exp. 


of  the  recoil  of  the  elastic  tissue  of  the  lungs.  As  a  result,  the 
air  in  the  lungs  becomes  compressed,  its  pressure  rises  above  that 
of  'the  atmosphere,  and  a  rapid  outflow  of  air  takes  place,  which 
continues  until  atmospheric  pressure  is  again  restored;  that  is,  at 
the  end  of  the  expiration. 

The  cause  for  the  fall  of  intra-pulmonary  pressure  during  in- 
spiration and  the  rise  during  expiration  is  to  be  found  in  the  resist- 
ance offered  by  the  air-passages  to  the  movement  of  the  air,  through- 
out their  entire  extent,  and  especially  at  the  level  of  the  vocal  bands. 
The  greater  the  resistance,  from  whatever  cause,  physiologic  or 
pathologic,  the  greater  the  variations  of  the  pressure. 

In  quiet  inspiration  the  fall  of  pressure,  as  indicated  by  a  man- 
ometer inserted  into  one  nostril,  seldom  amounts  to  more  than  1.5  mm. ; 
the  rise  in  expiration,  2.5  to  3  mm.  In  forcible  inspiratory  and  ex- 
piratory efl'orts  these  hmits  may  be  largely  exceeded.  Thus  it  was 
found  by  Bonders 
that  with  one  nostril 
closed  and  a  mercurial 
manometer  inserted 
into  the  other  the 
pressure  by  voluntary 
efforts  could  be  made 
to  fall  57  mm.  during 
inspiration  and  to  rise 
87  mm.  during  expira- 
tion. The  changes  in 
intra-pulmonary  pres- 
sure are  graphically 
represented  in  the  up- 
per half  of  Fig.  169. 

The  intra-thoracic 
pressure  also  varies  during  both  inspiration  and  expiration.  As  the 
intra-pulmonary  pressure  falls,  the  recoil  of  the  elastic  tissue  in- 
creases, with  the  result  of  diminishing  the  intra-thoracic  pressure, 
though  not  in  a  steadily  progressive  manner.  The  fall  of  intra- 
thoracic pressure  at  the  end  of  a  quiet  inspiration  amounts  to  about 
9  mm.  Hg.  In  forcible  inspiratory  efforts  this  fall  in  intra-thoracic 
pressure  may  amount  to  30  or  40  mm.  of  Hg.  As  the  intra- 
pulmonary  pressure  rises  above  the  atmospheric  pressure  during 
expiration,  the  recoil  of  the  elastic  tissue  is  again  opposed,  with  the 
result  of  increasing  the  intra-thoracic  pressure,  though  not  in  a 
steadily  progressive  manner.  The  changes  in  intra-thoracic  pressure 
are  graphically  represented  in  the  lower  half  of  Fig.  169. 

Respiratory  Movements. — ^As  previously  stated,' the  ventilation 
of  the  lungs  is  accomplished  by  an  alternate  increase  and  decrease  in 
the  capacity  of  the  thorax,  accompanied  by  corresponding  changes 


760  mm 


C  760  mm 


Intra-thoracic  pressure. 

Fig.  169. — Representing  the  Changes,  i,  in  the 
Intra-pulmonary,  and,  2,  in  the  Intr.\-tho- 
RAcic  Pressures  during  Inspiration  and  Ex- 
piration. 


364  TEXT-BOOK  OF  PHYSIOLOGY. 

in  the  lungs,  the  two  movements  being  known  as  inspiration  and 
expiration  respectively.  During  the  increase  in  the  thoracic  capacity, 
the  air  passively  flows  into  the  lungs;  during  the  decrease  in  the 
thoracic  capacity,  the  air  passively  flows  out  of  the  lungs.  In  both 
movements  the  lungs  play  an  entirely  passive  part,  their  movements 
being  detemiined  by  the  pressure  of  air  within  them  and  by  the  tho- 
racic walls,  with  which  they  are  in  close  contact. 

1.  Inspiration  is  an  active  process,  the  result  of  muscle  activity. 

2.  Expiration  is  a  passive  process,  the  result  mainly  of  the  recoil  of 

the  elastic  tissue  of  the  walls  of  the  thorax  and  abdomen  and  of 
the  elastic  tissue  of  the  lungs. 

In  inspiration  the  thorax  is  enlarged  in  all  its  diameters:  viz.,  ver- 
tical, transverse,  and  antero-posterior.  In  expiration  these  diameters 
are  again  decreased  as  the  thorax  returns  to  its  previous  condition. 

Inspiratory  Muscles. — The  muscles  which  from  their  origin, 
direction,  and  insertion  contribute  to  the  enlargement  or  expansion  of 
the  thorax  are  quite  numerous,  and  include  those  muscles  which 
enter  into  the  formation  of  the  thoracic  walls  (intrinsic  muscles), 
as  well  as  certain  muscles  which,  having  their  origin  elsewhere,  are 
attached  to  the  thoracic  walls  at  different  points  (extrinsic  muscles), 
though  the  extent  to  which  they  are  called  into  activity  depends 
on  the  necessity  for  either  tranquil  or  energetic  inspirations.  The 
gradations  between  a  minimum  and  a  maximum  inspiration  are 
very  slight,  and  it  is  difficult  to  state  at  what  particular  instant  any 
given  muscle  begins  to  act.  It  is  customary,  however,  to  divide 
the  muscles  into  two  groups:  (i)  Those  active  in  the  average  or 
ordinary  inspirations,  and  (2)  those  active  in  maximum  or  extra- 
ordinary inspirations.  Among  the  muscles  active  in  ordinary  in- 
spirations may  be  mentioned  the  diaphragm,  the  intercostales  externi, 
the  inter cartilagenei,  the  levatores  costariim,  the  scaleni,  and  the  ser- 
ratus  posticus  superior.  Among  the  muscles  active  in  extraordi- 
nary inspirations  may  be  mentioned,  in  addition  to  the  foregoing, 
the  sterno-cleido-mastoideus ,  the  trapezius,  and  the  pectorales  minor 
and  mijor. 

The  vertical  diameter  is  increased  by  the  contraction  and  descent 
of  the  diaphragm,  and  more  especially  of  its  lateral  muscular  portions. 
At  the  end  of  an  expiration  the  diaphragm  is  relaxed,  and  the  lower 
portion  closely  applied  to  the  walls  of  the  thorax.  At  the  beginning 
of  an  inspiration  the  muscle-fibers  contract,  shorten,  and  approxi- 
mate a  straight  line,  whereby  not  only  is  the  convexity  of  the  dia- 
phragm diminished,  but  that  portion  in  contact  with  the  thorax 
is  drawn  away,  thus  making  a  large  free  space  into  which  the  lat- 
eral and  posterior  portions  of  the  lungs  at  once  descend.  The 
attachment  of  the  central  tendon  of  the  diaphragm  to  the  peri- 
cardium prevents  any  marked  descent  of  this  portion  except  in  forcible 


RESPIRATION. 


;65 


inspirator}'    efforts    (Fig.    170).     The    vertical   diameters    are    thus 
enlarged,  though  unequally  in  dift'erent  regions  of  the  thorax. 

As  the  diaphragm  descends  it  displaces  the  abdominal  viscera, 
forcing  them  downward  and  outward  against  the  abdominal  walls, 
which  advance  and  become  more  convex.  In  forcible  inspiration 
the  diaphragm,  acting  from  the  central  tendon  as  the  more  fixed 
point,  would  draw  the  lower  portion  of  the  thorax  inward  were  this 
not  prevented  by  the  outward  pressure  of  the  displaced  viscera. 

The  antero- posterior  and  transverse  diameters  are  increased  by 
the  elevation  and  outward  rotation  of  the  ribs  and  an  advance  of  the 
sternum,  both  movements  made  possible  by  the  construction  and 
arrangement  of  the  costo-vertebral  and  costo-chondral  and  chondro- 
sternal  articulations.  The  construction  of 
these  articulations  is  such  as  to  permit  at 
the  first  a  shght  elevation  and  depression 
of  the  head  of  the  rib,  and  at  the  second  a 
ghding  of  the  tubercle  on  the  transverse 
process.  The  axis  around  which  the  rib 
rotates  practically  coincides  with  the  axis  of 
the  rib  neck,  which  in  the  upper  part  of  the 
thorax  is  almost  horizontal,  in  the  lower 
part  somewhat  sagittal  in  direction.  Hence 
when  the  ribs  are  elevated  the  upper  part  of 
the  thorax  increases  in  its  antero-posterior, 
the  lower  part  in  its  transverse  diameters. 
At  the  same  time,  the  lower  portion  of  the 
sternum  is  pushed  forward  and  upward  by 
the  elevation  of  the  anterior  extremity  of 
the  ribs  and  the  widening  of  the  angle  of  the 
costo-chondral  articulation.  With  the  ele- 
vation of  the  ribs  there  goes  an  eversion  or 
outward  rotation  which  gives  an  additional 

increase  to  the  transverse  diameters.  This  elevation  and  outward 
rotation  of  the  ribs  is  the  resultant  of  the  cooperation  of  the  follow- 
ing muscles,  viz.  :  the  intercostales  exierni,  the  intercartilagenei,  the 
levator  es  cost  arum,  the  scaleni  and  the  serratus  posticus  superior. 

The  action  of  the  external  intercostal  muscles  has  been  a  subject 
of  much  discussion.  Some  investigators  have  maintained  that  they 
are  elevators  of  the  ribs,  and  therefore  inspiratory;  others  that  they  are 
depressors  of  the  ribs,  and  therefore  expiratory  in  function.  At  the 
present  time  the  general  consensus  of  opinion  is  that  the  former  view 
is  the  one  most  in  accordance  with  the  facts.  The  situation  of  the 
muscles  and  the  shortness  of  their  fibers  render  it  extremely  diffi- 
cult to  obtain  myographic  tracings  of  their  action.  This  is  supposed, 
however,   to  be  disclosed   by  the  play  of  the  apparatus  suggested 


Fig.  170 

ING 


Diagram  show- 
Interval  BE- 
TWEEN THE  Position 
OF  the  Diaphragm 
IN  Expiration  (e,  e) 
AND  Inspiration  {i,  i). 
The  Increase  in 
Capacity  is  shown 
BY  THE  White 
Areas. — {Yeo.) 


365  TEXT-BOOK  OF  PHYSIOLOGY. 

originally  by  Bernouilli,  which  consists,  as  shown  in  Fig.  171,  of  a 
vertical  support  carrying  two  freely  movable  parallel  bars  united  at 
their  outer  extremities  by  a  short  vertical  strip,  representing  respec- 
tively the  vertebral  column,  two  adjoining  ribs,  and  a  piece  of  the 
sternum.  The  parallel  bars  are  joined  to  each  other  by  a  short 
elastic  band  having  the  direction  of  and  representing  the  external  in- 
tercostal muscles.  If  the  bars  are  depressed,  the  elastic  band  is 
elongated  and  made  tense.  On  releasing  the  bars  the  band  at  once 
recoils  and  elevates  them.  Although  the  elastic  force  is  the  same  in 
both  directions,  the  bars  are  yet  elevated  for  the  reason  that  in  ac- 
cordance with  the  parallelogram  of  forces  the  component  acting 
upward  on  the  long  arm  of  the  lever  preponderates  over  the  com- 
ponent acting  downward  on  the  short  arm  of  the  lever.  The  action 
of  the  band  is  supposed  to  disclose  and  illustrate  the  action  of  the 
muscle. 

The  intercartilaginei,  those  portions  of  the  intercostales  intern! 
which  occupy  the  space  between  the  costal  cartilages  from  the  sternum 
to  their  outer  extremity,  bear  the  same  relation  to  the  cartilages  in 
reference  to  the  sternum  that  the  external  intercostals  bear  to  the  ribs 
in  reference  to  the  vertebral  column;  that  is,  the  point  of  attachment 
to  the  cartilage  above,  lies  nearer  the  sternum,  nearer  the  fulcrum, 
than  the  point  of  attachment  below.  Hence  the  same  action  is  at- 
tributed to  them  as  to  the  external  intercostals:  viz.,  elevation  of  the 
cartilages  and  the  anterior  extremities  of  the  ribs. 

The  levatores  costarum,  as  is  evident  from  their  points  of  origin 
and  insertion,  elevate  the  ribs  posteriorly. 

The  scaleni  muscles,  anticus,  medius,  and  posticus,  arise  from  the 
transverse  processes  of  the  cervical  vertebrae,  and  after  pursuing  a 
downward  and  forward  direction  are  inserted  into  the  sternal  end  of 
the  first  and  second  ribs.  The  action  of  the  first  two,  at  least,  is  to 
elevate  the  first  rib  and  thus  estabHsh  a  fixed  point  from  which  the 
intercostal  muscles  can  act.  The  posticus  has  doubtless  a  similar 
action  on  the  second  rib. 

The  serratus  posticus  superior,  a  quadrilateral  sheet  of  muscle- 
fibers,  arises  mainly  from  the  spines  of  the  last  cervical  and  first  and 
second  thoracic  vertebrae.  The  anterior  extremity  is  serrated  and 
attached  to  the  outer  surfaces  of  the  second,  third,  fourth,  and  fifth 
ribs  beyond  the  angle.  The  action  of  the  muscle  is  the  elevation  of 
the  ribs  to  which  it  is  attached. 

In  forcible  or  extraordinary  inspirations,  whereby  the  capacity  of 
the  thorax  is  still  further  increased,  the  foregoing  muscles  are  rein- 
forced by  the  sternocleidomastoideus,  the  trapezius,  and  the  pec- 
torales  minor  and  major.  Their  functions  will  become  apparent 
from  a  consideration  of  their  origins  and  insertions. 


RESPIRATION. 


367 


Expiratory  Forces  and  Muscles. — Expiration,  as  previously 
stated,  is  a  passive  process  brought  about  by  the  recoil  of  the  elastic 
tissues  of  the  thoracic  and  abdominal  walls,  and  of  the  lungs,  aU  of 
which  have  been  stretched  and  made  tense  during  inspiration.  With 
the  cessation  of  the  inspirator}-  effort  the  elastic  forces,  assisted  by  the 
weight  of  the  ribs,  sternum,  and  soft  tissues,  return  the  thorax  to  its 
former  condition.  The  result  is  a  diminution  of  all  the  diameters  of 
the  thorax.  The  vertical  diameter  is  diminished  by  the  recoil  of 
the  tense  abdominal  walls,  the  replacement  of  the  abdominal  organs 
and  the  consequent  ascent  of  the  diaphragm  to  its  former  position. 
The  transverse  and  antero- posterior  diameters  are  diminished  by  the 
descent  of  the  ribs,  sternum,  and  lungs.  It  is  somewhat  uncertain 
if  a  normal  expirator}-  movement  necessitates  active  muscle  contrac- 


Fig.  171. — Diagram  illustrating  the  Action  of  A,  the  External  Intercostal 
AND  B,  the  Internal  Intercostal  Muscles.  V,  V.  Vertical  support.  R,  R'. 
Parallel  bars.  S.  Vertical  strip,  representing  respectively  the  vertebral  column, 
two  ribs,  and  sternum. 


tion.  If,  however,  there  is  any  impairment  of  the  elasticity  of  the 
lungs  or  ribs,  or  any  interference  with  the  free  exit  of  the  intra- 
puhnonan-  air,  it  is  highly  probable  that  the  elastic  forces  are  assisted 
by  the  internal  intercostal  and  triangularis  stemi  muscles. 

The  action  of  the  internal  intercostals  is  less  clearly  understood 
than  that  of  the  externals,  and  for  the  same  reasons.  If,  however, 
Bernouilli's  model  discloses  the  action  of  the  latter,  it  equally  well 
discloses  the  action  of  the  former.  Thus,  if  the  parallel  bars  be  joined 
by  an  elastic  band  having  the  direction  of  and  representing  the  inter- 
nal intercostals,  and  then  forcibly  elevated,  the  band  is  elongated  and 
made  tense.  On  releasing  the  bars,  the  elastic  band  at  once  recoils 
and  depresses  them.  Here,  again,  though  the  elastic  force  is  the  same 
in  both  directions,  the  bars  are  depressed,  for  the  reason  that  the 


368  TEXT-BOOK  OF  PHYSIOLOGY. 

component  acting  downward  on  the  long  arm  of  the  lever  pre- 
ponderates over  that  acting  upward  on  the  short  arm  of  the  lever. 
The  action  of  the  band  is  supposed  to  disclose  and  illustrate  the 
action  of  the  muscle. 

The  triangularis  sterni  muscle,  judging  from  its  anatomic  re- 
lations, in  all  probabihty  assists  in  expiration  by  depressing  the  car- 
tilages to  which  it  is  attached  and  as  a  further  result  the  anterior 
extremities  of  the  ribs. 

After  the  elastic  forces  have  ceased  to  act  and  the  normal  expira- 
tory movement  has  been  brought  to  a  close,  the  thorax  can  be,  to  a 
considerable  extent,  still  further  diminished  in  all  its  diameters  by 
the  contraction,  through  volitional  effort,  of  abdominal  and  thoracic 
muscles.  To  this  decrease  in  the  capacity  of  the  thorax,  as  a  result 
of  which  a  much  larger  volume  of  air  is  expelled  from  the  lungs  than 
during  passive  expiration,  the  term  forced  expiration  has  been  given. 
With  the  cessation  of  muscle  activity  the  elastic  forces  of  the  now- 
compressed  thoracic  walls,  aided  by  the  return  of  upward  displaced 
abdominal  organs,  at  once  restore  the  thoracic  walls  to  the  position 
they  had  attained  at  the  eni  of  passive  expiration.  Of  the  muscles 
active  in  forced  expiration  in  addition  to  the  intercostales  interni  and 
the  triangularis  sterni,  the  following  may  be  mentioned,  viz.:  the 
abdominales,  the  serratus  posticus  inferior,  and  the  quadratus  lum- 
borum. 

The  externus  abdominis  arises  by  a  series  of  muscle  slips  from  the 
outer  surface  of  the  lower  eight  ribs.  After  pursuing  an  oblique 
direction  downward  and  forward,  the  slips  blend  to  form  a  single 
muscle,  which  is  inserted  mainly  into  the  outer  Hp  of  the  anterior 
half  of  the  crest  of  the  ihum  and  into  the  central  abdominal  tendon 
or  aponeurosis. 

The  internus  abdominis  arises  mainly  from  the  anterior  two-thirds 
of  the  inner  crest  of  the  ihum  and  the  lumbar  fascia.  Its  fibers  pass 
upward  and  forward  to  be  inserted  into  the  cartilages  of  the  last  three 
ribs  and  into  the  central  abdominal  tendon. 

The  rectus  abdominis  arises  from  the  crest  of  the  pubes  and  is 
inserted  above  into  the  cartilages  of  the  fifth,  sixth,  and  seventh  ribs, 
and  occasionally  into  the  ensiform  cartilage. 

The  transversalis  arises  from  the  cartilages  of  the  last  six  ribs, 
the  lumbar  fascia,  and  the  anterior  half  of  the  crest  of  the  ilium. 
After  passing  transversely  across  the  abdomen,  the  fibers  are  inserted 
mainly  into  the  linea  alba. 

The  conjoint  action  of  these  muscles  is  to  diminish  the  convexity 
of  the  abdominal  walls  and  to  exert  a  pressure  on  the  abdominal 
organs.  These,  taking  the  Hne  of  least  resistance,  are  forced  upward 
against  the  inferior  surface  of  the  diaphragm,  which  in  consequence 
becomes  more  strongly  curved  and  ascends  higher  into  the  thorax. 


RESPIRATION.  369 

The  vertical  diameter  of  the  thorax  is  thus  diminished.  Acting  from 
the  pelvis  as  a  fixed  point,  these  muscles  will  also  draw  downward 
and  inward  the  lower  end  of  the  sternum  and  the  lower  ribs  and 
diminish  the  antero-posterior  and  transverse  diameters. 

The  serratus  posticus  inferior  arises  from  the  spines  of  the  last 
two  thoracic  and  first. two  lumbar  vertebrae.  The  fibers  pass  upward 
to  be  inserted  into  the  lower  border  of  the  last  four  or  five  ribs  beyond 
the  angle.  Their  action  is  to  depress  the  ribs  and  assist  in  expira- 
tion. 

The  quadratus  lumhorum  has  a  similar  action  on  the  last  rib. 

Movements  of  the  Lungs. — As  the  thorax  is  enlarging  in  all  its 
diameters  during  inspiration,  through  muscle  activity,  the  lungs  are 
correspondingly  enlarging  in  all  their  diameters,  by  virtue  of  their 
distensibility,  through  the  pressure  of  the  air  within  them.  The 
lungs  must  therefore  move  downward,  outward,  and  forward.  That 
this  is  the  case  is  made  evident  both  by  an  examination  of  the  lungs 
through  an  intercostal  space  after  removal  of  the  skin  and  intercostal 
muscles  and  by  the  methods  of  percussion.  The  inferior  border  of 
each  lung  descends  from  the  lower  border  of  the  sixth  to  the  eleventh 
rib,  inserting  itself  into  the  space  developed  between  the  thorax  and 
diaphragm  as  the  latter  contracts  and  is  drawn  away  from  the  former. 
In  consequence  of  the  lateral  expansion  the  anterior  border  of  each 
lung  advances  toward  the  middle  line  until  the  heart  is  almost  cov- 
ered. With  the  beginning  and  continuance  of  expiration  the  lungs 
exhibit  a  reverse  movement  which  continues  until  they  reach  their 
original  position.  At  all  times,  however,  the  movements  of  the  lungs 
are  entirely  passive  and  determined  by  the  movements  of  the  thorax. 

The  succession  of  events  in  the  thorax  at  the  time  of  a  respirator)' 
act  may  be  summarized  as  follows: 

During  Inspiration. 

1.  Enlargement   of   the   thoracic  diameters   by  muscle   action. 

2.  Expansion  of  intra-pulmonary   (alveolar)   air. 

3.  Expansion  of  the  lungs. 

4.  Lowering   of   the   intra-pulmonar}'   air   pressure   below   the 

atmospheric  air  pressure. 

5.  Increase   in   the   negativity   of    the   intra-thoracic   pressure. 

6.  Inflow  of  atmospheric  air,  in  consequence  of  its  higher  pres- 

sure, until   the  intra-pulmonary  air   pressure  rises   to  that 
of  the  atmosphere. 
During  Expiration. 

1.  Diminution  of  the  thoracic  diameters  by  the  action  of  elastic 

forces. 

2.  Recoil  of  the  lungs. 

3.  Compression  of  the  intra-pulmonary  (alveolar)  air. 

24 


37©  TEXT-BOOK  OF  PHYSIOLOGY. 

4.  Rise  of  intra-pulmonary  air  pressure  above  the  atmospheric 

air  pressure. 

5.  Decrease  in   the  negativity  of  the  intra- thoracic  pressure. 

6.  Outflow  of  intra-puknonary  air,  in  consequence  of  its  higher 

pressure,  until  the  intra-puhnonar)^  air  pressure  falls  to 
that  of  the  atmosphere. 

Respiratory  Movements  of  the  Upper  Air-passages. — The 
resistance  to  the  entrance  of  air  into  and  through  the  respiratory 
tract  is  much  diminished  by  respiratory  movements  of  the  nares  and 
larynx  which  are  associated  and  occur  synchronously  with  the  move- 
ment of  the  thorax. 

The  nares  at  each  inspiration  are  dilated  by  the  outward  move- 
ment of  their  alae  or  wings,  the  result  of  muscle  activity.  At  each 
expiration  they  are  diminished  by  the  return  of  their  cartilages  through 
the  play  of  elastic  forces.  The  larynx,  as  shown  by  observation 
with  the  laryngoscope,  exhibits  corresponding  movements  of  the  vocal 
membranes.  Their  introduction  at  this  level  naturally  narrows  the 
tract,  and  would  interfere  with  both  the  entrance  and  the  exit  of  air 
were  they  not  kept  widely  asunder  during  the  time  they  are  not  re- 
quired for  purposes  of  phonation.  This  is  accomphshed  by  the 
tonic  contraction  of  the  posterior  crico-arytenoid  muscles,  which  are 
entirely  respiratory  in  function. 

It  is  not  infrequently  stated  that  these  membranes  exhibit  consider- 
able oscillations,  outward  and  inward,  corresponding  to  the  periods 
of  inspiration  and  expiration.  The  statements  of  the  majority  of 
laryngologists  do  not  favor  this  view.  During  tranquil  breathing 
the  membranes  are  widely  separated  and  almost  stationary,  seldom 
moving  in  either  direction  more  than  a  few  milhmeters.  In  labored 
respirations  these  movements  are  naturally  increased  in  extent.  The 
reflex  movements  of  the  membranes  occasioned  by  the  unskilful  use 
of  the  laryngoscope,  especially  with  nervous  patients,  are  not  to  be  re- 
garded as  strictly  physiologic.  The  respiratory  space  in  quiet  breath- 
ing is  an  isoceles  triangle,  with  a  length  of  20  mm.  and  a  width  at  the 
base  of  15.5  mm. 

Respiratory  Types. — Observation  of  the  respiratory  movements 
in  the  two  sexes  shows  that  while  the  enlargement  of  the  thoracic 
cavity  is  accomphshed  both  by  the  descent  of  the  diaphragm  (as 
shown  by  the  protrusion  of  the  abdomen)  and  the  elevation  of  the 
thoracic  walls,  the  former  movement  preponderates  in  the  male,  the 
latter  in  the  female,  giving  rise  to  what  has  been  termed  in  the  one 
case  the  diaphragmatic  or  abdominal  and  in  the  other  the  thoracic  or 
costal  type  of  respiration.  The  cause  of  this  greater  mobility  and 
activity  of  the  thorax  in  the  female  has  been  a  subject  of  much  discus- 
sion. It  has  been  attributed,  on  the  one  hand,  to  the  necessity  for  a 
physiologic  adjustment  between  respiration  and  child-bearing,  and 


RESPIRATION. 


371 


therefore  a  specific  sex  peculiarity;  on  the  other  hand,  it  has  been 
attributed  to  persistent  constriction  of  the  waist,  in  consequence  of 
which  the  full  play  of  the  diaphragm  is  prevented  and  the  burden  of 
inspiration  is  thrown  on  the  thoracic  muscles.  It  has  been  assumed 
that  if  inspiration  were  confined  in  women  to  the  diaphragm,  there 
would  arise  in  the  latter  stages  of  gestation  such  an  increase  in  intra- 
abdominal pressure  that  not  only  would  respiratory  exchanges  be 
interfered  with,  but  fetal  fife  might  be  unfavorably  influenced,  if  not 
endangered.  Modern  investigations  have  not  confirmed  this  assump- 
tion, but,  on  the  contrary,  have  corroborated  the  view  that  the  pre- 
ponderance of  thoracic  movement  is  due  to  the  influences  of  dress 
restrictions,  for  with  their  removal  the  so-called  costal  type  of  breath- 
ing entirely  disappears.  While  gestation  may  lead  to  a  greater 
activity  of  the  thorax,  this  is  but  temporary,  for  with  its  termination 
there  is  a  return  to  the  diaphragmatic  type  of  breathing. 

Number  of  Respirations  per  Minute. — The  number  of  respira- 
tions which  occur  in  a  unit  of  time  varies  with  a  variety  of  conditions, 
the  most  important  of  which  is  age.  The  results  of  the  observa- 
tions of  Quetelet  on  this  point,  which  are  generally  accepted,  are  as 
follows : 


Age.  Respirations  per  Minute. 

o-  I  year, 44 

5  years,   26 

15-20      "       20 


Age.  Respirations  per  Minute. 

20-25  years, 18.7 

25-30      "      16.0 

30-50      "      18.0 


From  these  observations  it  may  be  assumed  that  the  average  number 
of  respirations  in  the  adult  is  eighteen  per  minute,  though  varying 
from  moment  to  moment  from  sixteen  to  twenty.  During  sleep, 
however,  the  respiratory  movements  often  diminish  in  number  as 
much  as  30  per  cent.,  at  the  same  time  diminishing  in  depth. 

Rhythm. — Each  respiratory  act  takes  place  normally  in  a  regular 
methodic  manner,  each  event  occurring  in  a  definite  sequence  and 
occupying  the  same 
relative  period  of 
time.  This  rhythm, 
however,  is  not  infre- 
quently temporarily 
disturbed  by  emo- 
tions, volitional  acts, 
muscle  activity,  pho- 
nation,  changes  in  the 
composition     of     the 

blood,  etc. ;  with  the  removal  of  these  disturbing  factors,  the  respiratory 
mechanism  soon  returns  to  its  normal  condition. 

A  graphic  representation  of  the  excursions  of  the  thoracic  walls, 
rhythmic  or  otherwise,  is  obtained  by  fastening  to  the  thorax  an 


Fig.  172. — Pneumograph. — (Fitz.) 


372  TEXT-BOOK  OF  PHYSIOLOGY. 

apparatus,  a  stethomeier  or  a  pneumograph,  which  by  means  of  a  tam- 
bour takes  up  and  transmits  the  movement  to  a  second  tambour 
provided  with  a  recording  lever.  A  simple  form  of  pneumograph, 
suggested  by  Fitz  (Fig.  172),  consists  of  a  coil  of  wire  two  and  a 
half  centimeters  in  diameter  and  about  40  centimeters  in  length, 
enclosed  by  thin  rubber  tubing,  one  end  of  which  is  closed,  the 
other  placed  in  communication  with  either  a  tambour  and  lever 
or  with  a  piston  recorder.  By  means  of  an  inelastic  cord  or  chain 
the  apparatus  is  securely  fastened  to  the  chest.  With  each  inspira- 
tion the  spring  is  elongated,  the  air  within  the  system  is  rarefied, 
and  as  a  result  the  lever  falls;  with  each  expiration  the  reverse 
conditions  obtain  and  the  lever  rises.  If  the  lever  be  applied  to 
the  recording  surface  of  a  moving  cyhnder,  a  curve  of  the  thoracic 
movement,  a  pneumaiogram,  is  obtained  (Fig.  173),  from  which  it 
is  apparent  that  inspiration  takes  place  more  abruptly  and  occupies 
a  shorter  period  of  time  than  expiration;  that  expiration  immediately 
follows  inspiration,  but  that  there  is  a  slight  pause  between  the  end 
of  the  expiration  and  the  beginning  of  the  inspiration.     The  time 

relations  of  the  two  movements 
can  be  obtained  by  a  magnet-sig- 
nal actuated  by  an  electric  current 
interrupted  once  a  second.  The 
ratio  of  inspiration  to  expiration 
has  been  represented  as  5  to  6, 
or  6  to  8. 
173— A  pneumatogram— (,4//er  Volumes  of  Air  Breathed. 

Marey.)  — 'pj^g  volumes  of  air  which  enter 

and  leave  the  lungs  with  each 
inspiration  and  expiration  naturally  vary  with  the  extent  of  the 
movement,  though  four  at  least  may  be  determined:  (i)  that  of  an 
ordinary  inspiration;  (2)  that  of  an  ordinary  expiration;  (3)  that  of 
a  forced  inspiration;  (4)  that  of  a  forced  expiration. 

The  apparatus  employed  for  the  determination  of  these  different 
volumes  is  the  spirometer,  a  modification  of  the  gasometer.  The 
form  introduced  by  Jonathan  Hutchinson  (Fig.  174)  consists  of  tw^o 
metallic  cylinders,  one  (a)  containing  water,  the  other  (b)  containing 
air,  the  latter  being  inserted  into  the  former.  The  air  cylinder  is 
balanced  by  weights  so  accurately  that  it  remains  stationary  in  any 
position.  A  tube,  penetrating  the  base  of  the  water  cyhnder,  is  con- 
tinued upward  through  and  above  the  level  of  the  water.  The 
air-space  above  is  thus  placed  in  free  communication  with  the  ex- 
ternal air.  A  stopcock  at  the  outer  end  of  this  tube  prevents  the 
escape  of  the  air  when  this  is  not  desirable.  To  the  free  end  of  the 
tube  a  rubber  tube  provided  with  a  suitable  mouthpiece  is  attached, 
through  which  air  can  be  breathed  into  or  out  of  the  air-cylinder. 


RESPIRATION. 


373 


With  each  inspiration  the  air-cyhnder  descends;  with  each  expiration 
it  ascends.  A  scale,  on  one  of  the  side  supports,  graduated  in 
cubic  inches  or  centimeters,  indicates  the  volume  of  air  inspired  or 
expired. 

With  this  apparatus  Hutchinson,  from  a  long  series  of  observa- 
tions, deiined  and  determined  the  above-mentioned  four  volumes 
as  follows: 

1.  The  tidal  volume,  that  which  flows  into  and  out  of  the  lungs  with 

each  inspiration  and   expiration,  which  varies  from  20  to  30 
cubic  inches  (312  to  468  c.c). 

2.  The  complemental  volume,  that  which  flows  into  the  lungs,  in  addi- 

tion to  the  tidal  volume,  as  a  result  of 
a  forcible  inspiration,  and  which 
amounts  to  about  no  cubic  inches 
(1748  c.c). 

3.  The  reserve  volume,  that  which  flows 

out  of  the  lungs,  in  addition  to  the 
tidal  volume,  as  a  result  of  a  forcible 
expiration,  and  which  amounts  to 
about  100  cubic  inches  (1562  c.c). 

After  the  expulsion  of  the  reserve 
volume  there  yet  remains  in  the  lungs 
an  unknown  volume  of  air  which  serves 
the  mechanic  function  of  distending 
the  air-cells  and  alveolar  passages, 
thus  maintaining  the  conditions  essen- 
tial to  the  free  movement  of  blood 
through  the  capillaries  and  to  the  ex- 
changes of  gases  between  the  blood 
and  alveolar  air.  As  this  air  can  not 
be  displaced  by  voHtional  effort,  but 
resides  permanently  in  the  alveoli  and 
bronchial  tubes  though  constantly  un-  yig. 
dergoing  renewal,  it  was  termed — 

4.  The   residual   volume,   the   amount   of 

which  is  difficult  of  determination,  but  has  been  estimated 
by  different  observers  at  914  c.c,  1562  c.c,  1980  c.c. 
The  Vital  Capacity  of  the  Lungs. — From  foregoing  statements 
it  is  clear  that  the  thorax  and  lungs  are  capable  of  a  maximum  degree 
of  expansion,  at  which  moment  the  lungs  contain  their  maximum 
volume  of  air.  This  volume,  whatever  it  may  be,  represents  the 
entire  capacity  of  the  lungs  in  the  physiologic  condition,  and  includes 
the  tidal,  the  complemental,  the  reserve,  and  the  residual  volumes. 
Mr.  Hutchinson,  however,  defined  the  vital  or  respiratory  capacity 
of  the  lungs  as  the  amount  of  air  which  can  be  expelled  by  the  most 


74. — Spirometer.— 
{Hutchinson.) 


374 


TEXT-BOOK  OF  PHYSIOLOGY. 


Fig.  175. — Pneumatograph. — {Gad.) 


forcible  expiration  after  the  most  forcible  inspiration,  and  which 
therefore  excludes  the  residual  volume.  The  vital  capacity  was  sup- 
posed to  be  an  indication  of  an  individual's  respiratory  power, 
not  only  in  physiologic  but  also  in  pathologic  conditions.  Though 
averaging  about  230  cubic  inches  (3593  c.c.)  for  an  individual  5  feet 

7  inches  in  height,  the  vital 
capacity  varies  with  a  number 
of  conditions,  the  most  im- 
portant of  which  is  stature. 
It  is  found  that  between  5  and 
6  feet  the  capacity  increases  8 
inches  (125  c.c.)  for  each  inch 
increase  in  height. 

The  volume  changes  of  the 
thorax  indicated  by  the  vol- 
umes of  air  entering  and  leav- 
ing the  lungs  can  be  not  only  determined  but  graphically  represented 
by  means  of  an  apparatus  similar  in  principle  to  the  spirometer,  de- 
vised by  Gad  and  known  as  the  pneumatograph  or  aero plethy sinograph 
(Fig.  175).  This  consists  of  a  quadrangular  box  with  double  walls, 
the  space  between  which  is  filled 
with  water.  The  center  of  the 
box  is  an  air  chamber.  A  thin- 
walled  mica  box  sinks  into  the 
water.  Posteriorly  it  is  attached 
to  and  rotates  around  an  axis, 
which  permits  of  an  elevation  or 
depression  of  the  anterior  portion. 
It  is  also  carefully  counterpoised. 
A  light  lever  attached  to  the  mica 
box  records  its  movements.  The 
interior  of  the  box  communicates 
by  a  tube  with  a  large  reservoir 
into  which  the  individual 
breathes,  the  object  being  to 
prevent  a  too  rapid  vitiation  of 
the  air.  Inspiration  causes  the 
lever  to  descend,  expiration  to  as- 
cend.     Previous   graduation    of 

the  apparatus  is  necessary  to  determine  the  volumes  breathed.  A 
graphic  record  of  the  volume  changes  is  shown  in  Fig.  176. 

Respiratory  Sounds. — On  applying  the  ear  over  the  trachea 
and  bronchi  there  is  heard  during  both  inspiration  and  expiration  a 
well-defined  sound,  loud,  harsh,  and  blowing  in  character,  which 
from  its  situation  is  known  as  the  bronchial  sound.     It  is  especially 


Fig.  176. — Representing  the  Volume 
Changes  of  the  Thorax  and 
Lungs    (Diagrammatic). 


RESPIRATION. 


375 


well  heard  between  the  scapulae  above  the  fourth  dorsal  vertebra. 
This  sound  is  produced  in  the  larynx,  for  with  its  separation  from 
the  trachea  the  sound  disappears.  The  cause  of  the  sound  is  to 
be  found  in  the  narrowing  of  the  air-passage  at  the  level  of  the 
vocal  membranes,  though  the  mechanism  of  its  production  is  un- 
certain. On  applying  the  ear  to  almost  any  portion  of  the  chest- 
wall,  but  especially  to  the  infrascapular  area,  there  is  heard  during 
both  inspiration  and  expiration  a  dehcate,  sighing,  rusthng  sound, 
which  from  its  supposed  seat  of  origin,  the  air-vesicles  or  -cells,  is 
known  as  the  vesicular  sound.  This  sound  is  supposed  to  be  due 
to  the  sudden  expansion  of  the  air-cells  during  inspiration  and  to  the 
friction  of  the  air  in  the  alveolar  passages. 


THE  CHEMISTRY  OF  RESPIRATION. 

The  general  nutritive  process  as  it  takes  place  in  the  tissues  in- 
volves the  assimilation  of  oxygen  and  the  evolution  of  carbon  dioxid. 
The  former  is  the  first,  the  latter  the  last,  of  a  series  of  chemic  changes 
the  continuance  of  which  is  essential  to  the  maintenance  of  all  hfe 
phenomena.  A  constant  supply  of  oxygen  and  an  equally  constant 
removal  of  carbon  dioxid  are  necessary  conditions  for  tissue  activity. 
The  respiratory  movements  constitute  the  means  by  which  the  oxygen 
of  the  air  is  brought  into,  and  the  carbon  dioxid  expelled  from,  the 
lungs  into  the  surrounding  air.  The  blood  is  the  medium  by  which 
the  oxygen  is  transported  from  the  lungs  to  the  tissues  and  the  carbon 
dioxid  from  the  tissues  to  the  lungs. 

The  exchanges  between  blood  and  tissues  constitute  internal 
respiration,  in  contradistinction  to  the  thoracic  movements  by  which 
the  air  is  brought  into  relation  with  the  blood,  and  which  constitute 
external  respiration.  The  transfer  of  the  oxygen  by  the  blood  from 
the  interior  of  the  lungs  to  the  tissues,  and  of  the  carbon  dioxid  from 
the  tissues  to  the  interior  of  the  lungs,  is  the  outcome  of  a  series  of 
chemic  changes  which  are  related  to  the  exchange  of  gases  between 
the  air  in  the  lungs  and  the  blood,  on  the  one  hand,  and  between 
the  blood  and  tissues,  on  the  other. 

In  consequence  of  the  many  and  complex  chemic  changes  which 
attend  these  gaseous  exchanges,  there  arise  changes  in  composition  of; 

1.  The  air  breathed. 

2.  The  blood,  both  arterial  and  venous. 

3.  The  tissue  elements  and  the  lymph  by  which  they  are  surrounded. 
The  investigation  of  the  nature  of  these  changes,  the  mechanism 

of  their  production,  and  their  quantitative  relations  constitutes  the 
subject-matter  of  the  chemistry  of  respiration. 


376  TEXT-BOOK  OF  PHYSIOLOGY. 


CHANGES  IN  THE  COMPOSITION  OF  THE  AIR. 

Experience  teaches  that  the  air  during  its  sojourn  in  the  lungs 
undergoes  such  a  change  in  composition  that  it  is  rendered  unfit  for 
further  breathing.  Chemic  analysis  has  shown  that  this  change 
involves  a  loss  of  oxygen,  a  gain  in  carbon  dioxid,  watery  vapor 
and  organic  matter.  For  the  correct  understanding  of  the  phenom- 
ena of  respiration  it  is  essential,  that  not  only  the  character  but  the 
extent  of  these  changes  be  known.  This  necessitates  an  analysis  of 
both  the  inspired  and  expired  airs,  from  a  comparison  of  which  certain 
deductions  can  be  made. 

The  results  which  have  been  obtained  are  represented  in  the 
following  table: 

Inspired  Air.  Expired  Air. 

(Oxygen,  20.80.  f Oxygen,  16.02. 
Carbon  dioxid, traces.  100  |  Carbon  dioxid, 4-38. 
Nitrogen,  79.20.  vols.  -I  Nitrogen,  79.60. 
Watery  vapor, variable.                            Watery  vapor, saturated. 

i  Organic  matter. 

These  analyses  indicate  that  under  ordinary  conditions  the  air 
loses  oxygen  to  the  extent  of  4.78  per  cent,  and  gains  carbon  dioxid 
to  the  extent  of  4.38  per  cent. ;  that  it  gains  in  nitrogen  to  the  extent 
of  0.4  per  cent,  and  in  watery  vapor  from  its  initial  amount  to  the 
point  of  saturation,  as  well  as  in  organic  matter.  It  is  to  these  changes 
in  their  totality  that  those  disturbances  of  physiologic  activity  are 
to  be  attributed  which  arise  when  expired  air  is  re-breathed  for  any 
length  of  time  without  having  undergone  renovation. 

Special  forms  of  apparatus  have  been  devised  for  the  collection 
and  analysis  of  gases.  Their  construction  as  well  as  the  methods 
of  analysis  involved  are  complicated  and  need  not  be  described  in 
this  connection.  The  presence  of  the  carbon  dioxid,  however,  may 
be  readily  shown  by  breathing  through  a  glass  tube  into  a  vessel  con- 
taining barium  or  calcium  hydrate.  The  turbidity  which  immediately 
follows  is  due  to  the  formation  of  barium  or  calcium  carbonate,  which 
can  be  due  only  to  the  presence  of  carbon  dioxid.  That  this  turbidity 
is  not  due  to  the  carbon  dioxid  normally  present  in  the  air  is  shown  by 
the  fact  that  the  solution  remains  clear  until  the  passage  of  the  atmos- 
pheric air  has  been  maintained  for  some  time.  From  the  percentage 
loss  of  oxygen  and  gain  in  carbon  dioxid,  the  total  oxygen  absorbed  and 
carbon  dioxid  exhaled  may  be  approximately  calculated.  Thus,  if 
the  volume  of  air  breathed  daily  be  accepted  at  either  10,800  or  12,- 
240  liters,  and  the  percentage  loss  of  oxygen  be  4.78,  the  total  oxygen 
absorbed  may  be  obtained  by  the  rule  of  simple  proportion,  e.  g.: 

100   :  4.78   ::  10,800   :  x  =   516  liters 
Or 

100   :  4.78   ::  12,240   :  x  =   585  liters. 


RESPIRATION.  377 

By  the  same  method  the  total  carbon  dioxid  exhaled  is  found  to  be 
either  473  or  526  liters;  volumes  in  both  instances  which  agree  very 
well  with  volumes  obtained  by  other  methods. 

From  the  fact  that  when  one  volume  of  oxygen  combines  with 
carbon  it  gives  rise  to  but  one  volume  of  carbon  dioxid,  it  is  evident 
that  of  the  oxygen  absorbed  the  greater  portion  by  far  is  utilized  in 
the  oxidation  of  the  carbon,  while  the  smaller  portion  is  utilized  in 
the  oxidation  of  other  substances,  but  especially  hydrogen,  as  shown 
by  the  increase  in  water  eliminated  beyond  that  consumed.  These 
amounts,  however,  are  not  fixed  but  variable,  and  depend  on  the 
quality  and  quantity  of  the  foods,  exercise,  etc.  The  ratio  of  the 
volume  of  the  carbon  dioxid  exhaled  to  the  volume  of  oxygen  absorbed 
is  known  as  the  respiratory  quotient,  and  is  usually  represented  by 

CO  > 

the  symbol  q-.  Thus  in  the  foregoing  analysis  the  respiratory 
quotient  is  0.916. 

The  gain  in  nitrogen  is  a  variable  factor,  ranging  from  zero  to 
0.9  per  cent.  This  gain  is  probably  of  accidental  occurrence,  due  to 
absorption  from  the  large  intestine,  in  which  decomposition  of 
nitrogen-holding  compounds  is  taking  place.  It  is  generally  believed 
that  free  nitrogen  plays  no  part  in  any  phenomenon  of  combination 
or  decomposition  within  the  body. 

The  gain  in  watery  vapor  will  depend  on  the  amount  previously 
present  in  the  air.  This  is  conditioned  by  the  temperature.  With 
a  rise  in  temperature  the  percentage  of  water  increases;  with  a  fall, 
it  decreases.  By  breathing  into  a  vessel  containing  pumice  stone 
saturated  with  sulphuric  acid,  the  vapor  may  be  collected.  The 
difference  observed  between  the  weight  before  and  after  breathing 
is  an  indication  of  the  amount  by  weight  of  water  exhaled  during  the 
time  of  breathing.  It  has  been  calculated  that  the  amount  of  water 
exhaled  daily  approximates  500  grams.  Though  invisible  at  ordinary 
temperatures,  it  becomes  visible  at  low  temperature  as  soon  as  it 
emerges  from  the  respiratory  tract.  The  loss  of  heat  is  followed  by 
a  condensation  of  the  vapor,  which  appears  at  once  as  a  cloudy  pre- 
cipitate. 

The  gain  in  organic  matter  is  also  variable.  The  amount  present 
is  not  sufficient  to  permit  of  a  thorough  chemic  analysis,  but  there  are 
reasons  for  beheving  that  it  belongs  to  the  proteid  group  of  bodies. 
If  it  accumulates  in  the  air,  especially  at  high  temperatures,  it  readily 
undergoes  decomposition,  with  the  production  of  offensive  odors. 
Traces  of  free  ammonia  have  also  been  found  in  the  expired  air. 
In  addition  to  these  chemic  changes,  the  air  experiences  physical 
changes;  e.  ^.,  a  rise  in  temperature  and  an  increase  in  volume.  The 
rise  in  temperature  can  be  shown  by  breathing  through  a  suitable 
mouthpiece  into  a  glass  tube  containing  a  thermometer.  By  this 
means  it  has  been  shown  that  inspired  air  at  20°  C.  rises  in  tern- 


378  TEXT-BOOK  OF  PHYSIOLOGY. 

perature  to  37°  C;  at  6.3°  to  29.8°  C.  The  increase  in  the  tem- 
perature will  depend  upon  that  of  the  air  inspired  and  the  time  it 
remains  in  the  lungs.  If  retained  a  sufficient  length  of  time  it  will 
always  become  that  of  the  body.  As  a  result  of  the  heat  absorption 
the  expired  air  increases  in  volume  about  one-ninth  over  that  of  the 
inspired  air.  When  corrected  for  temperature  and  pressure  and 
freed  from  aqueous  vapor,  the  volume  of  the  expired  air  is  less  than 
that  of  the  inspired  air  by  about  one-two  hundred  and  fiftieth. 

The  Composition  of  the  Alveolar  Air. — The  foregoing  state- 
ment of  the  composition  of  the  expired  air,  derived  in  part  from  the 
upper  air-passages,  trachea,  and  bronchi,  does  not  necessarily  repre- 
sent the  composition  of  the  alveolar  air.  It  is  very  probable  that  the 
percentage  of  carbon  dioxid  is  greater,  the  percentage  of  oxygen  less, 
in  the  latter  than  in  the  former.  This  is  made  evident  by  collecting 
in  several  portions  the  expired  air  as  it  escapes  from  the  respiratory 
tract  and  subjecting  it  to  analysis.  The  last  portion  always  contains 
a  larger  amount  of  carbon  dioxid  and  a  smaller  amount  of  oxygen 
than  the  first  portion.  The  determination  of  the  composition  of  the 
alveolar  air  is  extremely  difficult.  It  has  been  estimated  to  contain 
from  5  to  6  per  cent,  of  carbon  dioxid  and  from  14  to  18  per  cent.'__of 
oxygen. 

Pulmonary  Ventilation. — It  is  owing  largely  to  this  inequahty 
of  volumes  and  consequently  of  the  "  partial  pressures"  of  these  two 
gases  in  the  trachea  and  alveoli  that  the  degree  of  ventilation  necessary 
to  exchange  of  gases  between  lungs  and  air  is  maintained.  Though 
the  respiratory  movements  doubtless  create  currents  in  the  air-passages 
which  carry,  on  the  one  hand,  a  portion  of  the  inspired  air  directly 
into  the  alveoh,  and,  on  the  other  hand,  carry  a  portion  of  the  alveolar 
air  directly  out  of  the  body,  other  portions  find  their  way  into  and  out 
of  the  alveoli  in  accordance  with  the  laws  of  diffusion.  If  the  tension 
of  the  oxygen  in  the  trachea  is  158  mm.  Hg  and  in  the  alveoh  114  mm. 
Hg,  diffusion  downward  will  take  place.  Equilibrium,  however, 
is  never  estabhshed,  as  the  oxygen  is  continually  disappearing  by 
passing  into  the  blood.  On  the  contrary,  if  the  carbon  dioxid  tension 
in  the  alveoli  is  38  to  40  mm.  Hg,  and  in  the  trachea  0.3  mm.  Hg, 
diffusion  will  take  place  upward.  EquiUbrium  will  never  be  estab- 
hshed, however,  as  the  carbon  dioxid  is  constantly  coming  out  of 
the  blood.  Pulmonary  ventilation  may  also  be  aided  by  those 
alternate  changes  in  volume  of  the  heart,  great  vessels,  and  lungs 
occurring  as  the  result  of  the  heart-beat  and  producing  the  so-called 
cardio-pneumatic  movements. 

CHANGES  IN  THE  COMPOSITION  OF  THE  BLOOD.* 

The  blood  which  flows  into  the  lungs  through  the  pulmonary 
artery  is  dark  bluish-red,  that  which  flows  from  the  lungs  into  the 


RESPIRATION.  379 

pulmonary  veins  is  scarlet  red,  in  color.  The  blood  is  changed,  while 
flowing  through  the  lung  capillaries,  from  the  venous  to  the  arterial 
condition.  As  the  air  in  the  lungs  gains  carbon  dioxid  and  loses 
oxygen,  it  is  fair  to  assume  that  what  the  air  gains  the  blood  loses, 
and  what  the  air  loses  the  blood  gains.  In  other  words,  the  blood, 
while  passing  through  the  lungs,  is  changed  from  venous  to  arterial 
by  the  loss  of  carbon  dioxid  and  the  gain  of  oxygen.  The  change 
in  color  of  venous  blood  from  dark  bluish  to  scarlet  red  is  strikingly 
shown  by  shaking  it  in  a  test-tube  with  oxygen  or  atmospheric  air. 

The  blood  which  flows  into  the  tissues  through  the  arteries  is  red, 
that  which  flows  from  the  tissues  into  the  veins  is  bluish,  in  color. 
The  blood  while  flowing  through  the  tissue  capillaries  is  changed 
from  the  arterial  to  the  venous  condition.  Since  arterial  blood  when 
deprived  of  oxygen  becomes  bluish-red,  the  indication  is  that  the 
change  in  color  is  associated  with,  if  not  entirely  due  to,  the  escape 
of  oxygen  into  the  tissues.  The  constant  ehmination  of  carbon 
dioxid  from  the  blood  into  the  lungs  indicates  that  the  carbon 
dioxid  is  as  constantly  passing  from  the  tissues  through  the  capillary 
walls  into  the  blood. 

These  considerations  are  confirmed  by  the  results  of  analyses 
which  have  been  made  of  both  venous  and  arterial  blood.  The 
presence  of  gas  in  the  blood  is  demonstrated  by  subjecting  it  under 
appropriate  conditions  to  the  vacuum  of  the  mercurial  air-pump, 
into  which  it  at  once  escapes.  From  100  volumes,  an  average  of  60 
volumes  of  gas  at  standard  pressure,  760  mm.  Hg  and  temperature 
0°  C,  can  thus  be  obtained. 

Gases  of  the  Blood. — An  analysis  of  the  volumes  of  gas  removed 
from  both  venous  and  arterial  blood  shows  that  each  consists  of 
oxygen,  carbon  dioxid,  and  nitrogen,  though  in  different  amounts. 
An  average  composition  of  the  gases  extracted  from  dog's  blood 
obtained  from  the  right  ventricle  and  carotid  artery  is  given  in  the 
following  table: 


1,1     J  fOxvs^en, 0-12  vols.  »  ..    •  1  ui     j  f  Oxvgen, 20  vols. 

enous  blood     ^   -^       ,.     . ,  ^     _     „  Arterial  blood     ^   ^-P       ,■     .,  <, 

,         i  Carbon  dioxid,     4=;  1         i  Carbon  dioxid,     40 

100    vols.  .-,  ^^       ,{  100    vols.  -.TV  " 

t>^itrogen, i-  2  I. Nitrogen,   —      1-2 


The  changes  produced  in  the  blood  by  respiration,  both  external 
and  internal,  become  apparent  from  a  comparison  of  these  analyses. 
The  venous  blood  while  passing  through  the  lungs  gains  from  eight 
to  eleven  volumes  per  cent,  of  oxygen  and  loses  five  volumes  per 
cent,  of  carbon  dioxid.  The  arterial  blood  while  passing  through 
the  tissues  loses  ox}^gen  and  gains  carbon  dioxid  in  corresponding 
amounts.     The  volume  of  nitrogen  is  not  appreciably  changed. 

The  Relation  of  the  Gases  in  the  Blood. — The  mechanism 
by  w^hich  the  gases  become  associated  with  the  blood  at  the  moment 


38o  TEXT-BOOK  OF  PHYSIOLOGY. 

of  their  entrance  into  it,  and  again  become  dissociated  just  prior  to 
their  exit  from  it,  as  well  as  their  relation  while  in  transit,  will  be  more 
readily  understood  after  reference  to  &  few  elementary  facts  relative 
to  the  absorption  of  gases  in  general  and  the  conditions  of  temperature 
and  pressure  by  which  it  is  influenced. 

It  is  well  known  that  Hc^uids  will  take  up,  absorb,  or  dissolve 
unequal  volumes  of  different  gases  in  accordance  with  their  solu- 
bilities and  with  variations  in  temperature  and  pressure.  Water, 
for  example,  will  absorb  oxygen,  carbon  dioxid,  and  nitrogen  as  well 
as  other  gases  in  amounts  varying  with  the  foregoing  conditions. 
The  amount  of  any  gas  absorbed  by  one  volume  of  a  liquid  at  a  tem- 
perature of  o°  C.  and  a  pressure  of  760  mm.  Hg  is  known  as  the  co- 
efficient of  absorption.  The  coefficient  of  absorption  of  i  volume 
of  distilled  water  for  oxygen  is  0.0489  volume;  of  carbon  dioxid, 
1.797  volumes;  of  nitrogen,  0.023  volume.  With  a  rise  in  tem- 
perature, however,  the  absorptive  power  of  water  for  each  one  of 
these  gases  diminishes.  On  the  contrary,  as  the  pressure  rises  the 
quantity  of  the  gas  absorbed  increases,  and  as  it  falls,  decreases. 
In  all  gaseous  determinations,  therefore,  it  is  always  necessary,  for 
purposes  of  comparison,  to  reduce  the  obtained  volumes  to  standard 
temperature  (0°  C.)  and  pressure  (760  mm.  Hg). 

If  water  be  exposed  to  atmospheric  air  consisting  of  oxygen, 
carbon  dioxid,  and  nitrogen  in  the  ordinary  proportions,  at  any 
given  temperature  and  pressure,  the  water  will  absorb  unequal 
volumes  of  each  of  the  three  gases.  The  pressure  under  which 
each  gas  is  absorbed  is  a  part  only,  however,  of  the  total  atmos- 
pheric pressure  at  the  time.  The  pressure  exerted  by  any  one  of 
these  three  gases  is  known  as  its  partial  pressure,  and  depends  on  the 
percentage  volume  of  the  gas  present.  If  atmospheric  air  contains 
at  standard  pressure  and  temperature  79.15  volumes  per  cent,  of 
nitrogen,  its  partial  pressure  will  be  ^{^  of  760,  or  601.54  mm.  Hg; 
if  the  air  contains  0.04  volume  per  cent,  of  carbon  dioxid  and  20.85 
volumes  per  cent,  of  oxygen,  the  partial  pressure  of  each  will  be  0.30 
mm.  Hg  and  158.46  mm.  Hg  respectively.  The  absorption  of  each 
gas  is  independent  of  all  the  rest,  and  is  the  same  for  nitrogen,  for 
example,  as  if  it  alone  were  present  at  a  pressure  of  601.54  mm.  Hg. 

Again,  if  water  holding  in  solution  a  certain  volume  of  a  gas — 
carbon  dioxid,  for  example — be  exposed  to  an  atmosphere  containing 
but  0.04  volume  per  cent,  of  carbon  dioxid,  and  having  therefore  a 
pressure  of  but  0.3  mm.  Hg,  the  gas  will  at  once  begin  to  leave  the 
water,  and  continue  to  do  so  until  the  pressure  of  the  carbon  dioxid 
in  the  atmosphere  balances  the  tension  of  the  gas  in  the  water,  at  which 
moment  the  escape  of  the  gas  ceases.  The  tension  of  a  gas  in  a  liquid 
is  equal  to  that  pressure  in  milhmeters  of  mercury  of  the  same  gas  in 
the  atmosphere  which  is  required  to  keep  it  in  solution.     Pressure 


RESPIRATION.  3S1 

and  tension  are  therefore  in  this  case  convertible  terms.  What  is 
true  for  the  carbon  choxid  is  true  for  any  other  gas  that  may  be  in 
solution.  It  will  be  recalled  that  the  blood  yields  up  its  gases  when 
subjected  to  the  vacuum  of  the  mercurial  pump;  that  is,  to  a  diminu- 
tion or  complete  removal  of  the  atmospheric  pressure.  From  this 
it  might  be  inferred  that  the  gases  are  merely  held  in  solution  by 
pressure,  and  at  once  escape  the  moment  they  are  exposed  to  a 
space  in  which  there  is  a  very  slight  or  a  total  absence  of  pressure. 
It  is  therefore  necessary  to  test  this  supposed  condition  of  the  gases 
in  the  blood  by  subjecting  the  latter  to  gradually  diminishing  pres- 
sures, with  a  view  of  determining  in  how  far  the  evolution  of  the  gases 
follows  the  law  of  partial  pressures.  For  convenience  the  conditions 
of  each  gas  will  be  considered  separately. 

Oxygen. — If  blood  is  subjected  to  a  succession  of  pressures  pro- 
gressively less  than  the  standard,  it  is  found  that  though  oxygen  is 
evolved,  its  evolution  is  not  in  accordance  with  the  law  of  partial 
pressures ;  that  is,  in  proportion  to  the  diminution  of  pressure.  Within 
wide  limits — e.  g.,  from  760  to  238  mm.  atmospheric  pressure,  to 
which  correspond  oxygen  pressures  of  160  and  50  mm.  respec- 
tively— there  is  but  a  shght  increase  in  the  amount  of  oxygen  evolved ; 
and  it  is  not  until  the  pressure  of  the  oxygen  falls  to  about  40  to  3c 
mm.  that  it  begins  to  be  liberated  in  large  amounts.  From  this  on, 
the  oxygen  continues  to  be  liberated  with  decreasing  pressures,  until 
the  zero  point  is  reached,  when  all  gaseous  discharge  ceases.  Co- 
incidently  the  blood  changes  in  color  from  a  bright  red  to  a  deep  bluish- 
red.  It  is  evident  from  the  results  of  this  procedure  that  the  con- 
dition of  the  oxygen  in  the  blood  is  but  to  a  shght  extent  one  of 
physical  absorption.  The  indications  are  that  it  partakes  of  the 
nature  of  a  chemic  combination. 

If  the  red  corpuscles  be  removed  from  the  blood  and  the  plasma 
alone  be  treated  in  the  manner  above  described,  it  will  be  found  that  the 
oxygen  liberated  now  follows  the  law  of  partial  pressure.  The  amount 
so  liberated,  however,  is  small — about  one  per  cent,  of  the  total  oxygen 
of  the  blood.  The  agent  therefore  which  holds  the  oxygen  in  com- 
bination is  the  red  corpuscle,  and  more  especially  the  hemoglobin, 
which  constitutes  about  94  per  cent,  of  its  volume.  This  is  proved  by 
the  fact  that  a  solution  of  gas-free  hemoglobin  of  a  strength  equivalent 
to  that  of  the  blood  (14  per  cent.),  exposed  to  gradually  increasing 
pressures  from  zero  up  to  30  or  40  mm.  oxygen  pressure,  will  absorb 
large  quantities  of  oxygen;  beyond  this  point  the  amount  absorbed 
is  again  small  in  comparison.  At  70  mm.  pressure  the  hemoglobin 
is  almost  saturated.  Coincidently  with  this  absorption  the  hemo- 
globin changes  in  color  from  dark  blue  to  bright  red;  changes  from 
hemoglobin  to  oxyhemoglobin.  The  reverse  method,  that  of  subjecting 
oxyhemoglobin  to  gradually  diminishing  pressures,  yields  opposite 


382  TEXT-BOOK  OF  PHYSIOLOGY. 

results.  As  one  gram  of  hemoglobin  combines  with  1.59  c.c.  of 
oxygen,  and  as  the  percentage  of  hemoglobin  is  13.50  to  14,  it  is 
evident  that  there  is  sufficient  hemoglobin  to  combine  with  practically 
all  the  oxygen  usually  present  in  the  blood. 

The  relation  of  the  oxygen  in  the  blood  is  therefore  partly  physi- 
cal, partly  chemical.  One  per  cent,  is  physically  absorbed  by  or 
dissolved  in  the  plasma;  the  remainder  is  chemically  combined  with 
the  hemoglobin. 

The  association  or  combination  of  oxygen  is  favored  by  a  pressure 
of  at  least  from  30  to  50  mm.  Hg  and  upward;  the  dissociation,  by 
diminution  of  pressure.  In  the  conversion  of  hemoglobin  into  oxy- 
hemoglobin two  antagonistic  forces  are  at  work,  heat  and  chemic 
affinity.  The  former  endeavors  to  prevent,  the  latter  to  favor, 
the  union.  Chemic  affinity  increases  with  the  influence  of  mass,  that  is, 
in  proportion  to  the  number  of  atoms  in  a  unit  of  volume,  with  the 
density  and  with  the  partial  pressure  of  the  oxygen.  Diminution  of 
pressure  reduces  the  mass  influence  and  permits  the  heat  to  bring 
about  dissociation  (Bunge).  The  following  table  by  Hiifner  shows 
the  relative  proportion  of  hemoglobin  and  oxyhemoglobin  in  blood 
containing  14  per  cent,  hemoglobin  and  exposed  to  air  at  gradually 
diminishing  pressures: 


HERic  Pressure 
!   MM.   Hg. 

Partial  Pressure  of 
Oxygen  in  mm.  Hg. 

Hemoglobin 
Percentage. 

Oxyhemoglobin 
Percentage. 

760 
524.8 
357-8 
2380 

159-3 
no 

.   75 
50 

1.49 
2.14 

3-II 
4.60 

98.51 
97.86 
96.89 
95-4° 

119  3 
47-7      . 
23.8 

25 
10 

5 

8.79 
19.36 
32-51 

91.21 
80.64 
67.49 

0.0 

0.0 

I03.00 

o.oo 

Carbon  Dioxid. — The  blood  yields  up  its  contained  carbon 
dioxid  to  the  vacuum  of  the  gas-pump  as  completely  as  it  does  its 
oxygen.  The  same  is  not  the  case,  however,  if  the  red  corpuscles  are 
first  removed  and  the  experiment  made  with  either  plasma  or  serum. 
Even  at  zero  pressure  the  fluid  contains  carbon  dioxid,  as  shown  by 
its  liberation  on  the  addition  of  some  weak  acid,  as  tartaric  or  phos- 
phoric, an  indication  that  it  exists  in  a  state  of  firm  combination. 
The  same  result  follows  the  addition  of  the  red  blood-corpuscles, 
which  act  in  a  manner  similar  to  the  acids  just  mentioned.  This 
property  of  the  corpuscles  has* been  attributed  to  hemoglobin,  and 
especially  when  in  the  state  of  oxyhemoglobin.  It  is  for  this  reason 
that  blood  yields  all  its  carbon  dioxid  to  the  vacuum  of  the  gas-pump. 

The  Hmit  of  pressure  at  which  the  plasma  ceases  to  physically 
absorb  carbon  dioxid  and  begins  to  chemically  combine  it  is  not  very 
clearly  defined.  It  has  been  estimated  that  of  the  entire  amount, 
38  to  45  volumes,  only  about  2.5  volumes  are  so  absorbed,  the  re- 
mainder beino;  in  a  condition  of  both  loose  and  stable  combination. 


RESPIRATION.  383 

An  analysis  of  the  serum,  and  presumably  of  the  plasma,  shows 
the  presence  of  sodium  salts,  with  which  the  carbon  dioxid  could 
enter  into  combination,  viz.:  sodium  carbonate  and  dibasic  sodium 
phosphate.  The  sodium  is  thus  partly  divided  between  carbonic  acid 
and  phosphoric  acid.  The  amount  of  the  sodium  which  falls  to 
carbon  dioxid  will  depend  on  the  mass  influence  of  the  latter;  that 
is,  its  partial  pressure. 

At  its  origin  in  the  tissues  the  carbon  dioxid  acquires  a  consider- 
able tension,  and  its  mass  influence  is  correspondingly  large.  On 
entering  the  blood  it  combines  with  sodium  carbonate,  with  the 
formation  of  sodium  bicarbonate,  as  shown  in  the  following  equation : 

NajCOa  +   CO2  +   HjO   =   2NaHC03. 

At  the  same  time,  having  a  greater  mass  influence  than  the  phos- 
phoric acid,  it  wdll  withdraw  from  the  dibasic  sodium  phosphate 
one-half  of  its  sodium,  with  the  formation  of  sochum  bicarbonate  and 
monobasic  sodium  phosphate,  as  shown  in  the  following  equation: 

NajHPO^  +   CO2  -f   H2O   =  NaHCOj  -t-   NaHjPO,. 

With  the  diffusion  of  the  carbon  dioxid  from  the  blood  into  the 
alveoH  its  tension  in  the  venous  blood  falls,  its  mass  influence  dimin- 
ishes, while  that  of  the  phosphoric  acid  relatively  increases.  As  a 
result,  the  sodium  is  withdrawn  from  the  sodium  bicarbonate,  an 
additional  liberation  of  carbon  dioxid  takes  place  and  dibasic  sodium 
phosphate  is  re-formed.  The  association  or  combination  of  the 
carbon  dioxid  with  the  basic  salts  depends  on  its  partial  pressure;  its 
dissociation  in  the  lungs,  on  a  diminution  of  pressure. 

Nitrogen. — This  gas  exists  in  both  arterial  and  venous  blood 
in  a  state  of  solution.  There  is  no  evidence  that  it  enters  into  com- 
bination with  any  other  element. 

Tension  of  the  Gases  in  the  Blood. — It  will  be  recalled  that  a 
Hquid  holding  in  solution  one  or  more  gases  will  on  exposure  to  an 
atmosphere  composed  of  the  same  gases  either  give  up  or  absorb 
volumes  varying  in  amount  and  in  accordance  with  their  partial 
pressures  until  equihbrium  is  established.  If  the  pressure  of  any  one 
gas  in  the  atmosphere  is  greater  than  in  the  liquid,  it  is  absorbed; 
if  the  pressure  is  less,  it  is  discharged.  Knowing  the  pressure  of  the 
gases  in  percentages  of  an  atmosphere,  at  the  beginning  and  the  end 
of  an  experiment,  the  original  tension  or  pressure  of  the  gases  in  the 
liquid  can  be  easily  calculated.  On  this  principle  various  forms  of 
apparatus  known  as  aerotonometers  have  been  devised  by  which  the 
tension  of  the  gases  in  the  blood  can  be  determined. 

These  apphances  consist  essentially  of  a  glass  tube  containing 
oxygen  and  carbon  dioxid  in  known  amounts  and  tensions.     The 


384  TEXT-BOOK  OF  PHYSIOLOGY. 

blood  from  an  animal  is  then  allowed  to  flow  directly  from  an  artery 
or  vein  into  the  tube.  As  it  flows  down  its  sides  in  a  thin  layer  it 
presents  a  large  surface  to  the  action  of  the  contained  gases.  In  the 
aerotonometer  of  Fredericq  the  blood  made  non-coagulable  by  the 
injection  of  peptone  is  returned  from  the  opposite  extremity  of  the 
tube  to  the  animal.  This  enables  the  experiment  to  be  continued 
for  an  hour  or  more.  A  knowledge  of  the  tensions  of  the  blood  gases 
is  of  interest,  as  it  affords  a  clue  to  the  mechanism  by  which  the 
interchange  takes  place  between  the  lungs  and  the  blood,  on  the  one 
hand,  and  the  blood  and  tissues,  on  the  other.  The  results,  however, 
of  different  observers  are  not  sufliciently  in  accord  to  permit  of  positive 
deductions. 

In  the  well-known  and  generally  accepted  experiments  of  Strass- 
burger,  the  tension  of  the  oxygen  in  the  arterial  blood  of  the  dog 
was  found  to  be  29.64  mm.  Hg,  or  3.9  per  cent,  of  an  atmosphere,  and 
in  the  venous  blood  22.04  mm.  Hg,  or  2.9  per  cent.  The  tension 
of  the  carbon  dioxid  in  the  venous  blood  was  found  to  be  41.04  mm, 
Hg,  or  5.4  per  cent,  of  an  atmosphere,  and  in  the  arterial  blood  21.8 
mm.  Hg,  or  2.8  per  cent.  In  the  experiments  of  Fredericq  the  oxygen 
tension  in  the  arterial  blood  was  found  to  be  106  mm.  Hg,  or  14  per 
cent,  of  an  atmosphere. 


CHANGES  IN  THE  COMPOSITION  OF  THE  TISSUES  AND  LYMPH. 

From  previous  statements  the  inferences  can  be  drawn  that  the 
oxygen  leaves  the  blood  as  the  latter  flows  through  the  capillaries; 
that  it  passes  through  the  capillary  wall  into  the  surrounding  lymph 
and  so  to  the  tissue-cells;  that  it  oxidizes  food  materials  in  the  tissue- 
cells  whereby  the  potential  energy  of  the  former  is  hberated  as  kinetic 
energy;  that  the  carbon  dioxid  so  evolved  passes  into  the  lymph 
and  through  the  wall  of  the  capillary  into  the  blood. 

While  this  is  doubtless  the  case,  the  presence  of  free  oxygen  in  the 
tissues  can  not  be  demonstrated  by  the  usual  methods  of  gas  analysis. 
Only  in  the  saHva  and  in  the  blood  of  the  placental  umbiHcal  vein  can 
it  be  shown  that  oxygen  has  directly  passed  through  the  capiUary  wall. 
For  this  reason  it  has  been  claimed  by  a  few  investigators  that  oxygen 
does  not  leave  the  blood,  but  that  the  field  of  its  activity  as  an  oxidizing 
agent  is  limited  to  the  blood-current,  where  it  meets  with  and  oxidizes 
easily  reducible  substances  entering  from  the  tissues.  On  this  view 
the  potential  energy  of  the  food  would  be  liberated  by  mere  decom- 
position or  cleavage  in  consequence  of  cell  activity. 

Nevertheless  many  facts  from  the  fields  of  comparative  physi- 
ology and  physiologic  chemistry  combine  to  support  the  view  that 
oxygen  is  absolutely  necessary  to  the  maintenance  of  the  life  of  all 
tissue-cells.     Though  thev  will  continue  to  manifest  their  character- 


RESPIRATION.  385 

istic  activities — e.  g.,  contraction  on  the  part  of  a  muscle,  secretion  by  a 
gland,  the  conduction  of  a  nerve  impulse  by  the  nerve,  etc. — for  a 
variable  length  of  time  after  oxygen  is  prevented  from  gaining  access 
to  them,  nevertheless  they  will  in  due  time  die. 

The  necessity  for  oxygen  on  the  part  of  the  tissues  and  the  avidity 
with  which  they  absorb  it,  is  shown  by  their  power  of  reducing  pig- 
ments such  as  alizarine  blue.  If  this  pigment  be  injected  into  the 
blood-vessels  of  an  animal  and  the  animal  killed  in  about  ten  minutes, 
it  will  be  found  that  while  the  blood  exhibits  a  deep  blue  color  the 
tissues  present  their  usual  colors.  But  after  exposure  to  the  air  or  to 
free  oxygen  the  latter  also  acquire  the  characteristic  blue  color.  The 
explanation  offered  for  this  fact  is  that  the  tissues  in  their  need  for 
oxygen  absolutely  extract  it  from  the  pigment,  reducing  it  to  a  color- 
less compound,  which,  however,  on  exposure  recombines  with  oxygen 
and  regains  the  original  color. 

Though  free  oxygen  can  not  be  shown  to  be  present  in  the  tissues, 
there  are  many  reasons  for  beheving  that  it  is  continually  passing  into 
them  by  way  of  the  lymph-stream.  Its  rapid  disappearance  would 
indicate  that  it  is  immediately  utihzed  for  the  production  of  carbon 
dioxid  (which  is  improbable  on  other  grounds),  or  that  the  tissues 
possess  a  capacity  for  oxygen  storage,  of  placing  it  in  reserve  under 
some  combination  or  other,  by  which  it  can  be  securely  retained 
until  required  for  oxidation  purposes.  This  is  rendered  probable 
from  the  fact  that  the  carbon  dioxid  evolved  at  any  given  moment  is 
not  necessarily  dependent  on  the  oxygen  just  absorbed,  for  if  oxygen 
be  withheld  from  a  nutritive  fluid  which  is  being  artificially  circulated 
through  a  recently  isolated  organ,  carbon  dioxid  will  continue  to  be 
discharged  for  some  time.  A  muscle,  or  even  a  living  animal, — e.  g., 
a  frog, — placed  in  an  atmosphere  of  pure  nitrogen  will  remain 
active  and  evolve  COj  for  even  several  hours. 

Naturally  the  absorption  of  oxygen  and  the  discharge  of  carbon 
dioxid  and  the  changes  of  composition  which  are  incident  to  nutri- 
tion will  be  most  marked  in  those  tissues  characterized  by  the 
greatest  degree  of  physiologic  activity.  ^luscle-tissue  exhibits  these 
changes  to  a  greater  degree  than  bone.  Tissues  with  inter- 
mediate degrees  of  activity  should  exhibit  corresponding  degrees 
of  respiratory  change.  Experiment  confirms  this  view.  Thus,  100 
grams  each  of  muscle,  spleen,  and  broken  bone  from  a  recently 
Hving  animal  exposed  to  the  air  for  twenty-four  hours  absorbed 
respectively  50.8  c.c,  27.3  c.c,  and  17.2  c.c.  of  oxygen,  while  each 
discharged  during  the  same  period  56.8  c.c,  15.4  c.c,  and  8.1  c.c  of 
carbon  dioxid  respectively.  In  another  series  of  experiments  by  a 
different  observer  100  grams  of  muscle  absorbed  in  three  hours 
23  c.c.  of  oxygen,  and  100  grams  of  bone  5  c.c.  of  oxygen.  Both 
tissues  discharged   carbon  clioxid  in   amounts  proportional  to  the 


386 


TEXT-BOOK  OF  PHYSIOLOGY. 


ATMOSPHERIC  AIR. 
0-I58    MM    MG,OR  20.85  P.C 
CO     0.3   MM  HG    OR   0.04-  PC 


O-TEN  SION  

2.Z.  0+  M  M  HG  OR 
2.. 9    P.C. 


CO_  TENSION 
2. 
■♦I.04-    M  M   H  G  OR 
5.4    PC. 


AL-VEOLUS 


oxygen  absorbed.  The  same  respiratory  changes  may  be  more 
satisfactorily  demonstrated  by  passing  blood  through  the  tissues  of 
isolated  organs  and  the  tissues  of  recently  living  animals.  The 
analysis  of  the  blood  before  and  after  perfusion  shows  a  loss  of  oxygen 
and  a  gain  in  carbon  dioxid. 

Tension  of  the  Gases  in  the  Tissues. — As  the  presence  of  free 
oxygen  can  not  be  demonstrated,  its  tension  there  must  be  regarded 
as  nil.  The  tension  of  the  carbon  dioxid  is  quite  high,  though 
difficult  of  exact  determination.  It  has  been  estimated  at  from 
45  to  68  mm.  Hg,  or  from  6  to  9  per  cent,  of  an  atmosphere. 

■  The  variations 
of  tension  or  pres- 
sure of  these  two 
gases  in  the  lungs, 
in  different  parts  of 
the  vascular  ap- 
paratus, and  in  the 
tissues,  and  their  re- 
lations to  each 
other,  are  shown  in 
Fig.  177,  expressed 
in  mm.  Hg  and  per- 
centages of  an  at- 
mosphere. 

The  Mechan- 
ism of  the  Gas- 
eous Exchange. — 
In  these  pressure 
differences  s  u  f  f  i  - 
cient  cause  is  found 
for  the  exchange  of 
the  gases.  The 
oxygen  pressure  in 
the  alveoli  being  in 
excess  of  that  in  the 
blood,  the  gas  passes 
through  the  thin  al- 
veolo-capillary  wall  into  the  plasma.  As  the  pressure  in  the  plasma 
rises,  the  oxygen  combines  with  the  hemoglobin,  until  the  latter  is  al- 
most saturated.  On  passing  into  the  systemic  capillaries  the  blood 
enters  a  region  in  which  the  oxygen  tension  of  the  surrounding 
tissues  is  nil.  At  once  a  dissociation  of  the  oxyhemoglobin  and 
oxygen  takes  place,  after  which  the  latter  passes  through  the  capillary 
wall  into  the  plasma  and  so  to  the  tissue-cells,  in  which  it  is  stored 
and  utilized.     The  sojourn  of  the  blood  in  the  capillaries  being  of 


VENOUS 
BI.OOD 


ARTERIAL 
BLOOD 


O  -TENSION 

o.  00   M  M   H  a 

CO   -  TENSION 


-TENSION 
29. S*  M  M  HC  OR 
3  9  P.C. 


CO  —TENSION 


TISSUES 


Fig.  177. — Diagram  showing  the  Relative  Tension 
OF  Oxygen  and  Carbon  Dioxid  in  the  Lungs, 
IN  the  Blood,  and  in  the  Tissues. 


RESPIRATION.  387 

short  duration,  the  oxyhemoglobin  can  part  with  but  a  portion  of  its 
oxygen,  sufficient,  however,  to  satisfy  the  needs  of  the  tissues. 

The  carbon  dioxid  pressure  in  the  tissues  being  in  excess  of  that 
in  the  blood,  it  passes  through  the  capillary  wall  into  the  blood,  where 
it  exists  in  the  free  and  combined  states.  On  passing  into  the  pul- 
monic capillaries  the  blood  enters  a  region  in  which  the  carbon 
dioxid  in  the  alveoli  is  less  than  in  the  blood.  At  once  a  diffusion  and 
dissociation  of  the  carbon  dioxid  takes  place  through  the  alveolo- 
capillary  wall  until  equilibrium  is  established.  This,  however,  is  of 
very  short  duration,  for  the  carbon  dioxid  so  eliminated  is  rapidly 
removed  from  the  lungs  by  the  respiratory  movements. 

While  diffusion,  in  response  to  physical  and  chemic  conditions, 
thus  plays  a  large  part  in,  and  is  suthcient  to  account  for,  the  ex- 
changes of  gases,  it  is  possible  that  the  alveolar  or  respiratory  epithe- 
lium may  also  play  an  essential  role.  It  is  believed  by  some  in- 
vestigators that  it  is  active  in  both  the  absorption  of  oxygen  and  the 
excretion  of  carbon  dioxid.  This  view  has  been  suggested  as  a 
means  of  interpreting  the  results  of  the  experiments  of  more  recent 
investigators,  made  with  a  view  of  determining  the  tension  of  the 
blood  gases.  It  was  found  by  Bohr  that  the  tension  of  the  oxygen 
in  arterial  blood  was  often  as  high  as  loi  to  144  mm.  Hg,  and  in 
many  instances  higher  than  the  tension  of  the  oxyen  in  the  trachea, 
while  the  carbon  dioxid  tension  in  the  trachea  was  higher  than  in 
the  blood.  Haldane  and  Smith  by  a  different  method  found  an  oxy- 
gen tension  in  the  arterial  blood  of  200  mm.  Hg.  If  these  results 
should  prove  to  be  correct,  though  they  are  at  present  subject  to  con- 
siderable criticism  and  not  generally  accepted,  some  other  force  than 
diffusion  would  have  to  be  found  to  explain  the  facts.  It  would  then 
remain  to  determine  in  how  far  the  alveolar  epithelium  could  be 
regarded  as  an  active  agent  in  both  absorption  and  excretion  in 
opposition  to  pressure. 


THE  TOTAL  RESPIRATORY  EXCHANGE. 

The  total  quantities  of  oxygen  absorbed  and  carbon  dioxid  dis- 
charged by  a  human  being  in  twenty-four  hours  are  measures  of  the 
intensity  of  the  respiratory  process,  and  an  indication  of  the  extent  and 
character  of  the  chemic  changes  attending  all  life  phenomena.  Their 
determination  and  their  relation  to  each  other  are  matters  of  interest 
and  importance.  The  quantities  which  have  been  obtained  by  differ- 
ent observers  are  the  outcome  of  calculations  based  on  certain  groups 
of  data  and  of  experiments  made  with  special  forms  of  apparatus. 

Thus  from  the  total  air  breathed  daily,  estimated  from  the  amounts 
obtained  during  a  longer  or  shorter  period,  of  experiments  with  spiro- 
metric  apparatus,  and  from  the  percentage  loss  of  oxygen  and  gain  of 


388  TEXT-BOOK  OF  PHYSIOLOGY. 

carbon  dioxid  shown  by  an  analysis  of  the  respired  air,  it  can  be  cal- 
culated at  least  approximately  what  the  total  amounts  of  oxygen  ab- 
sorbed and  carbon  dioxid  exhaled  must  be.  If  it  be  assumed  that 
the  minimum  daily  volume  of  air  breathed  is  io,8co  liters  and  the 
maximum  volume  12,240  liters,  and  the  percentage  loss  of  oxygen  is 
4.78,  then  the  total  volume  of  oxygen  absorbed  is  516  liters  (735.17 
grams)  or  585  liters  (836.42  gramsj.  By  the  same  method  the  total 
carbon  dioxid  exhaled  daily  is  found  to  be  either  473  liters  (931.8  grams) 
or  526  liters  (1036  grams).  The  direct  experiments  which  have  been 
made  with  specially  devised  forms  of  apparatus,  both  on  human  beings 
and  animals,  have  yielded  similar  results.  With  those  forms  which 
are  adapted  for  both  human  beings  and  animals — Scharhng's,  Petten- 
kofer  and  Voit's — it  is  only  possible,  however,  to  determine  the  amount 
of  carbon  dioxid  and  water,  and  from  these  to  calculate  the  amount 
of  oxygen  absorbed.  This  is  done  by  deducting  the  loss  in  weight 
by  the  man  or  animal  during  the  experiment  from  the  combined 
weights  of  the  carbon  dioxid  and  water  discharged.  The  difference 
represents  the  oxygen  absorbed. 

The  Pettenkofer-Voit  apparatus  (Fig.  178)  consists  essentially 
of  a  chamber  large  enough  to  admit  a  man  and  capable  of  being 
made  air-tight  with  the  exception  of  an  inlet  for  air  for  breathing 
purposes.  The  respired  air  is  drawn  through  a  tube  and  measured 
by  a  large  meter  turned  by  a  water  or  gas  motor.  By  means  of  a  side 
tube  a  fractional  quantity  of  the  main  column  of  air  is  diverted  to  an 
absorption  apparatus  by  a  small  pump.  This  air  first  passes  into 
a  vessel  containing  H2S0^,  by  which  the  water  is  collected;  then 
into  long  tubes  containing  barium  hydroxid,  by  which  the  carbon 
dioxid  is  absorbed ;  thence  into  a  small  meter,  by  which  its  amount  is 
registered.  From  the  amount  of  water  and  carbon  dioxid  thus  ob- 
tained the  amounts  of  both  in  the  total  air  breathed  are  calculated. 
The  water  and  carbon  dioxid  previously  present  in  the  air  are  simulta- 
neously determined  by  a  corresponding  absorption  apparatus  and  de- 
ducted from  the  amounts  obtained  from  the  respired  air.  As  the 
apparatus  is  traversed  constantly  by  a  column  of  air  of  normal 
composition  and  the  waste  products  removed  as  rapidly  as  discharged, 
the  experiment  can  be  continued  for  periods  varying  from  six  to 
twenty-four  hours  without  detriment  to  the  subject  of  the  experiment. 

With  those  forms  adapted  only  for  animals — Regnault's  and 
Reiset's,  or  Jolyet  and  Rcgnard's — it  is  possible  to  determine  simul- 
taneously the  absorption  of  oxygen  and  the  discharge  of  carbon 
dioxid.  As  the  apparatus  employed  is  completely  closed,  the  carbon 
dioxid  must  be  removed  as  soon  as  discharged  and  the  oxygen  re- 
newed as  soon  as  absorbed.  The  former  is  accomplished  by  the  as- 
piratory  action  of  moving  bulbs  containing  an  alkah,  the  latter  by  a 
steadily  acting  pressure  on  a  reservoir  of  oxygen.     This  apparatus 


RESPIRATION. 


389 


>.-a 


390 


TEXT-BOOK  OF  PHYSIOLOGY. 


(Fig.  179)  consists  essentially  of  a  bell-jar  in  which  the  animal  is 
placed.  This  is  brought  into  connection  by  tubes,  on  the  one 
hand,  with  the  oxygen  reservoir,  and,  on  the  other  hand,  with  the 
aspiratory  bulbs,  kept  in  motion  by  some  form  of  motor.  The 
construction  of  each  of  these  forms  of  apparatus  is  so  complex,  the 
conduct  of  an  experiment  and  the  final  determination  of  the  results 


Fig.  179. — Regnault's  and  Reiset's  Respiration  Apparatus.  A.  Bell-jar  for  the 
reception  of  the  animal,  surrounded  by  a  compartment,  B,  containing  water. 
N,  N,  N.  Reservoirs  of  oxygen  communicating,  on  the  one  hand,  with  the  animal 
chamber,  and,  on  the  other  hand,  with  pressure  bottles,  P,  by  which  the  oxygen 
is  driven  into  the  animal  chamber.  G,  G.  Aspiratory  bulbs  containing  sodium 
hydro.xid  in  solution  for  the  absorption  of  the  carbon  dioxid.  The  bulbs  are 
given  an  alternate  up-and-down  movement  by  a  falling  weight  or  electric  motor. 

SO  complicated,  that  a  detailed  description  would  be  out  of  place  in 
a  work  of  this  character.* 

Among  the  results  obtained  by  these  and  other  methods  a  few  are 
given  in  the  following  table: 


*  Both  forms  of  apparatus  are  in  use  in  the  Physiological  Laboratory  of  the  Jeffer- 
son Medical  College  and  are  fully  described  by  Prof.  H.  C.  Chapman  in  his  text- 
book on  Physiology,  to  which  the  reader  is  directed  for  further  information. 


RESPIRATION.  391 


Oxygen  Absorbed. 

Observer. 

Carbon  Dioxid  Discharged 

746  grams. 

Vierordt. 

876  grams. 

700        " 

Pettenkofer  and  \'oit. 

800 

663        " 

Speck. 

770 

The  amounts  of  oxygen  absorbed  in  Pettenkofer  and  Voit's  experi- 
ments varied  from  594  to  1072  grams;  of  carbon  dioxid  exhaled,  from 
686  to  1285  grams. 

In  all  these  results  it  is  evident  on  examination  that  the  volume 
of  oxygen  absorbed  is  always  greater  than  the  volume  of  carbon 
dioxid  exhaled,  or,  what  amounts  to  the  same  thing,  the  weight  of  the 
oxygen  absorbed  is  always  greater  than  the  weight  of  the  oxygen 
entering  into  the  formation  of  the  carbon  dioxid  exhaled.  The  reason 
for  this  difference  between  the  amounts  of  oxygen  in  the  inspired  air 
and  in  the  CO2  exhaled  is  found  in  the  fact  that  on  a  mixed  diet — one 
containing  fat — a  portion  of  the  oxygen  is  utiHzed  in  the  oxidation 
of  the  hydrogen  of  the  fat  with  the  formation  of  water.  Under  such 
a  diet  the  respiratory  quotient  is  always  less  than  unity,  usually 
0.907.  On  a  purely  carbohydrate  diet — one  in  which  there  is  no 
surplus  hydrogen — all  the  oxygen  will  combine  with  carbon  and  be 
returned  as  carbon  dioxid,  and  hence  the  respiratory  quotient  will 
be  unity.  The  respiratory  quotient  therefore  indicates  the  extent  to 
which  the  oxygen  absorbed  is  utilized  in  oxidizing  carbon,  on  the  one 
hand,  and  hydrogen,  on  the  other. 

Since  the  total  oxygen  absorbed  and  carbon  dioxid  discharged 
will  vary  considerably  with  the  size  of  'the  animal,  it  is  customary, 
for  purposes  of  comparison,  to  reduce  all  total  results  to  the  unit  of 
body- weight  (one  kilogram)  and  to  the  unit  of  time  (one  hour). 

Respiratory  Activity. — The  activity  or  the  intensity  of  the 
respiratory  process  may  be  measured  either  by  the  oxygen  absorbed 
or  the  carbon  dioxid  discharged.  But  as  the  carbon  dioxid  is  more 
easily  estimated  than  the  oxygen,  it  is  usually  taken  as  the  index  of 
the  activity,  though  there  are  reasons  for  believing  that  it  would  be 
more  accurately  indicated  or  represented  by  the  oxygen. 

Whatever  factor  may  be  accepted  as  the  measure,  it  is  certain  that 
the  respiratory  activity  varies  in  different  tissues  in  accordance  with 
their  functional  activities,  being  least  in  bones  and  greatest  in  muscles. 
This  is  shown  by  the  relative  amounts  of  oxygen  absorbed  and  carbon 
dioxid  discharged  by  equal  amounts  of  each  of  these  and  other  tissues 
in  twenty-four  hours,  as  shown  in  the  following  table: 

QUANTITY    OF    O    AND    CO 2    ABSORBED    AND    EXHALED    DURING 
TWENTY-FOUR    HOURS,    IN    CUBIC    CENTIMETERS. 

By  100  Grams  of:  Oxygen  Absorbed.  Carbonic  Acid  Exhaled. 

Muscle, 50.8  c.c;  56.8  c.c. 

Brain,    

Kidneys, 

Spleen, 

Testicles, 

Pounded  bones, 


4.S-8 

42.8  ' 

37-0 

15.6  ' 

2  7-.S 

15-4  ' 

18.3 

27.5   ' 

17.2 

8.1   ' 

392  TEXT-BOOK  OF  PHYSIOLOGY. 

The  total  respiratory  change  therefore  of  the  body  as  a  whole  is  the 
resultant  of  the  respiratory  changes  of  its  individual  organs  and 
tissues,  and  is  conditioned  by  all  influences  which  retard  or  hasten 
their  activity.  Among  these  influences  the  more  imi)ortant  are  the 
following: 

Muscle  Activity.^ — As  the  muscles  constitute  a  large  part  of  the 
body,  about  40  per  cent.,  and  as  muscle-tissue  absorbs  and  discharges 
relatively  large  quantities  of  oxygen  and  carbon  dioxid,  it  is  readily 
apparent  that  an  increase  in  their  activity  would  be  followed  or 
attended  by  an  increase  in  the  respiratory  exchange.  In  passing 
from  a  condition  of  body  repose  to  one  of  marked  activity  there  ought 
to  be  an  increase  in  the  amount  of  oxygen  absorbed  and  COj  dis- 
charged. Pettenkofer  and  Voit  found  that  a  man  in  repose  who 
absorbed  daily  807.8  grams  of  oxygen  and  discharged  930  grams 
COj  absorbed  during  work  1006  grams  of  oxygen  and  discharged 
1 137  grams  of  CO,.  Edward  Smith,  who  estimated  only  the  COj, 
found  that  a  man  in  repose  who  discharged  carbon  dioxid  at  the  rate 
of  161. 6  c.c.  per  minute  increased  the  amount  while  walking  at  the 
rate  of  two  and  three  miles  an  hour  to  569  c.c.  and  851  c.c.  respec- 
tively.    Similar  results  have  been  obtained  by  other  investigators. 

Digestive  Activity. — The  activity  of  the  alimentary  canal, 
involving  contraction  of  its  muscle  coat  through  its  entire  length 
as  well  as  secretion  of  its  related  glands  called  forth  by  the  inges- 
tion of  food,  materially  influences  the  absorption  of  oxygen  and 
discharge  of  carbon  dioxid,  independent  of  the  increase  due  to  the 
oxidation  of  food  materials  after  absorption.  It  was  found  that  in  a 
fasting  man  a  dose  of  sodium  sulphate  increased  the  absorption  of 
oxygen  as  much  as  17  per  cent,  and  the  discharge  of  CO2  24  per  cent. 
(Lowy).  It  is  difficult  to  determine  how  much  of  the  increase  after 
a  meal  is  therefore  due  to  food  oxidation  and  how  much  to  functional 
activity  of  the  canal  itself.  The  consumption  of  nitrogenized 
meals,  however,  has  a  greater  effect  than  non-nitrogenized  meals. 

Temperature. — A  rise  in  temperature  of  the  surrounding  air 
has  as  an  effect  a  diminution  in  the  amounts  of  oxygen  consumed  and 
carbon  dioxid  discharged.  A  fall  in  temperature  has  the  opposite 
effect.  Thus  a  cat  at  a  temperature  of  — 3.2°  C.  consumed  during  a 
period  of  six  hours  21.39  grams  of  oxygen  and  discharged  22  gram.s 
of  carbon  dioxid,  while  at  a  temperature  of  29.6°  C.  the  correspond- 
ing amounts  for  the  same  period  of  time  were  for  oxygen  13.9  grams 
and  for  carbon  dioxid  13.12  grams.  Lavoisier  and  Sequin,  having 
reference  only  to  the  oxygen,  found  that  a  man  at  a  temperature  of 
15°  C.  consumed  38.31  grams  of  oxygen,  while  at  a  temperature  of 
32.8°  C.  the  corresponding  amount  was  but  35  grams.  Similar 
results  have  been  obtained  by  other  observers  with  different  animals. 
The  explanation  of  these  facts  is  to  be  found  in  the  increased  activity 


RESPIRATION.  393 

of  all  physiologic  mechanisms  coincident  with  a  fall,  and  in  the 
decreased  activity,  coincident  with  a  rise  in  temperature.  The  lower 
temperatures  act  as  a  stimulus  to  the  peripheral  terminations  of  the 
nerve  system,  bringing  about  reflexly  increased  activity  of  the  body 
at  large.  The  muscles  especially  are  not  only  reflexly  but  vohtionally 
excited  to  greater  activity.  This  leads  naturally  to  an  increase  in 
the  consumption  of  oxygen  and  in  the  production  of  carbon  dioxid 
and  in  the  evolution  of  heat. 

In  cold-blooded  animals  the  respiratory  exchange  is  influenced  in 
a  manner  the  reverse  of  that  observed  in  warm-blooded  animals. 
With  a  rise  of  external  temperature  and  a  corresponding  ris^  of 
body-temperature  the  discharge  of  carbon  dioxid  steadily  increases. 
Thus  a  frog  in  an  atmosphere  at  0°  C.  with  a  body-temperature  of 
1°  C.  discharged  per  kilogram  per  hour  4.31  c.c.  of  carbon  dioxid; 
in  an  atmosphere  of  35°  C.  with  a  body-temperature  of  34°  C.  there 
was  discharged  325  c.c.  per  kilo  per  hour.  Intermediate  tempera- 
tures were  attended  by  corresponding  increases  in  the  amounts  of 
COj  discharged.  The  reason  for  this  difference  in  the  two  classes 
of  animals  is  probably  to  be  found  in  the  want,  in  the  cold-blooded 
animals,  of  a  self-adjusting  heat-regulating  mechanism. 

Age. — In  early  youth,  as  a  result  partly  of  the  more  pronounced 
activity  of  the  nutritive  energies  and  partly  of  a  cutaneous  surface 
relatively  greater,  as  compared  with  the  mass  of  the  body,  than  in 
adult  life,  the  absorption  of  oxygen  and  the  discharge  of  carbon 
dioxid  are  greater  both  absolutely  and  relatively.  Thus,  in  a  boy  of 
nine  and  a  half  years  with  a  weight  of  22  kilograms  it  was  found  that 
in  twenty-four  hours  there  was  a  discharge  of  carbon  dioxid  amounting 
to  488  grams,  or  0.92  gram  per  kilo  per  hour,  and  in  man  with  a 
weight  of  65.5  kilograms  there  was  a  discharge  of  804.72  grams,  or  0.51 
gram  per  kilo  per  hour. 


MODIFICATIONS  OF  RESPIRATORY  RHYTHM. 

The  character  of  the  respiratory  movements  is  materially  modified 
by  a  change  in  the  quantitative  and  qualitative  composition  of  the 
air  and  blood  as  well  as  by  changes  of  a  pathologic  nature  of  the  re- 
spiratory apparatus  itself. 

Eupnea." — So  long  as  the  air  retains  its  normal  composition  and 
the  respiratory  mechanism  its  structural  integrity,  so  long  do  the 
respiratory  movements  exhibit  a  normal  rhythm  and  frequency. 
To  the  condition  of  easy  tranquil  breathing  the  term  eupnea  is  given. 
In  this  condition  the  percentages  of  oxygen  and  carbon  dioxid  in  the 
blood  are  such  as  to  favor  at  least  the  rhythmic  discharge  of  nerve 
impulses  to  the  respiratory  muscles,  of  sufficient  energy  and  frequency 
for  the  maintenance  of  normal  respiration. 


394  TEXT-BOOK  OF  PHYSIOLOGY. 

Hyperpnea. — The  normal  rate  of  the  respiratory  movements  is 
increased  by  a  rise  in  body-temperature,  by  active  exercise,  and  by 
emotional  states.  Whatever  the  cause,  the  increase  in  rate  and  prob- 
ably in  depth  is  termed  hyperpnea. 

Febrile  states  characterized  by  a  rise  in  the  temperature  of  the 
blood  increase  considerably  the  respiratory  activity.  This  is  due  in 
all  probability  to  a  warming  of  the  respiratory  center,  in  consequence 
of  which  its  excitabihty  is  heightened;  for  surrounding  the  carotid 
arteries  with  warm  tubes  and  heating  the  blood  on  its  way  to  the 
medulla  has  the  same  effect.  It  is  also  possible,  however,  that  the 
high  temperature  of  febrile  conditions  may  interfere  with  the  absorb- 
ing power  of  hemoglobin,  and  thus  by  diminishing  the  quantity  of 
oxygen  absorbed  lead  to  more  frequent  respirations.  To  the  hy- 
perpnea induced  by  heat  the  term  thermo- polypnea  is  frequently 
given. 

Muscle  activity,  especially  if  it  is  violent  and  indulged  in  by 
those  unaccustomed  to  exercise,  is  generally  followed  by  increased 
rate  and  depth  of  breathing,  and  not  infrequently  it  is  attended  with 
such  extreme  difficulty  that  the  condition  approximates  that  of 
dyspnea.  This  condition  is  attributed  to  the  production  and  dis- 
charge into  the  blood  of  unknown  waste  products  which  act  as 
irritants  to  the  respiratory  center  and  thus  increase  its  activity. 
As  they  apparently  can  not  be  isolated  and  their  chemic  nature  deter- 
mined, it  is  presumable  that  they  are  speedily  oxidized  or  reduced 
in  the  blood.  Experiment  has  shown  that  the  increase  of  carbon 
dioxid  does  not  account  for  the  increased  rate  of  breathing.  Emo- 
tional states  temporarily  increase  respiratory  activity.  With  their 
disappearance  the  normal  condition  returns. 

Apnea. — If  the  respiratory  movements  be  voluntarily  increased 
in  frequency  and  depth  for  a  short  time  it  will  be  found  on  cessation 
that  for  a  variable  length  of  time  the  respiratory  mechanism  remains 
in  a  condition  of  complete  rest  or  inaction.  To  this  complete  cessation 
of  activity  the  term  apnea  is  given.  The  same  phenomenon  is 
witnessed  in  animals  when  the  lungs  are  rapidly  inflated  with  air  by 
means  of  bellows.  At  one  time  this  was  attributed  to  an  excess  of 
oxygen  in  the  blood  (the  result  of  the  forced  ventilation  of  the  lungs), 
complete  saturation  of  the  plasma  and  hemoglobin,  in  consequence  of 
which  the  respiratory  center  remained  inactive.  This  has  been  dis- 
proved, however,  by  modern  chemic  analyses  of  the  blood.  The 
condition  is  now  attributed: 

1.  To  increased  ventilation  of  the  lungs  and  an  increased  percentage 

of  oxygen  in  the  alveoli,  as  a  result  of  which  the  normal  percentage 
of  oxygen  in  the  blood  can  be  maintained  for  a  longer  period  than 
usual. 

2.  To  a  stimulation  of  the  peripheral  terminations  of  the  pneumo- 


RESPIRATION.  395 

gastric  nerve  whereby  the  discharge  of  nerve  impulses  from  the 
respiratory  center  is  temporarily  inhibited.  Division  of  the 
pneumogastric  nerve  prevents  the  development  of  the  apneic 
condition. 

Dyspnea. — Excessive  and  laborious  respiratory  movements 
constitute  a  condition  known  as  dyspnea.  Movements  of  this  char- 
acter indicate  that  the  blood  is  deficient  in  oxygen  or  overcharged 
with  carbon  dioxid.  In  either  case  the  excitability  of  the  respiratory 
center  is  abnormally  heightened.  These  conditions  of  the  blood 
may  be  caused:  (i)  By  all  those  pathologic  conditions  of  the  respiratory 
apparatus  which  limit  the  free  entrance  of  oxygen  into  and  the  free 
exit  of  carbon  dioxid  from  the  blood;  (2)  by  those  alterations  in  the 
composition  of  the  air  and  subsequently  in  the  blood  which  arise  when 
the  individual  is  confined  in  a  space  of  moderate  size  with  imperfect 
ventilation.  The  want  of  oxygen  in  the  blood  gives  rise  to  more 
forcible  inspirations;  the  presence  of  CO2  in  excess,  to  more  forcible 
expirations — showing  that  the  former  condition  affects  the  inspiratory 
portion  of  the  respiratory  center,  the  latter  condition  the  expiratory 
portion.  A  deficiency  in  the  amount  or  quality  of  the  hemoglobin 
is  usually  attended  with  dyspnea. 

Asphyxia. — If  the  state  of  the  blood  observed  in  dyspnea  be  ex- 
aggerated,— that  is,  if  the  decrease  in  the  percentage  of  oxygen  and 
the  increase  in  the  percentage  of  carbon  dioxid  become  more 
marked, — the  respiratory  movements  become  more  laborious.  A 
continuance  of  this  changed  composition  of  the  blood  eventuates 
in  death.  Before  this  occurs  the  individual  exhibits  a  succession 
of  phenomena,  to  the  totality  of  which  the  term  asphyxia  is  given. 

Asphyxia  may  be  caused:  (i)  By  a  sudden  interference  with  the 
entrance  of  oxygen  into  and  the  exit  of  carbon  dioxid  from  the  blood, 
as  in  drowning,  occlusion  of  the  trachea  from  any  cause,  double 
pneumothorax,  etc.  (2)  By  confinement  in  a  small  space  the  air 
of  which  speedily  undergoes  a  loss  of  oxygen  and  an  accumulation  of 
carbon  dioxid.  In  the  first  instance  death  may  occur  in  a  few  minutes ; 
in  the  second  instance  it  may  be  postponed  several  hours  or  more, 
the  time  varying  with  the  size  of  the  space. 

The  succession  of  phenomena  presented  by  an  individual  in  the 
asphyxiated  condition  is  as  follows:  Increased  rate  and  depth  of  the 
respiratory  movements,  passing  rapidly  from  hyperpnea  to  dyspnea, 
with  an  active  contraction  of  all  the  muscles  concerned  in  respira- 
tion, ordinary  and  extraordinary;  a  blue  cyanosed  condition  of  the 
face  from  the  rapid  accumulation  of  carbon  dioxid  and  disappearance 
of  the  oxygen  of  the  blood;  a  diminution  in  the  depth  of  inspiration 
and  an  increase  in  the  force  and  extent  of  expiration,  followed  by 
general  convulsions;  collapse,  characterized  by  unconsciousness,  loss 
of  the  reflexes,  relaxation  of  the  muscles,  a  weak  action  of  the  heart, 


396  TEXT-BOOK  OF  PHYSIOLOGY. 

a  disappearance  of  the  pulse  and  death.  As  shown  by  observation 
of  the  circulatory  apparatus  in  artificially  induced  asphyxia,  there  is 
primarily  an  increase  in  the  activity  of  the  heart,  soon  followed  by 
retardation;  a  rise  of  blood-pressure  in  the  early  stages  and  a  fall  to 
zero  after  collapse  has  set  in.  The  retardation  and  final  cessation  of 
the  heart,  as  well  as  the  rise  of  the  blood-pressure,  are  to  be  attributed 
to  stimulation  of  the  cardio-inhibitory  and  vasomotor  centers  from 
the  accumulation  of  the  carbon  dioxid.  With  the  exhaustion  of  the 
nerve-centers,  there  is  a  general  relaxation  of  the  muscles  and  a  fall 
of  the  pressure. 


THE  NERVE  MECHANISM  OF  RESPIRATION. 

The  nerve  mechanism  by  which  the  respiratory  muscles  are  ex- 
cited to  action  is  extremely  complex  and  involves  the  action  of  both 
afferent  and  efferent  nerves  and  their  related  nerve-centers  in  the  cen- 
tral nerve  system.  For  the  free  introduction  of  air  into  the  lungs  it  is 
essential  that  the  nasal  and  laryngeal  passages  and  the  cavity  of  the 
thorax  be  simultaneously  enlarged.  The  muscles  by  which  these 
results  are  accomplished  have  already  been  mentioned  and  described. 
Their  simultaneous  and  coordinate  contraction  implies  the  coordinate 
activity  of  motor  nerves  and  their  centers;  thus,  the  nasal  and  laryn- 
geal muscles  (the  dilator  naris  and  the  posterior  crico-arytenoid) 
involve  the  activity  of  the  facial  and  inferior  laryngeal  nerves  re- 
spectively, the  centers  of  origin  of  which  lie  in  the  gray  matter  beneath 
the  floor  of  the  fourth  ventricle;  the  diaphragm  and  intercostal 
muscles  involve  respectively  the  activity  of  the  phrenic  and  inter- 
costal nerves,  the  centers  of  origin  of  which  lie  in  the  anterior  horn 
of  the  gray  matter  of  the  spinal  cord  at  a  level,  for  the  phrenic,  of  the 
fourth,  fifth,  and  sixth  cervical  nerves,  and  for  the  intercostals  at  the 
level  of  the  upper  thoracic  nerves.  Division  of  any  one  of  these  nerves 
is  followed  by  paralysis  of  its  related  muscle. 

The  coordinate  contraction  of  the  inspiratory  muscles  implies 
a  practically  simultaneous  discharge  of  nerve  impulses  from  each  of 
the  foregoing  nerve-centers,  accurately  graduated  in  intensity  in 
accordance  with  inspiratory  needs.  This  has  been  supposed  to 
necessitate  the  existence  in  the  central  nerve  system  of  a  single 
group  of  nerve-cells  from  which  nerve  impulses  are  rhythmically 
discharged  and  conducted  to  the  previously  mentioned  nerve-centers 
in  the  medulla  oblongata  and  spinal  cord,  by  which  they  are  in 
turn  excited  to  activity.  To  this  group  of  cells  the  term  "inspiratory 
center"  has  been  given.  For  the  free  exit  of  air  from  the  lungs  it  is 
not  only  essential  that  the  air-passages  be  open,  but  that  the  air  in 
the  lungs  be  compressed  until  its  pressure  rises  above  that  of  the 
atmosphere.     This  is  accomplished  by  the  recoil  of  the  elastic  tissue 


RESPIRATION.  397 

of  the  lungs  and  thorax,  the  return  of  the  displaced  abdominal  organs 
aided  by  atmospheric  pressure,  and  the  contraction  of  the  expiratory 
muscles.  In  how  far  muscle  action  is  necessary  for  expiratory  pur- 
poses will  depend  on  the  resistance  offered  to  the  outflow  of  air  and 
on  the  degree  of  efficiency  of  the  elastic  forces.  The  simultaneous  and 
coordinate  activity  of  the  expiratory  muscles  also  involves  the  action 
of  motor  nerves  and  nerve-centers.  The  simultaneous  and  coordinate 
discharge  of  nerve  impulses,  also  graduated  in  intensity  for  expiratory 
needs,  apparently  implies  the  existence  in  the  central  nerve  system 
of  a  single  center  from  which  nerve  impulses  are  rhythmically  dis- 
charged which  excite  and  coordinate  the  lower  nerve-centers.  To 
this  group  of  cells  the  term  "expiratory  center"  has  been  given. 
The  two  centers  taken  together  constitute  the  so-called  "respiratory 
center." 

The  existence,  however,  of  a  definite  group  of  cells  which  initiates 
the  respiratory  movements  has  not  as  yet  been  demonstrated.  Never- 
theless there  is  in  the  dorsal  portion  of  the  medulla  oblongata,  at  the 
level  of  the  sensory  end-nucleus  of  the  vagus  nerve,  a  region  the  sudden 
destruction  of  which  on  one  side  is  followed  by  a  cessation  of  respira- 
tory movements  on  the  corresponding  side,  though  they  continue  on 
the  opposite  side,  a  fact  which  indicates  that  the  area,  though  acting 
as  a  unit,  is  bilateral.  The  bilateral  character  of  the  area  is  also 
shown  by  the  continuance  of  the  respiratory  movements  on  both  sides 
after  longitudinal  division  of  the  medulla.  Destruction  of  the  entire 
region  is  followed  by  a  complete  cessation  of  respiratory  activity  and 
death  of  the  animal.  For  this  reason  the  term  "noeud  vital"  was 
apphed  to  it.  In  this  area  the  respiratory  center  was  located.  It  has, 
however,  been  shown  by  Gad  that  if  this  area  be  gradually  destroyed 
by  cauterization  the  respiratory  movements  do  not  cease,  but  con- 
tinue until  the  cauterization  has  reached  a  point  far  forward  in 
the  formatio  reticularis,  in  which  the  respiratory  center  was  assumed 
to  lie. 

Though  its  existence  has  not  been  anatomically  determined 
beyond  question,  it  is  permissible  to  speak  of  the  central  mechanism 
as  a  "center"  located  in  the  medulla  oblongata. 

The  activity  of  the  respiratory  center  has  long  been  described  as 
automatic  or  autochthonic  (Gad)  in  character,  expressive  of  the  idea 
that  the  rhythmic  discharge  of  nerve  impulses  is  conditioned  by  the 
composition  of  the  blood  or  lymph  by  which  it  is  surrounded.  Thus 
so  long  as  the  blood  retains  its  normal  composition  the  respiratory 
movements  are  normal.  If,  however,  the  blood  becomes  richer  in 
oxygen  and  poorer  in  carbon  dioxid,  the  rate  of  discharge  of  nerve 
impulses  and  the  inspiratory  movements  diminish  until  the  condition 
of  apnea  results.  If,  on  the  contrary,  the  blood  becomes  poorer  in 
oxygen  and  richer  in  carbon  dioxid,  the  reverse  condition  obtains: 


398  TEXT-BOOK  OF  PHYSIOLOGY. 

viz.,  an  increased  rate  of  discharge  of  nerve  impulses,  increased  fre- 
quency of  respiration,  hyperpnea,  and  dyspnea.  This  view  is  ap- 
parently supported  by  the  fact  that  after  division  of  the  fifth  and 
vagus  nerves  the  respiratory  movements  continue,  though  changed 
in  character,  becoming  less  frequent  and  deeper.  Whether  they 
would  continue  after  division  of  ail  afferent  nerves  it  is  impossible 
to  state,  since  from  the  nature  of  the  case  such  an  experiment  would 
be  most  difficult  to  perform. 

The  first  inspiration  after  birth  is  supposed  to  be  due  to  the  direct 
stimulation  of  the  respiratory  center  by  the  increase  in  the  carbon 
dioxid  present  in  the  blood,  though  it  may  be  aided  by  the  cooling 
of  the  skin  due  to  vaporization  of  the  amniotic  fluid. 

Whether  the  respiratory  center  is  automatic  in  character  or  not, 
it  may  be  influenced  directly  by  nerve  impulses  descending  from  the 
brain  in  consequence  of  volitional  acts  or  emotional  states,  and  in- 
directly by  nerve  impulses  reflected  to  it  from  the  general  periphery 
through  various  afferent  nerves,  in  consequence  of  agencies  acting 
on  their  peripheral  filaments:  e.  g.,  cold  applied  to  the  skin,  irritating 
gases  to  the  nasal  and  bronchial  mucous  membrane,  distention  and 
collapse  of  the  pulmonary  alveoli. 

Of  all  afferent  nerves,  the  vagus  appears  to  be  the  most  influential  in 
maintaining  the  rhythmic  discharge  of  nerve  impulses  from  the  respi- 
ratory center.  Thus,  if  while  the  animal  is  breathing  regularly  and 
quietly  both  vagi  are  cut,  the  respiratory  movements  become  much 
slower,  falling  perhaps  to  one-third  their  original  number  per  minute. 
If  the  central  end  of  the  divided  vagus  be  stimulated  with  weak 
faradic  currents,  the  respiratory  movements  are  increased  in  fre- 
quency and  their  depth  diminished  until  the  normal  rate  is  restored. 
With  the  cessation  of  the  stimulation  the  former  condition  at  once 
returns.  This  would  indicate  that  in  the  physiologic  state  afferent 
impulses  are  ascending  the  vagus  fibers  which  influence  the  rate  of 
discharge  from  the  respiratory  center.  If,  however,  the  stimulation 
is  increased  in  strength,  the  inspiratory  movement  gradually  so  ex- 
ceeds the  expiratory  that  the  muscles  pass  into  the  tetanic  state  and 
the  chest-walls  come  to  rest  in  the  condition  of  forced  inspiration. 
The  vagus  evidently  contains  fibers  which  augment  the  activity  of  the 
inspiratory  center  and  inhibit  the  activity  of  the  expiratory  center. 
If,  on  the  other  hand,  the  central  end  of  the  divided  superior  laryngeal 
nerve  be  stimulated  with  faradic  currents,  the  opposite  effect  is  pro- 
duced: viz.,  an  excess  of  the  expiratory  over  the  inspiratory  move- 
ment until  the  chest-waUs  come  to  rest  in  the  condition  of  forced 
expiration.  The  superior  laryngeal  nerve  evidently  contains  fibers 
which  augment  the  activity  of  the  expiratory  center  and  inhibit  the 
activity  of  the  inspiratory  center. 

The  same  result  not  infrequently  follows  stimulation  of  the  divided 


RESPIRATION.  399 

vagus  and  always  after  the  administration  of  large  doses  of  chloral. 
The  vagus  contains  two  classes  of  fibers,  one  of  which  augments  the 
activity  of  the  inspiratory  while  inhibiting  the  activity  of  the  ex- 
piratory; the  other  inhibiting  the  inspiratory  while  augmenting  the 
expiratory.  The  stimulus  adequate  to  the  excitation  of  the  nerve- 
fibers  in  the  physiologic  condition  was  formerly  believed  to  be  the 
chemic  action  of  carbon  dioxid;  it  is  now  believed  to  be  a  mechanic 
action,  the  result  of  the  alternate  distention  and  collapse  of  the  walls 
of  the  pulmonary  alveoli.  Thus,  it  has  been  shown  by  Head  that 
if  the  lungs  are  actively  inflated  there  will  be  produced  an  inhibition 
of  the  inspiratory  and  an  augmentation  of  the  expiratory  movement 
until  the  chest  comes  to  rest,  a  result  similar  in  all  respects  to  that 
produced  by  stimulation  of  the  superior  laryngeal  nerve.  On  the 
other  hand,  if  the  lungs  are  collapsed  by  the  artificial  withdrawal 
of  air,  there  will  be  produced  an  augmentation  of  the  inspiratory 
and  an  inhibition  of  the  expiratory  movements  until  the  chest  comes 
to  rest  in  extreme  inspiration,  a  result  similar  in  all  respects  to  that 
produced  by  powerful  stimulation  of  the  central  end  of  the  divided 
vagus.  These  facts  indicate  that  the  respiratory  mechanism  is  reflex 
and  self-regulating  in  character,  and  the  stimulus,  the  alternate 
collapse  and  distention  of  the  pulmonary  alveoli ;  collapse  augmenting 
the  inspiratory  and  inhibiting  the  expiratory,  distention,  inhibiting 
the  inspiratory  and  augmenting  the  expiratory  center. 


THE  EFFECT  OF  THE  RESPIRATORY  MOVEMENTS  ON  THE  FLOW  OF 

BLOOD    THROUGH   THE    INTRA-THORACIC    VESSELS   AND 

ON  THE  ARTERLAL  PRESSURE. 

I.  On  the  Intra-thoracic  Vessels. — The  forces  which  cause  the 
air  to  flow  into  and  out  of  the  lungs  will  at  the  same  time  and  in  the 
same  way  cause  the  blood  of  the  systemic  vessels  to  flow  into,  through, 
and  out  of  the  intra-thoracic  vessels.  From  the  tendency  of  the  pul- 
monary elastic  tissue  to  recoil,  the  blood-vessels  in  the  thorax  at  the 
end  of  an  expiration  sustain  a  positive  pressure,  about  six  millimeters 
of  mercury  less,  than  that  in  the  lungs,  or,  in  other  words,  a  pressure 
negative  to  that  of  the  atmosphere  by  six  millimeters.  As  a  result 
the  blood  in  the  systemic  vessels  standing  under  atmospheric  pressure 
will  flow  steadily  tow^ard  the  intra-thoracic  veins,  the  venae  cavag, 
and  the  right  side  of  the  heart;  i.  e.,  from  a  point  of  high  to  a  point  of 
low  pressure.  Since  during  inspiration,  with  the  increasing  tendency 
to  pulmonary  recoil,  the  positive  pressure  on  the  veins  and  heart  may 
diminish  by  thirty  millimeters  of  mercury,  the  blood  will  flow  in 
increased  volume  from  the  systemic  to  the  intra-thoracic  vessels. 
The  right  heart,  being  more  generally  filled  with  blood,  will  discharge 
a  larger  volume  with  each  contraction  into  the  pulmonary  artery. 


400  TEXT-BOOK  OF  PHYSIOLOGY. 

Coincident  with  these  effects  a  similar  effect  is  produced  in  the 
arterioles  and  capillaries  of  the  pulmonary  alveoli.  Situated  between 
the  two  elastic  layers  of  the  alveolar  wall,  embedded  in  a  meshwork 
of  connective  tissue,  the  pressure  to  which  they  are  subjected  at  the 
end  of  an  expiration  will  also  be  a  few  millimeters  less  than  that  of 
the  intra-pulmonary  air;  and  at  the  end  of  an  inspiration  it  will  be 
considerably  less.  With  the  inspiration  there  will  occur  a  dilatation 
of  the  vessels,  a  larger  flow  of  blood  through  them  and  into  the  pul- 
monary veins.  The  left  heart,  being  in  consequence  more  generously 
filled  with  blood,  will  discharge  a  larger  volume  into  the  aorta  at  each 
contraction.  During  expiration  the  flow  of  blood  through  the  intra- 
thoracic vessels  will  be  diminished  for  the  reverse  reasons. 

2.  On  the  Arterial  Pressure. — An  examination  of  a  tracing  of 
the  arterial  pressure  will  show  that  it  is  characterized  by  small  un- 
dulations due  to  the  cardiac  beat  and  large  undulations  due  to  the 
respiratory  movements,  inspiration  being  accompanied  by  a  rise, 
expiration  by  a  fall  of  pressure.  These  results  are  readily  accounted 
for  by  the  difl'erence  in  the  volume  of  blood  discharged  by  the  left 
heart  into  the  aorta  during  the  time  of  the  two  movements.  If  a 
tracing  of  the  respiratory  movements  and  of  the  blood-pressure  be 
taken  simultaneously,  it  will  be  found  that  the  rise  of  pressure  does  not 
exactly  correspond  with  inspiration,  nor  the  fall  of  pressure  with 
expiration ;  for  a  certain  period  after  the  beginning  of  an  inspiration 
the  pressure  continues  to  fall,  and  for  a  certain  period  after  the 
beginning  of  an  expiration  the  pressure  continues  to  rise.  During 
the  remainder  of  the  period,  however,  inspiration  is  attended  by 
a  rise,  expiration  by  a  faff  of  pressure.  The  explanation  of  these 
results  lies  in  the  fact  that  at  the  beginning  of  the  inspiration,  when 
the  vessels  dilate,  the  blood-flow  momentarily  slows;  the  left  heart 
continuing  to  discharge  small  volumes  into  the  aorta,  the  pressure 
continues  to  fall.  So  soon  as  the  left  heart  begins  to  be  better  filled,  the 
pressure  at  once  begins  to  rise,  x^t  the  end  of  an  expiration  the  flow 
of  blood  into  the  left  heart  continues  and  the  pressure  rises,  but  with 
the  return  of  the  intra-thoracic  pressure  the  vessels  diminish  in  caliber, 
the  volume  of  blood  transmitted  by  them  becomes  less,  the  output 
of  the  left  heart  dechnes,  and  the  pressure  faUs. 


CHAPTER   XIV. 
ANIMAL  HEAT. 

The  chemic  changes  which  take  place  in  the  tissues  and  organs 
of  the  living  body  and  which  underlie  all  manifestations  of  life  are 
attended  by  the  evolution  of  heat.  In  consequence  of  this  each 
animal  acquires  a  certain  body-temperature. 

In  man,  as  well  as  in  other  mammals  and  in  birds,  the  chemic 
changes  are  extremely  active  and  the  evolution  of  heat  very  great. 
Through  some  special  heat- regulating  mechanism,  by  wdiich  heat- 
production  and  heat-dissipation  are  kept  in  equilibrium,  these  animals 
have  acquired  and  maintain  within  limits  a  constant  temperature 
which  is  independent  of  and  generally  above  that  of  the  surrounding 
atmosphere.  As  the  temperature  of  these  animals  is  high  and  per- 
ceptible to  the  sense  of  touch,  they  were  originally  designated  "warm- 
blooded" animals.  As  this  temperature  is  constant  notwithstanding 
the  great  variations  in  external  temperature  during  the  summer  and 
winter  seasons,  they  are  more  appropriately  termed  constant-tem- 
peratured  or  homoiothermous  animals.  The  intensity  of  the  body- 
temperature  determined  by  the  insertion  of  a  thermometer  in  the 
rectum  varies  in  different  classes  of  mammals  from  37.2°  C.  to  40°  C. 
The  causes  of  this  variation  are  doubtless  connected  with  peculiarities 
or  organization.  In  birds  the  rectal  temperature  is  usually  higher, 
var}dng  from  40.9°  C.  in  the  pigeon  to  44°  C.  in  the  titmouse  and 
the  swift. 

In  reptiles,  amphibians,  and  fish  chemic  changes,  as  a  rule,  are 
not  very  active  and  heat-production  relatively  slight.  As  they  are 
devoid  of  a  sufficiently  active  heat-regulating  mechanism,  the  tem- 
perature of  the  body  is  largely  dependent  on  that  of  the  medium  in 
which  they  live,  though  it  is  always  one  or  more  degrees  above  it.  In 
winter  the  body-temperature  of  frogs,  for  example,  may  decline  as 
low  as  8.9°  C,  the  temperature  of  the  surrounding  medium  being 
6.7°  C.  When  subjected  to  temperatures  below  zero,  the  temperature 
of  the  body  may  fall  below  the  freezing-point  also,  when  the  lymph 
and  fluids  of  the  body  become  ice.  Though  apparently  dead,  the 
gradual  elevation  of  the  temperature  restores  their  vitality.  In 
summer-time,  on  the  contrary,  the  body-temperature  may  attain  to 
38°  C.  Similar  variations  have  been  observed  in  other  animals.  As 
the  temperature  of  these  animals  is  low  and  perceptibly  below  that  of 
our  own  bodies,  they  were  originally  termed  "cold-blooded"  animals; 
26  401 


402  TEXT-BOOK  OF  PHYSIOLOGY. 

as  their  temperature  is  inconstant,  varying  with  the  temperature  of 
the  surrounding  medium,  they  are  more  appropriately  termed  "varia- 
ble tempcratured  "  or  poikilo-thermous  animals. 


THE  TEMPERATURE  OF  THE  HUMAN  BODY. 

The  determination  of  the  temperature  of  the  human  body  under 
the  changing  conditions  of  life  is  a  matter  of  the  greatest  physiologic 
and  chnical  interest.  The  temperature  of  the  superficial  portions  of 
the  body  may  be  obtained  by  the  introduction  of  a  thermometer  into 
the  mouth,  the  rectum,  the  vagina,  or  the  axilla.  As  a  result  of  many 
observations  it  has  been  found  that  the  temperature  of  the  rectum  is, 
on  the  average,  37.2°  C;  that  of  the  mouth,  36.8°  C;  that  of  the 
axilla,  36.9°  C.  Owing  to  radiation  and  conduction  the  surface  tem- 
perature is  lower  than  that  of  either  the  mouth  or  rectum,  and  varies 
to  a  slight  extent  in  different  regions  of  the  body:  e.  g.,  at  a  room- 
temperature  of  20°  C.  the  skin  of  the  pectoral  region  has  a  tem- 
perature of  34.7°;  that  of  the  cheek,  34.4°;  that  of  the  calf,  33.6°; 
that  of  the  tip  of  the  ear,  only  28.8°,  etc. 

In  the  interior  of  the  body,  especially  in  organs  in  which  oxidation 
takes  place  rapidly,  and  which  at  the  same  time  are  protected  by 
their  anatomic  surroundings  from  rapid  radiation,  the  temperature 
is  higher  than  that  observed  in  the  rectum.  From  an  investigation 
of  the  temperature  of  the  blood  as  it  emerges  from  the  liver,  the 
muscles,  the  brain,  alimentary  canal  etc.,  it  is  evident  that  these 
organs  have  a  higher  temperature  than  the  rectum. 

As  the  chemic  changes  underlying  physiologic  activity  vary  in 
intensity  and  extent  in  different  regions  of  the  body,  there  would  be 
marked  variations  in  their  temperature  were  it  not  that  the  blood, 
having  a  large  capacity  for  heat-absorption,  distributes  the  heat 
almost  uniformly  to  all  portions  of  the  body,  so  that  at  a  short  distance 
beneath  the  surface  the  temperature  does  not  vary  but  within  a  few 
degrees. 

In  the  dog  the  temperature  of  the  blood  in  the  aorta  and  in  its 
principal  branches  is  approximately  38.3°  C.  In  passing  through 
the  systemic  capillaries  the  temperature  falls  from  radiation  and  con- 
duction to  surface  temperature,  to  again  rise  as  the  venous  blood 
approaches  the  deeper  regions  of  the  body.  In  the  neighborhood 
of  the  renal  veins  and  in  the  superior  vena  cava  the  temperature  is 
again  that  of  the  aorta.  In  the  portal  vein  the  temperature  rises  to 
40.2°  C. ;  in  the  hepatic  vein,  to  40.6°  C.  In  the  right  ventricle,  owing 
to  the  admixture  of  blood  from  different  localities  having  different 
temperatures,  the  temperature  falls  to  38.2°  to  40.4°.  In  passing 
through  the  pulmonary  capillaries  the  temperature  of  the  blood  again 
falls,  so  that  in  the  left  ventricle  it  will  register  from  38°  C.  to  40.2°  C 


ANIMAL  HEAT.  403 

There  is  thus  usually  a  difference  between  the  two  sides  of  the  heart 
of  about  0.2°  C. 

Variations  in  the  Mean  Temperature. — The  mean  tempera- 
ture of  the  human  body  for  twenty-four  hours,  which  for  the  mouth 
and  rectum  may  be  accepted  at  36.7°  C.  and  37.2°  C.  respectively,  is 
subject  to  variations  from  a  variety  of  circumstances,  such  as  age, 
periods  of  the  day,  food,  exercise,  etc. 

Age. — i\t  birth  the  temperature  of  the  infant  is  slightly  higher 
than  that  of  the  mother,  registering  in  the  rectum  about  37.5°  C.  In 
a  few  hours  it  rapidly  declines  to  about  36.5°,  to  be  followed  in  the 
course  of  twenty-four  hours  by  a  rise  to  the  normal  or  slightly  beyond. 
During  childhood  the  temperature  gradually  approximates  that  of 
the  adult.  In  old  age  the  temperature  rises,  as  a  rule,  and  attains  a 
maximum  at  eighty  years  of  37.4°  C. 

Periods  0}  the  Day. — The  observations  of  Jiirgensen  show  that 
there  is  a  diurnal  variation  in  the  mean  temperature  of  from  0.5°  C. 
to  1.5°  C,  the  maximum  occurring  late  in  the  afternoon,  from  5  to  7 
o'clock,  the  minimum  early  in  the  morning,  from  4  to  7  o'clock.  This 
diurnal  variation  in  the  mean  temperature  is  related  to  corresponding 
variations  in  many  other  physiologic  processes,  and  its  causes  are  to 
be  found  in  the  ordinary  habits  of  hfe  as  regards  the  time  of  meals, 
periods  of  exercise,  sleep,  etc. 

Food  and  Drink. — The  ingestion  of  a  hearty  meal  increases  the 
temperature  but  slightly — not  more  than  0.5°  C.  Insufficiency  of 
food  lowers  the  temperature^  total  withdrawal  of  food,  as  in  starva- 
tion, is  followed  by  a  steady  though  shght  decline,  until  just  preceding 
the  death  of  the  animal,  when  it  falls  abruptly  to  from  6°  to  8°  C. 
Cold  drinks  lower,  hot  drinks  raise  the  temperature.  Food  and 
drinks,  however,  only  temporarily  change  the  mean  temperature, 
and  after  a  short  period  equilibrium  is  restored  through  the  activity 
of  the  heat-regulating  mechanism.  Alcoholic  drinks  lower  the  tem- 
perature about  0.5°  C.  In  large  toxic  doses  in  persons  unaccustomed 
to  their  use  the  temperature  may  be  lowered  several  degrees.  This 
is  attributed  not  to  a  diminution  in  heat-production,  but  rather  to 
an  increase  in  heat-dissipation  (Reichert)  from  increased  action  of 
the  heart,  dilatation  of  the  blood-vessels  of  the  skin,  and  increased 
activity  of  the  sweat-glands. 

Exercise. — The  temperature  may  be  raised  by  active  muscular 
exercise  from  1°  to  1.5°  C.  as  a  result  of  increased  activity  in 
chemic  changes  in  the  muscles  themselves.  A  rise  beyond  this  point 
is  prevented  by  the  increased  activity  of  the  circulatory  apparatus, 
the  removal  of  the  heat  to  the  surface,  and  its  rapid  radiation. 

External  Temperature. — The  external  temperature  influences  but 
slightly  the  mean  temperature  of  the  human  body.  In  the  tropic 
as  well  as  in  the  arctic  regions,  notwithstanding  the  change  in  the 


404  TEXT-BOOK  OF  PHYSIOLOGY. 

temperature  of  the  air,  that  of  the  body  remains  almost  constant. 
The  same  is  true  for  the  seasonal  variations  in  the  temperature  of 
the  temperate  regions. 


THE  SOURCE  AND  TOTAL  QUANTITY  OF  HEAT  PRODUCED. 

The  Source  of  Heat. — The  immediate  source  of  the  body-heat 
is  to  be  found  in  the  chemic  changes  which  take  place  in  all  the  tissues 
and  organs  of  the  body.  Each  contraction  of  a  muscle,  each  act  of 
secretion,  each  exhibition  of  nerve-force,  is  accompanied  by  the 
evolution  of  heat.  The  chemic  changes  are  for  the  most  part  of  the 
nature  of  oxidations,  the  union  of  oxygen  v^ith  the  elements,  carbon 
and  hydrogen,  of  the  food  principles  either  before  or  after  they  have 
become  constituents  of  the  tissues.  The  ultimate  source  of  the  body- 
heat  is  the  latent  or  potential  energy  in  the  food  principles,  which  was 
absorbed  from  the  sun's  energy  and  stored  up  during  the  growth  of 
the  vegetable  world.  In  the  metaboHsm  of  the  animal  body  the  food 
principles  are  again  reduced  through  oxidation,  directly  or  indirectly, 
to  relatively  simple  bodies,  such  as  urea,  carbon  dioxid,  and  water, 
with  a  Hberation  of  a  large  portion  of  their  contained  energy  which 
manifests  itself  as  heat  and  mechanic  motion. 

The  Total  Quantity. — The  total  quantity  of  heat  liberated  in 
the  body  daily  may  be  approximately  determined  in  at  least  two  ways : 
(i)  By  determining  experimentally  the  heat  values  of  different  food 
principles  by  direct  oxidation;  (2)  by  collecting  and  measuring  with 
a  suitable  apparatus,  a  calorimeter,  the  heat  evolved  by  the  oxidation 
of  the  food  within,  and  dissipated  from,  the  body  daily. 

I.  Direct  Oxidation. — The  amount  of  heat  which  any  given  food 
principle  will  yield  can  be  determined  by  burning  a  definite  amount — 
e.  g.,  I  gram — to  carbon  dioxid  and  water  and  ascertaining  the  extent 
to  which  the  heat  thus  liberated  will  raise  the  temperature  of  a  given 
amount  of  water:  e.  g.,  1  kilogram.  The  amount  of  heat  may  be 
expressed  in  gram  or  kilogram  degrees  or  calories;  a  gram  calorie 
or  kilogram  calorie  being  the  amount  of  heat  required  to  raise  the 
temperature  of  a  gram  or  a  kilogram  (1000  grams)  of  water  1°  C. 
The  apparatus  employed  for  this  purpose  is  termed  a  calorimeter, 
which  consists  essentially  of  a  closed  chamber,  in  which  the  oxidation 
takes  place,  surrounded  by  a  water-jacket.  The  rise  in  temperature 
of  the  water  indicates  the  amount  of  heat  produced. 

The  results  obtained  by  investigators  employing  different  calor- 
imeters and  different  food  principles  of  the  same  class  vary,  though 
within  narrow  limits:  e.  g.,  i  gram  casein  yields  5.867  kilogram 
calories;  i  gram  of  lean  beef,  5.656;  i  gram  of  fat,  9.353,  9.423,  9.686 
calories;  i  gram  of  starch  or  sugar,  4.1 16,  4.182,  4.479,  etc.,  calories. 
These  results  are,  however,  physical  values,  and  indicate  the  quan- 


ANIMAL  HEAT.  405 

tity  of  heat  such  quantities  of  foods  give  rise  to  when  completely 
oxidized  to  carbonic  acid  and  water.  In  the  human  body  the  carbo- 
hydrates and  the  fats,  with  the  exception  of  the  small  portion  which 
escapes  digestion,  are  reduced  to  carbon  dioxid  and  water,  and  hence 
practically  liberate  as  much  heat  as  they  do  when  oxidized  outside 
the  body.  The  proteids,  however,  are  only  reduced  to  the  stage  of 
urea.  As  this  compound  is  capable  of  further  reduction  in  the  calor- 
imeter to  carbon  dioxid  and  water,  with  the  liberation  of  heat,  the 
quantity  of  heat  it  contains  must  therefore  be  deducted  from  the 
physical  heat  value  of  the  proteid.  According  to  Rubner,  i  gram 
of  urea  will  yield  2.523  kilogram  calories.  As  about  one-third  of  a 
gram  of  urea  results  from  the  oxidation  of  i  gram  of  proteid,  the 
amount  of  heat  to  be  deducted  from  the  heat  value  of  the  proteid  is 
J  of  2.523,  or  0.841  calorie.  It  has  also  been  shown  by  the  same  in- 
vestigator that  some  of  the  ingested  proteid  is  found  in  the  feces,  the 
heat  value  of  which  must  also  be  determined  and  deducted.  This 
having  been  done,  the  physiologic  heat  value  becomes  4.124  calories. 
The  following  estimates  give  approximately  the  number  of  kilo- 
gram calories  which  should  be  liberated  within  the  body  when  the 
proteid  is  burned  to  the  stage  of  urea,  and  the  fat  and  carbohydrate 
to  the  stage  of  carbon  dioxid  and  water: 

1  gram  of  proteid 4-124  calories 

I         "         fat 9.353 

I         "         carbohydrate 4. 116        " 

The  total  number  of  kilogram  calories  yielded  by  the  various  diet 
scales  can  be  readily  determined  by  multiplying  the  quantities  of  the 
food  principles  consumed  by  the  foregoing  factors.  The  diet  scale 
of  Vierordt,  for  example,  yields  the  following: 

120  grams  of  proteid 404.88  calories 

90  "  fat 841.77         " 

330  "  starch 1358.28        " 

Total,    2694.93         " 

The  total  calories  obtained  from  other  diet  scales  would  be  as  follows : 
Ranke's,  2335;  Voit's,  3387;  Moleschott's,  2984;  Atwater's,  3331; 
Hultgren's,  3436.  These  numbers  indicate  theoretically  the  total 
heat-production  in  the  body  daily. 

2.  Calorimetric  Measurements. — By  this  method  the  heat  dissi- 
pated from  the  body  of  an  animal  is  directly  collected  and  measured, 
and  the  amount  so  obtained  is  taken  as  a  measure  of  the  heat  evolved 
by  the  oxidation  of  the  food.  A  calorimeter  is  therefore  an  apparatus 
for  the  direct  estimation  of  the  quantity  of  heat  dissipated  from  the 
body  in  a  given  time.  The  substance  employed  for  collecting  and 
measuring  the  heat  is  either  water  or  air.     The  calorimeters  in 


4o6 


TEXT-BOOK  OF  PHYSIOLOGY. 


general  use  consist  essentially  of  two  metallic  boxes  placed  one  within 
the  other,  though  separated  by  a  space  sufficiently  large  to  hold  a 
definite  amount  of  water  (Fig.  i8o).  The  animal  is  placed  in  the 
inner  box,  which  is  also  provided  with  tubes  for  the  entrance  of  fresh 
and  the  exit  of  expired  air.  The  heat  radiated  is  absorbed  by  the 
water  and  its  temperature  raised.  To  prevent  loss  by  radiation  and  to 
render  it  independent  of  changes  in  the  surrounding  temperature  the 
calorimeter  is  surrounded  by  a  poorly  conducting  material,  such  as 
wool.  The  temperature  of  the  animal  is  taken  at  the  beginning  and 
the  end  of  the  experiment.  If  the  temperature  of  the  animal  remains 
the  same  at  the  end  of  the  experiment,  then  the  heat  absorbed  by  the 

water  represents  the 
amount  produced  by  the 
animal.  If,  on  the  con- 
trary, the  temperature  of 
the  animal  rises  or  falls, 
the  number  of  calories 
so  retained  or  lost  must 
be  added  to  or  sub- 
tracted from  the  amount 
absorbed  by  the  calor- 
imeter. In  the  determi- 
nation of  the  absolute 
amount  of  heat  retained 
lost    by   the   animal 


sc-sssEiKiimin 


or 


Fig 


.  1 80. — Water  Calorimetek  of  Dulong.  D 
and  D'.  Tubes  for  the  entrance  and  exit  of 
air.  T  and  T'.  Thermometers  for  ascertain- 
ing the  temperature  of  the  water.  S.  A 
mechanic  contrivance  for  stirring  the  water  for 
the  purpose  of  distributing  the  absorbed  heat 
uniformly.  To  prevent  the  escape  of  heat 
with  the  expired  air,  the  tube  D'  is  wound 
many  times  in  the  water-space  beneath  the 
animal  cage. 


above  or  below  the  initial 
temperature,  as  well  as 
that  absorbed  by  the 
materials  of  the  appara- 
tus in  these  various  in- 
stances, the  w^ater  equiv- 
alent of  the  tissues  of  the 
animal  and  the  materials 
of  the  calorimeter  must 
be  obtained,  and  then  added  to  or  subtracted  from,  as  the  case  may 
be,  the  amount  of  water  in  the  calorimeter,  and  the  amount  thus  ob- 
tained multiplied  by  its  rise  in  temperature.  In  properly  conducted 
experiments  in  which  the  sources  of  error  are  reduced  to  a  minimum 
there  is  a  very  close  correspondence  between  the  total  physiologic  heat 
value  of  the  food  and  the  amount  collected  by  the  calorimeter.  Thus, 
in  an  experiment  detailed  by  Rubner  a  dog  was  given  during  twelve 
days  228.06  grams  of  proteid  and  340.4  grams  of  fat  the  physical  heat 
value  of  which  was  estimated  at  4429  calories.  The  urine  and  feces 
during  this  period  were  collected  and  their  heat  value  determined, 
which   amounted   to   305    calories.     The   heat   which   theoretically 


ANIMAL  HEAT.  407 

should  have  been  produced  was  4124  calories.  During  the  experi- 
ment the  calorimeter  actually  absorbed  3958  calories,  a  difference 
between  the  theoretic  and  experimental  results  of  156  calories. 

Calorimetric  experiments  on  man  corresponding  to  those  made 
by  Rubner  on  dogs  have  not  been  successful,  owing  purely  to  tech- 
nical difficulties.  Various  attempts  have  been  made,  however,  to 
determine  the  daily  heat-dissipation.  Liebermeister  immersed  a 
man  in  a  bath  with  a  temperature  lower  than  that  of  the  man's  body. 
From  the  rise  in  temperature  of  the  water  it  was  calculated  that  the 
man  produced  daily  3525  calories.  Leyden  placed  the  leg  alone  of 
a  man  in  a  calorimeter.  In  one  hour  6  calories  were  absorbed. 
Assuming  that  the  total  superficial  area  of  the  body  was  fifteen  times 
that  of  the  leg,  he  calculated,  taking  into  consideration  various 
sources  of  error,  that  the  entire  body  would  produce  daily  2376  cal- 
ories. Ott,  employing  a  w^ater  calorimeter,  found  that  the  body  of  a 
man  produced  103  calories  during  an  afternoon,  or  at  the  rate  of 
2472  calories  daily.  These  and  similar  experiments,  while  not  free 
from  many  objections,  furnish  results  which  indicate  that  the  heat 
dissipated  from  the  body  approximates  the  physiologic  heat  values 
of  the  foods. 


HEAT-DISSIPATION  AND    REGULATION   OF  THE  TEMPERATURE. 

Heat-dissipation.— From  the  preceding  statements  it  is  evident 
that  the  body  is  continually  evolving  heat  in  amounts  daily  far  in 
excess  of  that  necessary  for  the  maintenance  of  the  body-temperature. 
Should  this  heat  be  retained,  the  temperature  of  the  body  would  be 
raised  at  the  end  of  twenty-four  hours  an  additional  18°  or  20°  C, — 
a  temperature  far  in  excess  of  that  compatible  with  the  maintenance 
of  physiologic  processes.  That  the  body  may  be  kept  at  the  mean 
temperature  of  37.8°  C.  it  is  essential  that  the  heat  evolved  be  dissi- 
pated as  fast  as  produced.  This  is  accomplished  in  several  ways: 
(i)  In  warming  the  food  and  drink  to  the  temperature  of  the  body. 
(2)  In  warming  the  inspired  air  to  the  same  temperature.  (3)  In 
the  evaporation  of  water  from  the  lungs.  (4)  In  evaporating  water 
from  the  skin.  (5)  In  radiation  and  conduction  from  the  skin. 
The  quantities  of  heat  lost  to  the  body  by  these  different  processes 
it  is  difficult  for  obvious  reasons  to  accurately  determine,  and  the 
estimates  usually  given  must  be  regarded  only  as  approximative. 

Assuming  2500  calories  to  be  an  average  amount  of  heat  liberated 
during  a  day  of  repose,  the  losses,  in  the  ways  stated  above,  may  be 
given  as  follows : 

I.  In  Warming  Food  and  Drink. — The  average  temperature  of  food 
and  drink  is  about  12°  C;  the  amount  of  both  together  is  about 
3  kilograms;  the  specific  heat  of  food  about  0.8.     The  absorption 


4o8  TEXT-BOOK  OF  PHYSIOLOGY. 

of  body-heat  therefore  amounts  to  3  X  0.8  X  25°  C.  =  60  calories 
=  2.8  per  cent.  With  the  removal  of  the  end-products  of  the 
foods  and  drink  from  the  body  an  equal  amount  of  heat  is  carried 
out. 

2.  In  Warming  the  Inspired  Air. — The  average  temperature  of  the 

air  is  12°  C;  the  amount  of  inspired  air,  about  15  kilograms; 
the  specific  heat  of  air,  0.26.  The  absorption  of  body-heat  by 
the  air  therefore  amounts  to  15  X  0.26  X  25°  =  97.5  =  3.8 
per  cent.  The  expired  air  removes  from  the  body  a  corre- 
sponding volume. 

3.  In  the  Evaporation  0}  Water  from  the  Lungs. — The  quantity  of 

water  evaporated  from  the  lungs  may  be  estimated  at  400  grams ; 
as  each  gram  requires  for  its  evaporation  0.582  calorie,  the 
quantity  of  heat  lost  by  this  channel  would  be  400  X  0.582  = 
232.8°  C.  =  9.4  per  cent. 

4.  In  the  Evaporation  0}  Water  from  the  Skin. — The  quantity  of 

water  evaporated  from  the  skin  may  be  estimated  at  660  grams, 
causing  a  loss  of  heat  by  this  channel  of  660  X  0.582  =  384.1° 
C.  =  15.3  per  cent. 

5.  In  Radiation  and  Conduction  from  the  Skin. — The  amount  of 

heat  lost  by  this  process  can  be  indirectly  determined  only  by 
subtracting  the  total  amount  lost  by  the  above-mentioned 
channels  from  the  total  amount  produced.  Thus,  2500 —  774-4 
=  1725.6  =  69  per  cent,  would  represent  the  average  amount  lost 
by  radiation  and  conduction. 

Regulation  of  the  Mean  Temperature. — In  order  that  the 
mean  temperature  of  the  body  may  remain  practically  constant,  the 
heat  produced  must  be  exactly  balanced  by  the  heat  dissipated.  Should 
there  be  any  want  of  correspondence  between  the  two  processes,  there 
would  arise  either  an  increase  or  a  decrease  in  the  mean  temperature. 
As  both  lieat-production  and  heat-dissipation  are  variable  factors, 
dependent  on  a  variety  of  internal  and  external  conditions,  their 
adjustment  is  accomphshed  by  a  complex  self-regulating  mechanism 
involving  muscular,  vascular,  and  secretory  elements,  coordinated  by 
the  nerve  system.  Heat-production  varies  in  intensity  and  amount, 
in  accordance  with  a  number  of  conditions,  but  principally  with 
variations  in  physiologic  activity,  the  quantity  and  quahty  of  the 
food,  and  changes  in  the  external  temperature.  All  physiologic  and 
especially  muscle  activity  is  attended  by  chemic  changes  and  the 
evolution  of  heat.  The  greater  the  activity,  the  larger  is  the  volume 
of  heat.  Foods  have  different  physiologic  heat  values.  If  the  food 
consumed  contains  much  potential  energy  and  the  quantity  con- 
sumed be  larger  than  the  average  daily  requirements,  there  will  be 
an  increase  in  heat-production.  A  lowering  of  the  external  tem- 
perature, as  in  the  winter  season,  leads  to  increased  heat-production 


ANIMAL  HEAT.  409 

through  stimulation  of  the  nerve-centers.  When  all  these  conditions, 
increased  muscular  activity,  increased  amount  of  food  of  high  poten- 
tial energy,  and  a  low  external  temperature  coexist,  heat-production 
attains  its  maximum,  amounting  to  as  much  as  4726  calories  daily 
(Hultgren). 

Heat-dissipation  varies  in  rapidity  in  accordance  with  variations 
of  a  number  of  factors,  but  principally  with  variations  in  the  external 
temperature  and  the  activity  of  the  perspiratory  apparatus.  The 
heat  is  dissipated  mainly  by  the  skin,  69  per  cent.,  in  consequence  of 
radiation  and  conduction  and  by  the  evaporation  of  the  sweat.  The 
loss  by  this  channel  as  well  as  from  the  lungs  is  dependent  for  the 
most  part  on  a  difference  of  temperature  of  the  surrounding  air  and 
of  the  body.  If  the  surrounding  temperature  is  high,  there  is  an 
increase  in  the  activity  of  both  the  circulatory  and  respiratory  mechan- 
isms, brought  about  by  the  central  nervous  system.  In  addition  to 
an  increased  action  of  the  heart,  the  blood-vessels  of  the  skin  dilate 
and  dehver  to  the  surface  a  larger  volume  of  blood  in  a  given  time, 
thus  increasing  the  conditions  favorable  to  radiation.  The  sweat- 
glands  at  the  same  time  are  stimulated  to  increased  activity,  and 
in  consequence  of  the  additional  volumes  of  blood  brought  to  the  skin 
a  larger  amount  of  sweat  is  secreted,  which  speedily  undergoes  evap- 
oration. As  each  gram  of  water  for  its  evaporation  requires  0.582  of 
a  calorie,  it  is  evident  that  increased  secretion  of  sweat  favors  heat- 
dissipation.  The  nerve-centers  influencing  the  activity  of  the  sweat- 
glands  may  be  stimulated  not  only  refiexly,  but  directly  by  an  excess 
of  heat  in  the  blood.  If,  however,  the  atmosphere  itself  possesses  a 
high  percentage  of  moisture,  evaporation  from  the  body  is  much 
diminished  and  the  value  of  sweating  as  a  means  of  lowering  the 
body-temperature  is  much  impaired.  Evaporation  is  hastened  by 
air  in  motion.  Hastened  respiratory  movements  and  the  dilatation 
of  blood-vessels  of  the  respiratory  surface  also  increase  the  evaporation 
of  water  from  the  lungs  and  thus  occasion  a  greater  loss  of  heat. 

If  the  external  temperature  falls  there  is  a  decrease  in  the  physio- 
logic activity  of  the  skin  from  a  contraction  of  the  blood-vessels,  a 
diminution  of  the  blood-supply,  and  a  cessation  in  the  secretion  of 
sweat.  The  blood,  being  prevented  from  coming  to  the  surface,  is 
retained  in  the  deeper  portion  of  the  body,  and  in  consequence  the 
conditions  for  radiation  are  diminished.  These  variations  in  the 
cutaneous  circulation  in  response  to  variations  in  the  external  tem- 
perature are  brought  about  by  the  vasomotor  nerve  mechanism ;  and 
as  they  take  place  with  extreme  promptness  heat-dissipation  and  heat- 
production  are  quickly  adjusted  and  the  mean  temperature  main- 
tained. 

Radiation  from  the  skin  is  modified  to  some  extent  by  clothing. 
An  excess  of  clothing  diminishes,  a  diminution  of  clothing  increases 


4IO  TEXT-BOOK  OF  PHYSIOLOGY. 

radiation.  Tlie  quality  of  clothing  is  also  an  important  factor. 
Wool  is  a  poor  conductor  of  heat  but  a  good  absorber  and  retainer  of 
moisture,  and  hence  is  adapted  for  cold  weather.  Linen  and  cotton 
possess  the  opposite  qualities,  and  hence  are  adapted  for  warm 
weather.  Radiation  from  the  skin  is  somewhat  interfered  with  by 
subcutaneous  fat,  the  extent  of  the  interference  being  dependent  on  its 
amount. 

The  foregoing  estimates  as  to  the  amounts  of  heat  produced  have 
reference  only  to  the  body  in  repose.  When  the  body  passes  into  a 
state  of  muscle  activity,  there  is  at  once  a  notable  increase  in  heat- 
production  in  consequence  of  the  increase  in  the  activity  of  the  chemic 
changes  which  underhe  body  activity,  as  shown  by  the  increase  in  the 
consumption  of  oxygen  and  the  production  of  carbon  dioxid.  Not 
all  of  the  potential  energy  set  free,  however,  appears  as  heat;  for 
if  the  muscles  are  engaged  in  doing  work  a  part  of  the  energy  which 
would  otherwise  manifest  itself  as  heat  is  converted  into  mechanic 
motion.  From  the  work  done  during  a  period  of  eight  hours  it  has 
been  estimated  that  about  500  calories  are  so  transformed  or  utihzed. 
Hirn  calculated  from  an  average  of  five  experiments  that  a  man 
weighing  67  kilos  in  repose  produced  154.4  calories  per  hour  and 
absorbed  30.7  grams  of  oxygen  per  hour;  but  when  engaged  in  active 
muscle  movements  produced  271.2  calories  and  absorbed  119.84 
grams  of  oxygen  per  hour.  The  increase  in  heat-production  per  hour 
during  activity  was  thus  almost  doubled,  though  the  sum  total  pro- 
duced daily  in  which  there  was  a  working  period  of  eight  or  ten  hours 
was  only  about  one-third  more  than  during  a  day  of  repose.  During 
sleep  there  is  a  greatly  diminished  heat-production,  not  more  than 
40  calories  per  hour  being  produced.  The  preceding  data  may  be 
tabulated  as  follows  (Martin) : 

Day  of  Rest.  Day  of  Work. 


Heat  units  (calor-1  ^sst  16  hrs.     Sleep  8  hrs.      Rest  8  hrs.    WorkShrs.    .Sleep8hrs. 
ies)  produced..  I        2470.4  320  1235.2  2160.6  320  ^ 

2790.4  3724-8 


CHAPTER    XV. 

SECRETION. 

Secretion. — As  the  blood  flows  through  the  capillaries  of  the 
body  certain  of  its  nutritive  principles  are  separated  by  the  activity 
of  the  epithelial  cells  of  the  capiUary  wall,  aided  by  the  physical 
processes  of  filtration  and  diffusion.  To  this  process  the  term 
secretion  may  be  applied.  This  separated  or  secreted  material  may 
be  utilized  in  several  ways: 

1.  For  the  repair  of  the  tissues,  for  growth,  for  the  hberation  of 

energy. 

2.  For  the  elaboration  or  production  by  specialized  organs  of  a 

variety  of  complex  fluids  of  widely  different  application.     The 
fluids  thus  formed  are  utilized  for  the  most  part  to  meet  some 
special  need  of  the  body.     All  such  fluids  are  termed  secretions. 
The  organs  concerned  in  the  elaboration  of  the  various  secretions 
are  covered  or  lined  by  epithelium  to  the  activity  of  which  the 
secretion  is  to  be  referred.     To  all  these  organs  the  term  gland  may 
be  applied.     As  these  fluids  are  poured  out  on  the  surface  of  the 
body,  they  have  been  termed  external  secretions:  e.  g.,  mucus,  saliva, 
gastric  juice,  milk,  sebaceous  matter,  etc.     Within  recent  years  it  has 
been  demonstrated  that  the  epithehum  of  glands  and  particularly 
of  those  which  do  not  possess  a  duct,  such  as  the  thyroid,  thymus, 
adrenals,  hypophysis,  etc.,  also  produces  certain  specific  constituents 
which  are  reabsorbed  into  the  blood,  and  which  in  some  unknown 
but  yet  favorable  way  influence  the  general  nutrition.     To  such  prod- 
ucts of  these  glands  the  term  internal  secretions  has  been  given. 

The  blood,  in  addition  to  its  nutritive  constituents,  contains  a 
number  of  principles,  derived  from  the  tissues,  which  are  to  be 
regarded  as  waste  products,  the  outcome  of  the  metabolic  activity 
of  the  tissues  and  of  no  further  use  to  the  body.  If  retained,  they 
would  seriously  if  not  fatally  interfere  with  the  normal  physiologic 
activities  of  the  different  tissues.  They  are  therefore  removed  by 
specialized  organs  after  their  separation  from  the  blood-stream.  The 
waste  products  in  solution  thus  removed  are  not  capable  of  being 
utilized  for  any  special  purpose,  and  are  therefore  termed  excre- 
tions: e.  g.,  urine,  perspiration,  etc.  Excretion,  however,  is  per- 
formed by  the  activities  of  epithehal  cells  aided  by  the  physical 
forces  of  diffusion  and  filtration;  and  though  a  distinction  is  made 

411 


412  TEXT-BOOK  OF  PHYSIOLOGY. 

between  the  two  classes  of  fluids,  no  sharp  Hne  can  be  drawn  between 
the  cell  processes  which  take  place  in  secretory  and  excretory  organs. 
All  secretory  organs  may  be  divided  into — 

1.  Epithelial. 

2.  Reticular  and  vascular,  the  latter  term  indicating  merely  their  re- 

lation to  blood-vessels. 

The  Epithelial  Secretory  Apparatus. — The  apparatus  essential 
to  the  production  of  a  secretion  is  a  dehcate  homogeneous  mem- 
brane, on  one  side  of  which  and  in  close  contact  is  a  layer  of  capillary 
blood-vessels,  lymphatics,  and  nerves;  on  the  other  side,  a  layer  of 
epithelial  cells,  the  physiologic  function  of  which  varies  in  different 
situations. 

The  epithelial  organs  may  consist  of  a  single  layer  of  cells  or  a 
group  of  cells,  and  may  be  subdivided  into — 

1.  Secreting  membranes. 

2.  Secreting  glands. 

The  secreting  membranes  are  the  mucous  membranes  lining 
the  gastro-intestinal,  the  pulmonary,  and  the  genito-urinary  tracts, 
and  the  serous  membranes  lining  closed  cavities,  such  as  the  pleural, 
pericardial,  peritoneal,  and  synovial  membranes. 

The  mucous  membranes  are  soft  and  velvety  in  character  and 
are  composed  of  a  condensed  connective  tissue  forming  a  basement 
membrane  beneath  which  is  a  layer  of  blood-vessels  and  muscle-fibers, 
and  on  which  is  a  layer  of  epithelium,  the  histologic  as  well  as  physio- 
logic characters  of  which  vary  in  different  situations.  The  mucus 
secreted  by  the  various  epithelial  forms  will  very  naturally  possess  a 
somewhat  different  composition,  according  to  the  locality  in  which  it 
is  formed.  In  a  general  way  it  may  be  said  that  mucus  is  a  pale, 
semitransparent,  alkaline  fluid,  containing  leukocytes  and  epithelial 
cells.  It  is  composed  chemically  of  water,  mineral  salts,  and  an 
albuminoid  body,  mucin,  to  the  presence  of  which  it  owes  its  vis- 
cidity. Much  of  the  mucus  is  secreted  by  the  goblet  cells  on  the 
surface  of  the  mucous  membranes.  The  principal  varieties  of  mucus 
are  the  nasal,  bronchial,  vaginal,  urinary,  gastro-intestinal. 

The  serous  membranes  are  composed  of  thin  membrane  formed 
by  a  condensation  of  connective  tissue  and  covered  by  a  single  layer 
of  large,  flat,  nucleated  cells  with  irregular  margins.  These  mem- 
branes enclose  what  are  practically  large  lymph  sacs  or  spaces,  and 
the  fluid  they  contain  resembles  lymph  in  all  respects  and  is  prac- 
tically identical  with  it.  It  serves  to  diminish  friction  when  the 
viscera  they  enclose  move  over  one  another.  The  most  important 
of  the  serous  membranes  are  the  pleural,  pericardial,  and  peritoneal. 

The  synovial  membranes  in  and  around  joints  resemble  serous 
membranes.  The  cells  covering  them,  however,  secrete  a  clear 
colorless  fluid  resembling  lymph,  but  differing  from  it  in  containing 


SECRETION.  413 

a  mucin-like  substance,  a  nucleo-albumin,  which  imparts  to  it  con- 
siderable viscidity.  This  synovial  fluid  serves  to  diminish  friction 
between  the  opposing  surfaces  of  the  bones  as  they  ghde  over  one 
another  during  movement. 

Other  secretions,  such  as  the  aqueous  and  vitreous  humors  of  the 
eye,  the  fluid  of  the  internal  ear,  the  cerebrospinal  fluid,  etc.,  will  be 
considered  in  connection  with  the  organs  with  which  they  are  asso- 
ciated, as  have  been  the  digestive  secretions. 

The  secreting  glands  are  formed  of  the  same  histologic  elements 
as  the  secreting  membranes.  They  are  formed  by  an  involution  of 
the  mucous  membrane  or  skin  the  epithehum  of  which  is  variously 
modified  structurally  and  functionally  in  the  various  situations  in 
which  they  are  formed.  Like  the  membranes  themselves,  the  glands 
are  invested  by  capillary  blood-vessels  and  supphed  with  lymphatics 
and  nerves,  of  which  the  latter  are  in  direct  connection  with  the  blood- 
vessels and  epithelial  cells.  The  interior  of  each  gland  is  in  com- 
munication with  the  free  surface  by  one  or  more  passageways  known 
as  ducts. 

These  glands  may  be  classified  according  as  the  involution  is 
cyhndrical  or  dilated  as — 

1.  Tubular.  The  tubular  glands  may  be  simple — e.  g.,  sweat- 
glands,  intestinal  glands,  fundus  glands  of  the  stomach;  or  compound 
— e.  g.,  kidney,  testicle,  saHvary,  and  lachrymal  glands. 

2.  Alveolar.  The  alveolar  glands  may  also  be  simple — e.  g., 
the  sebaceous  glands,  the  ovarian  folHcles,  meibomian  glands;  or 
compound,  as  the  mammary  glands  and  salivary  glands. 

For  the  production  of  a  secretion  it  is  necessary  that  the  plasma 
of  the  blood,  the  common  material,  be  dehvered  to  the  lymph-spaces 
with  which  the  epithelial  cells  are  in  close  relation.  The  processes 
involved  in  the  passage  of  the  plasma  across  the  capillary  wall  have 
already  been  considered  in  connection  with  the  production  of  lymph. 
They  include  the  physical  processes,  difi'usion,  filtration,  and  osmosis, 
combined  with  a  secretory  activity  of  the  cells  of  the  capillary  wall. 
The  question  as  to  which  of  these  processes  is  the  more  active  is  yet 
a  subject  of  investigation. 

As  the  chemic  composition  and  the  chemic  features  of  the  organic 
constituents  of  all  secretions  have  been  demonstrated  to  be  the  out- 
come of  metaboHc  processes  going  on  within  the  epithehal  cells,  it 
must  be  assumed  at  least  that  these  dift'erences  are  correlated  with 
differences  in  the  histologic  features  and  molecular  structure  of  the 
epithehum.  The  discharge  of  the  secretion  is,  as  a  rule,  intermittent ; 
that  is,  there  are  periods  of  activity  on  the  part  of  the  gland  fol- 
lowed by  periods  of  inactivity  or  rest.  In  rest  more  especially  the 
epithelial  cells,  after  the  assimilation  of  lymph,  accumulate  within 
themselves  such  characteristic  products  as  globules  of  mucin,  gran- 


414  TEXT-BOOK  OF  PHYSIOLOGY. 

ules  which  apparently  are  the  antecedents  of  the  digestive  enzymes, 
granules  of  glycogen,  globules  of  fat,  sugar,  and  proteid,  as  in  the 
case  of  the  mammary  gland.  In  how  far  all  these  compounds  are 
the  result  of  secretory  activity  or  of  a  cell  degeneration  and  disinte- 
gration it  is  impossible  to  state  in  the  light  of  present  knowledge. 
During  the  period  of  gland  rest  the  blood-supply  to  the  gland  is 
merely  sufficient  for  nutritive  purposes.  When  the  occasion  arises 
for  gland  activity,  the  blood-vessels,  under  the  influence  of  the 
vasomotor  nerves,  dilate  and  dehver  to  the  gland  an  amount  of  blood 
far  beyond  that  required  for  nutritive  purposes.  As  a  result  the  gland 
becomes  red  and  vascular  and  the  blood  emerging  by  the  veins  fre- 
quently retains  its  customary  arterial  color.  The  increased  blood-supply 
favors  a  rapid  transudation  of  water  and  salts  into  the  lymph-spaces 
where  they  are  speedily  absorbed  and  transmitted  by  the  epithelial 
cells  into  the  interior  of  the  gland  lumen.  Coincident  with  the  passage 
of  water  through  the  cell,  the  organic  constituents  are  extruded  from 
the  end  of  the  cell  bordering  the  lumen  to  become  dissolved,  or  in  the 
case  of  fat  to  be  suspended,  in  the  water.  The  secretion  thus  formed 
accumulates,  and  with  the  rise  of  pressure  which  inevitably  follows 
it  at  once  passes  into  the  ducts  to  be  discharged  on  the  surface  of  the 
mucous  membrane  or  skin,  as  the  case  may  be. 

Influence  of  the  Nerve  System. — The  activity  of  every  gland 
is  controlled  by  nerve-centers  situated  in  the  central  nerve  system. 
These  centers  may  be  excited  to  activity  either  by  impressions  made 
on  the  peripheral  terminations  or  by  emotional  states,  or,  possibly,  by 
changes  in  the  composition  of  the  blood  itself.  As  a  rule,  all  normal 
secretion  is  a  reflex  act  involving  the  usual  mechanism:  viz.,  a  sentient 
surface  (skin,  mucous  membrane,  or  sense-organ),  an  afferent  nerve,  an 
emissive  cell  from  which  emerges  an  efferent  nerve  to  be  distributed 
to  a  responsive  organ,  the  gland  epithelium. 

For  the  production  of  the  secretion  by  the  epithelial  cell  it 
is  beheved  by  some  experimenters  that  two  physiologically  dis- 
tinct, efferent  nerve-fibers  are  involved — one  stimulating  the  pro- 
duction of  the  organic  constituents  {trophic  nerves),  the  other  stimu- 
lating the  secretion  of  water  and  inorganic  salts  (secretor  nerves). 
The  evidence  for  the  influence  of  the  nerve  system  on  secretion  and 
the  mode  of  connection  of  the  nerve-fibers  with  the  gland-cells  have 
been  alluded  to  and  will  again  be  in  subsequent  chapters. 

The  reticular  and  vascular  glands,  though  not  possessing  any 
common  histologic  features,  are  grouped  together  merely  for  con- 
venience, and  will  be  considered  in  a  separate  chapter  in  connection 
with  the  problems  of  internal  secretion. 


SECRETION. 


415 


MAMMARY  GLANDS. 

The  mammary  glands,  which  secrete  the  milk,  are  two  more  or 
less  hemispheric  organs  situated  in  the  human  female  on  the  anterior 
surface  of  the  thorax.  Though  rudimentary  in  childhood,  they 
gradually  increase  in  size  as  puberty  approaches.  The  gland  pre- 
sents at  its  convexity  a  small  conical  eminence  termed  the  mammilla 
or  nipple,  surrounded  by  a  circular  area  of  pigmented  skin,  the 
areola.  The  gland  proper  is  covered  by  a  layer  of  adipose  tissue 
anteriorly  and  is  attached  posteriorly  to  the  pectoral  muscles  by  a 
network  of  fibrous  tissue. 

During    utero-gestation    the   mammary   glands    become    larger, 

firmer,  and  more  lobulated;  the 
areola  darkens  and  the  blood- 
vessels, especially  the  veins,  be- 
come more  prominent.  At  the 
period  of  lactation  the  gland  is 
the  seat  of  active  histologic  and 
physiologic  changes  correlated 
wdth  the  production  of  milk. 
At   the  close  of  lactation   these 


Fig.  181.  —  Mammary  Gland.  1. 
Lactiferous  ducts.  2.  Lobuli  of 
the   mammary  gland. 


Fig.  182.  —  Acini  of  the  Mammary 
Gland  of  a  Sheep  during  Lacta- 
tion, a.  Membrana  propria,  b. 
Secretory  epithelium. 


activities  cease,  the  glands  diminish  in  size,  undergo  involution,  and 
gradually  return  to  their  former  non-secreting  condition. 
^  Structure  of  the  Mammary  Gland. — Each  mammar}'  gland 
consists  of  an  aggregation  of  some  15  or  20  irregular  pyramidal  lobes, 
each  of  which  is  surrounded  by  a  framework  of  fibrous  tissue.  This 
tissue  affords  support  for  blood-vessels,  lymphatics,  and  nerves. 
Each  lobe  is  provided  with  a  single  excretory  duct,  the  lactiferous 
duct,  which  as  it  approaches  the  areola  expands  into  a  fusiform 
ampulla  or  reservoir.  At  the  base  of  the  nipple  the  ampullae  contract 
to  form  some  16  or  18  narrow  ducts,  which,  ascending  the  nipple, 
open  by  constricted  orifices  0.5  mm.  in  diameter  on  its  apex  (Fig.  181). 


4i6  TEXT-BOOK  OF  PHYSIOLOGY. 

On  tracing  the  lactiferous  duct  into  a  lobe,  it  is  found  to  divide 
and  subdivide  into  a  number  of  branches,  which  pass  into  smaller 
masses — the  lobules.  The  lobule  in  turn  is  composed  of  a  large 
number  of  tubular  acini  or  alveoli,  the  final  terminations  of  the  lobu- 
lar ducts.  Each  acinus  consists  of  a  basement  membrane  lined  by  a 
single  layer  of  low  cuboidal  epithehal  cells  (Fig.  182).  Externally 
the  acinus  is  surrounded  by  blood-vessels,  nerves,  and  lymphatics. 


MILK. 

Milk  as  obtained  during  active  lactation  is  an  opaque  bluish- 
white  fluid,  almost  inodorous,  with  a  sweet  taste,  an  alkahne  reaction, 
and  a  specific  gravity  of  from  1.025  to  1.040.  Examined  micro- 
scopically, it  is  seen  to  consist  of  a  clear  fluid,  the  milk  plasma,  hold- 
ing in  suspension  an  enormous  number  of  small,  highly  refractive 
oil-globules,  which  measure  on  the  average  about  yo  (TTrrr  of  3-^  inch 
in  diameter.  It  has  been  asserted  by  some  observers  that  each  globule 
is  surrounded  by  a  thin  proteid  envelope  which  enables  it  to  maintain 
the  discrete  form.     This,  however,  is  at  present  disbelieved. 

The  quantity  of  milk  secreted  daily  by  the  human  female  averages 
about  1200  c.c. 

Chemic  analysis  has  shown  that  the  milk  of  all  the  mammalia 
consists  of  all  the  different  classes  of  nutritive  principles,  though  in 
different  proportions,  which  are  necessary  to  the  growth  and  devel- 
opment of  the  body.  The  only  exception  appears  to  be  an  insuffi- 
cient amount  of  iron  for  the  formation  of  the  coloring-matter  of  the 
blood,  the  hemoglobin. 

Caseinogen  is  the  chief  proteid  constituent  of  milk.  Associated 
with  it,  however,  are  two  other  proteids,  lactalbumin  and  lactoglob- 
ulin,  both  of  which  are  present  in  but  small  quantity.  When  milk 
is  treated  with  acetic  acid,  sodium  chlorid,  or  magnesium  sulphate 
to  saturation,  the  caseinogen  is  precipitated  as  such,  and  after  the  re- 
moval of  th^  fat  with  which  it  is  entangled  may  be  collected  by  ap- 
propriate chemic  methods.  On  the  addition  of  rennet  prepared  from 
the  mucous  membrane  of  the  calf's  stomach,  which  contains  the 
enzyme  rennin  or  pexin,  the  caseinogen  undergoes  cleavage  into 
an  insoluble  proteid,  casein  or  tyrein,  and  a  small  quantity  of  a  new 
soluble  proteid.  To  this  process  the  term  coagulation  has  been  given. 
The  presence  of  calcium  phosphate  appears  to  be  essential  to  this 
process,  inasmuch  as  it  does  not  take  place  if  the  milk  be  completely 
decalcified  by  the  addition  of  potassium  oxalate.  After  coagulation^ 
the  more  or  less  solid  mass  of  milk  separates  into  a  liquid  portion, 
the  serum,  and  a  solid  portion,  the  coagulum.  The  former,  generally 
termed  whey,  consists  of  water,  salts,  lactalbumin,  sugar;  the  latter, 
the  curd,  consists  of  the  casein  and  entangled  fat.     Boiling  the  milk 


SECRETION. 


417 


retards  and  even  prevents  the  coagulation  by  rennet,  owing  to  the 
precipitation  of  the  calcium  phosphate.  When  milk  is  taken  into  the 
stomach,  it  is  probable  that  the  rennin  coagulates  the  caseinogen  in  a 
manner  similar  to,  if  not  identical  with,  this  process,  which  appears 
to  be  essential  to  the  normal  digestion  of  the  milk. 

The  fat  of  milk  is  more  or  less  solid  at  ordinary  temperatures. 
It  is  a  compound  of  olein,  palmitin,  and  stearin  with  small  quantities 
of  butyrin  and  caproin.  The  melting-point  of  butter  varies  between 
31°  and  34°  C.  When  milk  is  allowed  to  stand  for  some  time,  the 
fat-globules  rise  to  the  surface  and  form  a  thick  layer  known  as 
cream.  Churning  the  milk  or  cream  causes  the  fat-globules  to  run 
together  and  form  a  coherent  mass  termed  butter. 

Lactose  is  the  particular  form  of  sugar  characteristic  of  milk. 
It  belongs  to  the  saccharose  group  and  has  the  following  composition: 
Cj2H220jj.  Though  incapable  of  undergoing  fermentation  by  the 
action  of  the  yeast  plant,  it  is  readily  reduced  by  the  Bacillus 
acidi  iactici  to  lactic  acid  and  carbon  dioxid,  the  former  of  which 
imparts  to  milk  an  acid  reaction  and  a  sour  taste.  With  the  accumu- 
lation of  the  lactic  acid  the  caseinogen  is  precipitated  as  a  more  or  less 
consistent  mass. 

The  inorganic  salts  of  milk  are  chiefly  potassium,  sodium, 
calcium,  and  magnesium  phosphates  and  chlorids.  Iron  is  also 
present  in  small  amount.  The  following  table  of  Bunge  gives  the 
quantitative  amounts  of  these  constituents  in  both  human  and  cow's 
milk: 


In  iooo  Parts. 

POTAS-        „ 

siuM.       Sodium. 

Calcium. 

Magne- 
sium. 

Iron 

OXID. 

Phos- 
phoric 
Acid. 

Chlorin 

Human  milk, 

Cow's  milk, 

0.78          0.25 
1.76           I. II 

0-33 
1-59 

0.06        0.0036 
0.21         0.0030 

0.47 
I.Q7 

0-43 
1.69 

Mechanism  of  Milk  Secretion. — During  the  time  of  lactation 
the  mammar}^  gland  exhibits  periods  of  secretory  activity  which 
alternate  with  periods  of  repose.  Coincidently  with  these  periods 
certain  histologic  changes  take  place  in  the  secreting  epithelium. 
At  the  close  of  a  period  of  active  secretion  and  after  the  discharge  of 
the  milk  each  acinus  presents  the  following  features:  The  epithelial 
cells  are  short,  cubical,  nucleated,  and  border  a  relatively  wide 
lumen,  in  which  is  found  a  variable  quantity  of  milk.  After  the  gland 
has  rested  for  some  time  active  metabolism  again  begins.  The 
cells  grow  and  elongate;  the  nucleus  divides  into  two  or  three  new 
nuclei;  constriction  takes  place  and  the  inner  portion  is  detached 
and  discharged  into  the  lumen  of  the  acinus.  During  the  time  these 
changes  are  taking  place  oil-globules  make  their  appearance  in  the  cell 
27 


4iJ 


TEXT-BOOK  OF  PHYSIOLOGY. 


protoplasm,  some  of  which  are  discharged  separately  into  the  lumen, 
while  others  remain  for  a  time  associated  with  the  detached  portion  of 
the  cell  (Fig.  183).  From  these  histologic  changes  it  is  inferred  that 
the  caseinogen  and  fat  are  products  of  the  metabolism  of  the  cell 
protoplasm  and  not  derived  directly  through  the  lymph  from  the  blood. 
The  lactose  apparently  has  a  similar  origin,  as  appears  from  the  fact 
that  it  is  not  found  either  in  the  blood  or  any  other  tissue,  and  that  it 
is  formed  independently  of  carbohydrate  food.  The  water,  and 
especially  the  inorganic  salts,  are  the  result  of  secretory  activity,  rather 
than  of  diffusion  and  filtration.  This  is  rendered  probable  from  the 
fact  that  the  proportions  of  the  inorganic  salts  of  milk  are  more  closely 
allied  to  those  of  the  tissues  of  the  new-born  child  than  to  blood. 
With  the  passage  of  the  water  and  salts  into  the  lumen  of  the  acinus 

the  proteids  undergo  disintegration  and 
solution  and  the  liquid  assumes  the 
characteristics  of  milk. 

The  discharge  of  milk  is  occasioned 
by  the  suction  efforts  on  the  part  of  the 
child,  aided  by  atmospheric  pressure  and 
the  contractions  of  the  non-striated  mus- 
cle-fibers of  the  lactiferous  ducts. 

Influence  of  the  Nerve  System. 
— Judging  from  analogy,  it  is  probable 
that  the  secretion  of  milk  is  regulated 
by  influences  emanating  from  the  nerve 
system,  though  the  exact  nerve-channels 
for  the  transmission  of  such  influences 
have  not  been  determined  experimentally. 
Various  attempts  have  been  made  to 
isolate  and  study  these  nerves,  but  the 
results  are  inconclusive.  It  is  well  known 
tliat  emotional  states  on  the  part  of  the  mother  modify  the  quantity 
as  well  as  quahty  of  milk,  indicating  a  connection  between  the  gland- 
cells  and  the  central  organs  of  the  nerve  system.  Nerve  terminals 
have  been  discovered  in  and  around  the  epithehal  cells — a  fact 
\vhich  supports  this  view. 

Colostrum. — Within  a  day  or  two  after  parturition  the  alveoli 
become  filled  with  a  fluid  which  in  some  respects  resembles  milk 
and  which  has  been  termed  colostrum.  This  is  a  watery  fluid  con- 
taining disintegrated  epithehal  cells,  fat-globules,  as  wefl  as  colos- 
trum corpuscles,  which  are  probably  emigrated  leukocytes.  Colos- 
trum is  distinguished  from  milk  in  being  richer  in  sugar  and  inorganic 
salts.  It  is  said  to  possess  constituents  which  act  as  a  laxative  to 
the  young  child. 


Fig.  183. — Section  of  the 
Mammary  Gland  of  a 
Cat  in,  the  Early 
Stages  of  Lactation. 
A.  Cavity  of  alveoli  filled 
with  granules  and  globules 
of  fat.  I,  2,  3.  Epithe- 
lium in  various  stages  of 
milk-formation. — ( Yeo.) 


SECRETION. 


419 


THE  LIVER. 

The  liver  is  a  large  gland  situated  in  the  upper  and  right  side 
of  the  abdominal  cavity,  where  it  is  held  in  position  largely  by  hga- 
ments  formed  by  redupHcations  of  the  peritoneal  investment.  In 
the  adult  it  weighs,  freed  of  blood,  from  1300  to  1700  grams.  The 
liver  is  connected  with  the  duodenal  portion  of  the  intestine  by  the 
hepatic  duct.  It  receives  blood  both  from  the  hepatic  artery  and  from 
the  portal  vein,  and  in  this  respect  differs  from  all  other  glands  in 
the  body.  The  epithehal  structures  of  the  Uver  are  inclosed  by  a 
firm  fibrous  membrane,  known  as  Ghsson's  capsule.  At  the  trans- 
verse fissure  it  invests  and  follows  the  blood-vessels,  which  there 
enter,  in  all  their  ramifications  through  the  gland. 

Structure  of  the  Liver. — The  fiver  is  composed  of  an  enor- 
number     of 


ev 
2- 


Vi 


nious 

small  masses,  rounded, 
ovoid,  or  polygonal 
in  shape,  called  lo- 
bules, measuring 
about  one  milhmeter 
in  diameter  and  sepa- 
rated from  one  an- 
other by  a  narrow 
space  in  which  are 
to  be  found  blood- 
vessels, lymphatics, 
hepatic  ducts,  sup- 
ported by  connective 
tissue.  In  the  pig  this 
space  and  its  con- 
tained elements  is 
quite  distinct,  sharply 
marking  out  the 
border  of  the  lobule  (Fig.  184).  This  is  not  so  apparent  in  man. 
Each  lobule  is  made  up  of  irregular  or  polygonal  shaped  cells  measur- 
ing about  30  to  40  micromiUimeters  in  diameter.  These  cells  are 
arranged  in  a  radial  manner  from  the  center  to  the  circumference  of 
the  lobule  (Fig.  185).  Each  cell  possesses  one  and  at  times  two  nuclei. 
There  is  no  evidence  for  the  existence  of  a  distinct  cell-wall.  The 
cell  protoplasm  frequently  contains  globules  of  fat,  granules  of  a 
proteid  nature,  granules  of  glycogen,  pigment  material,  etc.  The 
appearance  presented  by  the  cell  will  vary  considerably,  according  to 
the  time  it  is  observed.  Thus  there  may  be  a  complete  absence  of 
these  constituents,  when  the  cell  may  present  a  series  of  vacuoles 
separated   by  bands  of  protoplasm.     The   cells  are  the  secreting 


Fig.  1S4. — Section  of  Liver  of  Pig,  showing  very 
DiAGRAiiMATiCALLY  THE  LoBULES.  a.  Interlobu- 
lar connective  tissue,  b,  c.  Branches  of  portal  vein 
and  of  hepatic  artery,  d.  Bile-ducts.  e.  Intra- 
lobular vein. — {Piersol.) 


420 


TEXT-BOOK  OF  PHYSIOLOGY. 


Trabeculae  of 
hepatic  cells. 


Central  vein. 


which   uhimately   oc- 
space    between    the 


structures  of  the  hver,  and  hence  are  in  close  relation  with  capillary 
blood-vessels,  lymphatic  spaces,  nerves,  and  irregular  channels  or 
passageways.  The  latter  running  between  the  epithelial  cells  may 
be  compared  to  the  lumen  of  other  secreting  glands. 

Blood-vessels    and   Their    Distribution. — The    blood-vessels 
which  are  in  relation  with  the  hver  are  : 

1.  The  portal  vein. 

2.  The  hepatic  artery. 

3.  The  hepatic  vein. 

The  portal  vein  and  the  hepatic  artery  enter  the  hver  at  the  trans- 
verse fissure.     After  penetrating  its  substance  they  divide  and  sub- 
divide into  smaller  and  smaller 
branches, 
cupy    the 

lobules,  completely  surrounding 
and  hmiting  them.  From  their 
situation  they  are  termed  inter- 
lohidar  veins  and  arteries. 

The  interlobular  veins  give  off 
small  capillary  vessels  which 
penetrate  the  lobule  at  all  points 
of  its  surface.  These  capillaries, 
though  frequently  anastomosing, 
form  a  radial  meshwork  which 
converges  toward  the  center  of 
the  lobule.  In  the  meshes  of 
this  plexus  are  found,  arranged 
in  a  corresponding  radial  man- 
ner, the  hver  cells.  The  inter- 
lobular arteries  are  distributed  to 
the  walls  of  the  portal  vein,  to 
the  connective  tissue,  and  finally 
terminate  in  the  portal  vein  capil- 
laries. The  intralobular  capil- 
laries thus  receive  and  transmit 
blood  which  is  an  admixture  of  both  arterial  and  venous  blood.  In 
the  center  of  each  lobule  there  is  a  large  vein,  formed  by  the  union  of 
the  intralobular  capillaries,  known  as  the  intralohilar  vein,  which 
collects  ah  the  blood  of  the  lobule  and  transmits  it  through  the  lobule 
to  an  underlying  or  sublobular  vein  (Fig.  186).  These  latter  vessels, 
uniting  and  reuniting,  ultimately  form  the  hepatic  vein,  which  empties 
the  blood  into  the  inferior  vena  cava. 

Bile  Capillaries  and  Hepatic  Ducts. — The  bile  capillaries  are 
narrow  channels  which  penetrate  the  lobule  in  all  directions  and  are 
generally  found  running  along  the  sides  of  the  ceHs.     These  channels. 


Interlobular  vein.     Hepatic  duct. 

Fig.  185. — Scheme  of  a  Hepatic  Lob- 
ule,  REPRESENTED  IN  TRANSVERSE 

Section  below  and,  by  Partial 
Leveling,  in  Longitudinal  Sec- 
tion above.  In  the  left  half  the 
blood-vessels  are  drawn;  in  the  right 
half  only  the  cords  of  hepatic  cells. 
X  20.— {Stohr.) 


SECRETION.  421 

which  are  devoid  of  walls,  receive  from  the  cells  some  of  the  products 
of  their  secretory  activity,  and  hence  are  comparable  to  the  lumen  of 
the  alveoH  of  other  secreting  glands.  At  the  periphery  of  the  lobules 
the  bile  capillaries  communicate  with  larger  channels  which  are  the 
beginnings  of  the  hepatic  or  bile-ducts  lying  in  the  interlobular 
spaces.  The  interlobular  bile-ducts  possess  a  distinct  wall  Hned  by 
flattened  epitheHum.  There  is,  however,  no  distinct  Hne  of  demarca- 
tion between  the  cells  of  the  interlobular  ducts  and  the  secreting 
cells  of  the  Uver  proper,  as  the  two  blend  insensibly,  the  one  into 
the  other.  As  the  hepatic  ducts  increase  in  size  they  gradually 
acquire  the  structure  characteristic  of  the  main  hepatic  duct:  viz.,  a 
mucous,  a  muscle,  and  a  fibrous  coat. 


Fig.  186. — Transverse  Section  of  a  Single  Hepatic  Lobule,  i.  Intralobular 
vein,  cut  across.  2,  2,  2,  2.  Afferent  branches  of  the  intralobular  vein.  3,  3, 
3'  3'  3>  3>  3'  3'  3-  Interlobular  branches  of  the  portal  vein,  with  its  capillary 
branches,  forming  the  lobular  plexus,  extending  to  the  radicles  of  the  intralobular 

vein. — (Sappey.) 

Nerves. — Experimental  investigations  have  demonstrated  that 
the  Hver  is  supplied  with  nerves  derived  from  the  central  nerve 
system.  The  route  of  these  nerves  is  probably  by  way  of  the  splanch- 
nics  and  the  vagi.  Many  of  the  nerves  which  enter  the  liver  are 
vasomotor  in  function ;  as  to  whether  others  are  secretory  in  character 
is  yet  a  subject  of  investigation.  It  has  been  asserted  that  nerve 
terminals  have  been  demonstrated  running  between  the  cells  and 
even  penetrating  their  substance.  This  fact  would  indicate  that  the 
metaboHc  processes  of  the  liver  are  under  the  control  of  the  central 
nerve  system. 

Functions  of  the  Liver.— The  anatomic  and  histologic  pecu- 


422  TEXT-BOOK  OF  PHYSIOLOGY. 

liarities  of  the  liver  would  indicate  that  it  has  a  variety  of  relations  to 
the  general  processes  of  the  body.  Experimental  investigation  has 
brought  some  of  these  relations  to  light.  Though  its  physiologic 
actions  are  not  yet  v^holly  understood,  it  may  be  said  that  it — 

1.  Secretes  bile. 

2.  Produces  and  stores  glycogen. 

3.  Assists  in  the  formation  of  urea. 

Secretion  of  Bile. — The  physical  properties  and  chemic  com- 
position of  the  bile  have  already  been  considered  (page  203). 
The  characteristic  salts  of  the  bile,  sodium  glycocholate  and  tauro- 
cholate,  do  not  pre-exist  in  the  blood,  and  therefore  must  be  formed 
by  the  hver  cells  out  of  materials  derived  from  the  blood  of  the 
intralobular  capillaries.  The  antecedents  of  the  bile  salts,  glycocoU 
and  taurin,  are  crystalhzable  nitrogenized  compounds,  and  known 
chemically  as  amido-acetic  and  amido-ethylsulphonic  acids.  Their 
chemic  composition  indicates  that  they  are  derivatives  of  the  proteids 
or  the  albuminoids,  though  the  intermediate  stages  in  their  produc- 
tion are  unknown.  The  origin  of  the  cholahc  acid  with  which  they 
are  combined  is  equally  obscure.  The  bile  salts  as  they  are  found 
in  the  bile  are  produced  in  the  hver  cells  by  metabolic  activity. 

The  primary  coloring-matter  of  the  bile,  bihrubin,  has  been  shown 
to  be  a  derivative  of  hematin,  a  product  of  the  disintegration  of  hemo- 
globin. It  is  supposed  that  the  hver  cells  bring  about  this  change 
by  combining  water  with  hematin,  with  the  abstraction  of  iron.  The 
product  thus  formed  is  bilirubin,  which  is  excreted,  while  the  iron  is 
for  the  most  part  retained. 

Cholesterin  is  a  waste  product  derived  largely  from  the  nerve- 
tissue.  It  is  brought  to  the  liver  and  simply  excreted  by  the  cells. 
The  remaining  constituents  of  the  bile,  water  and  inorganic  salts,  are 
secreted  here  as  in  all  other  glands. 

When  once  formed,  the  hver  cells  discharge  these  various  com- 
pounds into  the  channels  by  which  they  are  surrounded;  they  then 
pass  into  the  open  mouths  of  the  bile-ducts  at  the  periphery  of  the 
lobules.  Under  the  increasing  pressure  which  arises  from  the  secre- 
tion and  accumulation  of  bile,  this  fluid  flows  from  the  smaller  into  the 
larger  bile-ducts,  and  finally  is  emptied  either  directly  into  the  in- 
testine or  into  the  gall-bladder,  where  it  is  stored  until  required  for 
digestive  purposes.  The  secretion  of  bile,  as  observed  by  means  of 
a  biliary  fistula,  is  continuous  and  not  intermittent,  though  the  rate 
of  flow  is  subject  to  considerable  variation. 

The  hver  cells,  as  far  as  the  secretion  of  bile  is  concerned,  appear 
to  be  independent  of  the  nerve  system.  Their  activity,  however, 
is  stimulated  by  the  increased  blood-supply  which  arises  during 
digestion  in  consequence  of  the  dilatation  of  the  intestinal  vessels, 
since  it  is  at  this  period  that  the  rate  of  discharge  is  the  greatest. 


SECRETION.  423 

The  same  results  have  been  shown  by  experiment.  Thus,  division 
of  the  splanchnic  nerves  is  followed  by  an  increased  discharge  of 
bile,  apparently  due  to  the  dilatation  of  the  portal  vessels;  stimula- 
tion of  their  peripheral  ends  is  followed  by  a  decreased  discharge  of 
bile  in  consequence  of  the  contraction  of  the  portal  vessels.  The 
bile  salts  appear  to  be  the  most  efficient  stimulants  to  the  activity  of 
the  liver  cells,  for  their  administration  and  absorption  is  followed  by 
an. increase  not  only  in  the  amount  of  water,  but  of  the  inorganic 
salts  and  other  soUd  constituents  as  well. 

The  flow  of  bile  from  the  bile  capillaries  to  the  main  hepatic 
duct,  though  primarily  dependent  on  differences  of  pressure,  is  aided 
by  the  contraction  of  the  muscular  walls  of  the  bile-ducts  and  the 
inspiratory  movements  of  the  diaphragm,  xlny  obstacle  to  the  dis- 
charge of  bile  leads  to  its  accumulation,  a  rise  of  pressure  beyond 
that  of  the  capillary  blood-vessels,  and  a  reabsorption  by  the  lymph- 
atics of  the  bile  constituents.  After  their  discharge  into  the  blood 
from  the  thoracic  duct  these  constituents  are  deposited  in  various 
tissues,  giving  rise  to  the  phenomena  of  jaundice. 

The  Production  of  Glycogen. — In  1857  Bernard  discovered  the 
fact  that  the  hver  normally  during  Hfe  produces  a  sugar-forming 
substance,  analogous  in  its  chemic  composition  to  starch,  to  which  he 
gave  the  name  glycogen.  This  substance  can  be  obtained  by  the 
following  method:  Small  pieces  of  the  liver  of  an  animal  recently 
killed,  preferably  after  a  meal  rich  in  carbohydrates,  are  placed  in 
acidulated  boihng  water  for  a  few  minutes;  then  rubbed  up  in  a 
mortar  with  sand,  again  boiled,  after  which  the  proteids  are  removed 
by  filtration.  The  filtrate  thus  obtained  is  opalescent  and  resembles 
a  solution  of  starch.  The  glycogen  may  be  precipitated  from  this 
solution  with  alcohol  as  a  white  amorphous  powder,  soluble  in 
water.  Chemic  analysis  shows  that  it  consists  of  CgH^gOj,  or  a 
multiple  of  it. 

When  either  the  original  solution  obtained  by  boihng  or  a  solution 
of  this  amorphous  powder  is  treated  with  iodin,  it  strikes  a  port- wine 
color.  When  digested  with  saHva,  pancreatic  juice,  or  boiled  with 
dilute  acids,  the  solution  becomes  clear,  and  testing  with  Fehhng's 
solution  reveals  the  presence  of  sugar. 

If  the  liver  be  allowed  to  remain  in  the  body  of  an  animal  for  a 
period  of  twenty-four  hours  before  the  decoction  is  made  as  above 
described,  it  will  be  found  that  the  solution  contains  only  a  small 
amount  of  glycogen  but  a  relatively  large  amount  of  sugar.  The 
inference  drawn  is  that  after  death  the  glycogen  is  transformed  by 
some  agent,  possibly  a  ferment,  into  sugar  (dextrose). 

The  presence  of  glycogen  in  the  hver  cells  can  be  shown  micro- 
scopically in  the  form  of  discrete  hyaline  and  refractive  granules. 
As  they  are  soluble  in  water  they  can  be  readily  dissolved  out  from 


424  TEXT-BOOK  OF  PHYSIOLOGY. 

the  cells,  leaving  small  vacuoles  separated  from  one  another  by 
strands  of  cell  substance.  The  amount  of  glycogen  in  a  well-fed 
animal  varies  from  1.5  to  4  per  cent,  of  the  total  vv^eight  of  the  liver. 
The  production  of  glycogen  is  dependent  very  largely  on  the  con- 
sumption of  carbohydrates,  the  greater  the  amount  of  sugar  and 
starch  in  the  food,  the  greater  being  the  production  of  glycogen. 
On  a  pure  proteid  diet  it  is  still  produced,  though  in  small  amounts. 

Glycogen  is  present  also  in  muscles,  in  the  placenta,  in  the  tissues 
of  the  embryo — wherever,  indeed,  active  tissue  changes  and  growth 
are  taking  place. 

The  facts  connected  with  the  formation  of  glycogen,  as  well  as  its 
disposition  as  at  present  generally  accepted,  may  be  stated  as  follows : 
The  dextrose  into  which  the  carlDohydrates  are  converted  by  the 
action  of  the  digestive  fluids  is  absorbed  into  the  blood  of  the  portal 
vein  and  carried  direct  into  the  liver,  where  by  the  action  of  the  cells 
it  is  abstracted,  dehydrated,  and  temporarily  deposited  under  the 
form  of  the  non-diffusible  body  glycogen.  At  a  subsequent  period  and 
in  proportion  to  the  needs  of  the  system  the  liver  cells,  through  the 
agency  of  a  ferment,  transform  the  glycogen  into  dextrose,  return  it  to 
the  circulation,  by  which  it  is  transported  to  the  systemic  capillaries, 
where  it  disappears.  The  blood  of  the  hepatic  vein  therefore  contains 
more  sugar  than  the  blood  of  any  other  part  of  the  body,  and  the 
blood  of  the  arteries  more  than  the  blood  of  the  other  veins.  Should 
there  be  a  failure  on  the  part  of  the  liver  cells  to  abstract  the  sugar, 
it  would  pass  through  the  liver  into  the  general  circulation,  from  which 
it  would  be  eliminated  by  the  kidneys.  The  final  fate  of  the  sugar 
is  uncertain.  It  is,  however,  probable  that  after  its  delivery  to  the 
muscles,  for  example,  it  may  be  directly  oxidized,  or  stored  as 
glycogen,  or  possibly  utihzed  in  the  formation  of  living  material. 
Ultimately,  however,  through  oxidation  it  yields  heat,  and  contributes 
to  the  production  of  muscle  energy. 

In  opposition  to  this  view,  Dr.  Pavy,  after  years  of  accurate  ex- 
perimentation, states  that  the  blood  on  the  cardiac  side  of  the  liver 
never  under  normal  circumstances  contains  a  larger  percentage  of 
sugar  than  is  to  be  found  in  any  part  of  the  circulation,  except  in  the 
portal  vein.  He  states  that  glycogen  is  never  reconverted  into  sugar, 
and  denies  that  the  liver  produces  sugar,  to  be  discharged  into  the 
blood ;  that  the  function  of  the  liver  is  merely  to  arrest  the  passage  of 
sugar,  and  so  to  shield  the  general  circulation  from  an  excess ;  that  the 
sugar  which  arises  in  the  liver  after  death  is  a  postmortem  product 
and  not  an  illustration  of  what  takes  place  during  life.  Dr.  Pavy, 
having  apparently  demonstrated  the  glucoside  constitution  of  proteid 
material  in  general,  accounts  for  the  presence  of  glycogen  in  muscles 
and  other  tissues  on  the  assumption  that  during  the  cleavage  of  the 
proteid  molecule  the  carbohydrate  element  is  set  free  and  temporarily 


SECRETION.  425 

stored  as  glycogen.  He  thus  accounts  for  the  production  of  sugar 
in  the  body,  even  in  the  absence  of  all  sugar  and  starch  from  the  food. 
Pavy  believes  that  the  glycogen  produced  in  the  liver  is  utiHzed  in  the 
formation  of  fat  and  the  synthesis  of  complex  proteids  necessar}'  to 
the  construction  of  the  tissues. 

Influence  of  the  Nerve  System. — The  nerve  system  influ- 
ences in  some  way  the  glycogenic  function  of  the  liver.  It  was 
discovered  by  Bernard  that  puncture  of  the  floor  of  the  fourth  ven- 
tricle at  a  point  between  the  acoustic  and  vagus  nerves,  near  the  middle 
line  is  followed  within  an  hour  or  two  by  the  appearance  of  sugar  in 
the  urine,  which  lasted  for  twenty-four  or  thirty-six  hours.  To  this 
area,  in  close  relation  to  the  vasomotor  center,  he  gave  the  name 
"diabetic  area."  The  quantity  of  sugar  excreted  in  the  urine  will 
depend  on  the  amount  of  sugar  previously  present  in  the  liver. 
Through  some  agency  the  stored-up  glycogen  is  rapidly  converted 
into  sugar  discharged  into  the  blood  and  eliminated  by  the  kidneys. 
There  is  no  positive  evidence  that  the  puncture  destroys  the  vaso- 
motor center  for  the  blood-vessels  of  the  liver,  or  that  there  is  any 
change  in  the  relation  of  the  blood-vessels  to  the  liver-cells.  Never- 
theless powerful  stimulation  of  the  sciatic  nerve,  or  the  central  end  of 
the  vagus  which  impairs  or  depresses  the  vasomotor  center,  will  give 
rise  to  a  similar  production  and  elimination  of  sugar.  The  pathway 
for  the  passage  of  these  influences  beyond  the  first  thoracic  ganglion 
is  unknown,  but  that  it  is  not  by  way  of  the  splanchnics  or  the  vagi 
is  evident  from  the  fact  that  division  of  either  of  these  nerves  is  not 
followed  by  the  appearance  of  sugar  in  the  urine. 

Diabetes. — Diabetes  is  a  chronic  disease  characterized  by  the 
appearance  of  sugar  in  the  urine  in  variable  amounts.  This  patho- 
logic condition  has  usually  been  associated  with  derangements  of  the 
glycogenic  function  of  the  liver,  though  doubtless  derangements  of 
other  organic  functions  will  produce  the  same  condition.  At  the 
present  time  it  is  believed  that  the  excretion  of  sugar  by  the  kidneys 
depends  on  two  causes:  (i)  An  ineffectual  abstraction  and  storage 
of  sugar  due  to  some  impairment  in  the  activity  of  the  liver  cells;  (2) 
a  rapid  cleavage  of  the  proteid  constituents  of  the  tissues,  in  conse- 
quence of  some  profound  alteration  in  the  nutritive  process,  whereby 
their  glucose  radicals  are  Hberated  in  unusual  amounts.  The  physi- 
ologic mechanism  by  which  the  normal  metabolism  of  the  carbohy- 
drates is  regulated  is  unknown.  That  it  is  complex  in  character  is 
shown  by  the  phenomena  which  follow  not  only  puncture  of  the 
medulla,  but  also  removal  of  the  pancreas  and  the  administration  of 
various  poisons. 

Removal  of  the  pancreas  from  the  body  of  a  dog  or  other  ani- 
mal is  at  once  followed  by  a  rise  in  the  percentage  of  sugar  in  the 
blood  and  its  elimination  by  the  kidneys.     In  a  short  time  acetone. 


426  TEXT-BOOK  OF  PHYSIOLOGY. 

aceto-acetic  and  oxybutyric  acids  make  their  appearance,  attended 
by  the  usual  symptoms  characteristic  of  glycosuria  in  man.  The 
quantity  of  sugar  excreted  and  the  gravity  of  the  attendant  symptoms 
may  be  much  diminished  by  allowing  a  portion  of  the  gland  to  remain 
in  situ,  even  though  its  capacity  for  the  production  of  pancreatic 
juice  is  entirely  abolished.  Transplantation  of  the  pancreas  to  the 
subcutaneous  tissue  or  to  the  abdominal  cavity  will  practically  pre- 
vent the  glycosuria.  The  explanations  which  have  been  offered  as 
to  the  manner  in  which  the  pancreatic  tissue  prevents  and  its  absence 
gives  rise  to  the  excretion  of  sugar  are  purely  hypothetical.  It  has 
been  claimed  by  some  investigators  that  the  pancreas  secretes  a 
specific  material,  which  enters  the  blood  and  promotes  oxidation  of 
the  sugar.  In  the  absence  of  this  material  the  sugar  accumulates, 
and  is  finally  ehminated  by  the  kidneys.  Since  the  discovery  of  the 
islands  of  Langerhans  it  has  been  suggested  by  some  investigators  that 
the  production  of  the  material  which  regulates  carbohydrate  metabo- 
Usm  should  be  attributed  to  them  rather  than  to  the  pancreas  as  a 
whole.  The  sugar  excreted  doubtless  in  part  comes  from  the  glycogen 
of  the  liver,  as  this  disappears  in  a  short  time.  But  as  sugar  con- 
tinues to  be  excreted,  even  though  all  carbohydrates  be  withdrawn 
from  the  food,  the  conclusion  is  justifiable  that  it  arises  in  conse- 
quence of  increased  proteid  metabohsm.  This  supposition  is 
strengthened  by  the  fact  that  the  quantity  of  urea  excreted  rises  and 
falls  with  the  quantity  of  sugar  excreted. 

Phloridzin,  a  glucoside  obtained  from  the  root  bark  of  the  cherry 
and  plum  tree,  gives  rise  to  the  appearance  of  sugar  in  the  urine,  in 
amounts  beyond  that  which  might  come  from  the  glucose  normally 
present  in  the  blood  or  from  the  glycogen  of  the  liver.  As  there  is  a 
concomitant  increase  in  the  amount  of  urea  excreted,  the  supposition 
is  that  phloridzin  increases  proteid  metabolism. 

Curara,  in  doses  sufficient  to  paralyze  the  muscles,  also  gives  rise 
to  the  appearance  of  sugar  in  the  urine.  This  is  not  due,  however, 
to  an  increased  production  on  the  part  of  the  liver,  but  rather  to  a 
want  of  consumption  on  the  part  of  the  muscles,  due  to  their  inac- 
tivity. The  accumulation  of  the  sugar  in  the  blood  which  takes  place 
for  this  reason  leads  very  promptly  to  its  removal  by  the  kidneys. 

The  Formation  of  Urea. — It  is  now  generally  beheved  that  the 
hver  is  the  most  active  of  all  the  organs  which  may  be  engaged  in  the 
production  of  urea.  This  belief  is  based  on  numerous  physiologic 
and  pathologic  data.  The  compounds  out  of  which  the  hepatic  cells 
construct  urea  have  been  for  chemic  reasons  asserted  to  be  the 
ammonium  salts,  e.  g.,  the  carbonate,  lactate,  carbamate,  which  are 
constantly  present  in  the  blood.  These  salts,  which  result  from 
proteid  metabolism,  are  absorbed  from  the  tissues,  carried  to  the 
liver,  and  there  synthesized  to  urea.     This  supposition  is  supported 


SECRETION.  427 

by  an  experiment  as  follows:  The  liver  of  an  animal  recently  living 
is  removed  from  the  body  and  its  vessels  perfused  continuously  with 
blood  (the  urea  content  of  which  is  known)  containing  the  ammonium 
salts.  An  analysis  of  this  blood  shows,  after  a  time,  a  diminution 
of  these  salts,  and  a  large  increase  in  the  amount  of  the  urea.  The 
leucin  and  tyrosin  which  result  from  the  prolonged  action  of  pan- 
creatic juice  on  hemi-peptone  are  also  capable  of  being  converted  to 
urea  by  the  hepatic  cells,  and  in  all  probability  are  so  disposed  of. 
Destructive  diseases  of  the  liver — e.  g.,  acute  yellow  atrophy,  sup- 
puration, cirrhosis — largely  diminish  the  production  of  urea,  but  in- 
crease the  quantities  of  the  ammonium  salts  in  the  urine.  The  same 
is  true  when  the  liver  cells  are  destroyed  during  acute  phosphorus- 
poisoning. 

VASCULAR  OR  DUCTLESS  GLANDS. 
INTERNAL  SECRETIONS. 

The  metaboHsm  of  the  body  generally,  as  well  as  that  of  individual 
organs,  has  been  shown  to  be  related  not  only  to  the  physiologic  ac- 
tivity of  such  organs  as  the  hver  and  pancreas,  but  also  to  the  activity 
of  the  so-called  vascular  or  ductless  glands.  The  influence  of  the 
pancreas  in  regulating  the  production  of  glycogen  by  the  liver,  and 
the  influence  of  the  hver  in  the  maintenance  of  the  general  metabo- 
lism through  the  production  of  glycogen  and  the  formation  of  urea, 
are  now  established  facts.  That  the  vascular  or  ductless  glands  to 
an  equal  extent,  though  perhaps  in  a  different  way,  assist  in  the  main- 
tenance of  physiologic  processes,  appears  certain  from  the  results 
of  animal  experimentation.  The  explanation  given  for  the  influence 
of  these  glands  is  that  they  produce  specific  substances,  which  are 
poured  into  the  blood  or  lymph  and  carried  direct  to  the  tissues,  to 
the  activities  of  which  they  appear  to  be  essential;  for  without  these 
substances  the  nutrition  of  the  tissues  declines  and  in  a  short  time 
a  fatal  termination  ensues. 

Inasmuch  as  these  partly  unknown  substances  are  formed  by  cell 
activity  and  are  poured  into  the  interstices  of  the  tissues,  they  have 
been  termed  "internal  secretions."  Though  the  term  internal  secre- 
tions is  appHcable  to  all  substances  which  arise  in  consequence  of 
tissue  metabohsm,  and  which,  after  being  poured  into  the  blood, 
influence  in  varying  degrees  and  ways  physiologic  processes,  yet  the 
term  in  this  connection  will  be  applied  only  to  the  secretions  of  the 
thyroid  gland,  hypophysis  cerebri,  and  adrenal  bodies. 

Thyroid  Gland. — The  thyroid  gland  or  body  consists  of  two  lobes 
situated  on  the  lateral  aspect  of  the  upper  part  of  the  trachea.  Each 
lobe  is  pyriform  in  shape,  the  base  being  directed  downward  and  on  a 
level  with  the  fifth  or  sixth  tracheal  ring.  The  lobe  is  about  50  mm. 
in  length,  20  mm.  in  breadth,  and  25  mm.  in  thickness.     As  a  rule, 


428 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  lobes  are  united  by  a  narrow  band  or  isthmus  of  the  same  tissue. 
The  gland  is  reddish  in  color,  and  abundantly  suppHed  with  blood- 
vessels and  lymphatics. 

Microscopic  examination  shows  that  the  thyroid  consists  of  an 
enormous  number  of  closed  sacs  or  vesicles,  variable  in  size,  the 
largest  not  measuring  more  than  o.i  mm.  in  diameter.  Each  sac  is 
composed  of  a  thin  homogeneous  membrane  hned  by  cuboid  epithe- 
lium. The  interior  of  the  sac  in  adult  Hfe  contains  a  transparent, 
viscid  fluid  containing  albumin  and  termed  "colloid"  substance. 
Externally,  the  sacs  are  surrounded  by  a  plexus  of  capillary  blood- 
vessels and  lymphatics.  The  individual  sacs  are  united  and  sup- 
ported by  connective  tissue,  which  forms,  in  addition,  a  covering 

for  the  entire  gland. 
^  Function  of  the  Thy- 

roid.— The  knowledge  at 
present  possessed  as  to  the 
function  of  the  thyroid 
gland,  especially  in  mam- 
mals, is  the  outcome  of  a 
study  of  the  effects  which 
follow  its  arrested  develop- 
ment in  the  child,  its  de- 
generation in  the  adult, 
and  its  extirpation  in  the 
human  being  as  well  as  in 
animals.  The  results,  how- 
ever, which  follow  its  ex- 
tirpation are  not  always 
uniform  in  all  animals, 
though  sufficient  reasons 
for  the  lack  of  uniformity 
can  not  always  be  assigned. 
Cretinism,  a  condition  characterized  by  a  want  of  physical  and 
mental  development,  is  associated  with,  if  not  directly  dependent  on, 
a  congenital  absence  of  the  thyroid  or  its  arrested  development  dur- 
ing the  early  years  of  childhood. 

Myxedema,  a  condition  of  the  skin  in  which  there  is  a  hyperplasia 
of  the  connective  tissue,  of  an  embryonic  type,  rich  in  mucin,  is  gener- 
ally regarded  as  one  of  the  effects  of  degenerative  processes  in  the 
thyroid.  Partly  in  consequence  of  this  change  in  the  skin  the  face 
becomes  broader,  swollen,  and  flattened,  giving  rise  to  a  loss  of  ex- 
pression. At  the  same  time  the  mind  becomes  dull,  clouded,  even 
approximating  the  idiotic  type.  This  supposed  infiltration  of  the 
skin  with  mucin  was  termed  myxedema  by  Ord,  who  at  the  same 
time  associated  it  with  a  change  in  the  structure  of  the  thyroid  as  a 
result  of  which  it  became  functionally  useless. 


Fig.  187. — View  of  Thyroid  Body.  i.  Thy- 
roid isthmus.  2.  Median  portion  of  crico- 
thyroid membrane.  3.  Crico-thyroid 
muscle.  4.  Lateral  lobe  of  thyroid  body. — 
{After  Morris.) 


SECRETION. 


429 


Extirpation  of  the  thyroid,  for  rehef  from  symptoms  due  to  grave 
pathologic  changes,  has  been  followed  in  human  beings  by  symptoms 
similar  to  those  of  myxedema.  To  this  condition  the  terms  operative 
myxedema  and  cachexia  strumipriva  have  been  applied. 

After  the  pubhcation  of  the  history  of  the  myxedema  which  fol- 
lowed surgical  removal  of  the  thyroid,  Schiff,  in  1887,  repeated  his 
earlier  experiments  on  dogs,  and  found  again  that  removal  of  the 
thyroid  was  speedily  followed  by  tremors,  convulsions,  and  death. 
Similar  experiments  were  made  by  Horsley  on  monkeys,  with  results 
which  resembled  those  characteristic  of  myxedema.  Among  the 
symptoms  which 
developed  within  a 
few  days  after  the 
removal  of  the  gland 
may  be  mentioned 
loss  of  appetite ;  fib- 
rillar contractions 
of  muscles;  tremors 
and  spasms;  mu- 
cinoid  degeneration 
of  the  skin,  giving 
rise  to  pulSness  of 
the  eyehds  and  face 
and  to  a  swollen 
condition  of  the 
abdomen ;  hebetude 
of  mind,  frequently 
terminating  in 
idiocy ;  fall  of  blood- 
pressure;  dyspnea; 
albuminuria ;  atro- 
phy of  the  tissues, 
followed  by  death 
of  the  animal  in  the 
course  of  from  five  to  eight  weeks.  The  complexus  of  symptoms 
observed  in  monkeys  was  divided  by  Horsley  into  three  stages:  viz., 
the  neurotic,  the  mucinoid,  and  the  atrophic. 

It  is  evident  that  the  presence  of  the  thyroid  is  essential  to 
the  normal  activity  of  the  tissues  generally.  As  to  the  manner 
in  which  it  exerts  its  favorable  influence,  there  is  some  difference 
of  opinion.  The  view  that  the  gland  removes  from  the  blood  cer- 
tain toxic  bodies,  rendering  them  innocuous  and  thus  preserving 
the  body  from  a  species  of  auto-intoxication,  is  gradually  yielding  to 
the  more  probable  view  that  the  epithelium  is  engaged  in  the  secre- 
tion of  a  specific  material,  which  finds  its  way  into  the  blood  or  lymph 


Fig.  188. — A  Lobule  from  a  Thin  Section  of  the 
Thyroid  Gland  of  an  Adult  Man.  1.  Colloid 
substance.  2.  Epithelium.  3.  Tangential  section 
of  a  tubule,  the  epithelium  viewed  from  the  surface. 
4.  Tubule  in  transverse  section.  5.  Connective  tissue. 
—{Stohr.) 


430  TEXT-BOOK  OF  PHYSIOLOGY. 

and  in  some  unknown  way  influences  favorably  tissue  metabolism. 
This  view  of  the  function  of  the  thyroid  is  supported  by  the  fact  that 
successful  grafting  of  a  portion  of  the  thyroid  beneath  the  skin  or  in 
the  abdominal  cavity  will  prevent  the  usual  symptoms  which  follow 
thyroidectomy.  The  same  result  is  obtained  by  the  intravenous 
injection  of  thyroid  juice  or  by  the  administration  of  the  raw  gland. 
It  was  shown  by  Murray  that  myxedematous  patients  could  be  bene- 
fited, and  even  cured,  by  feeding  them  with  fresh  thyroids  or  with 
the  dry  extract. 

The  chemic  features  of  the  material  secreted  and  obtained  from 
the  structures  of  the  thyroid  indicate  that  it  is  a  complex  proteid  con- 
taining iodin,  which,  under  the  influence  of  various  reagents,  under- 
goes cleavage,  giving  rise  to  a  non-proteid  residue,  which  carries  with 
it  the  iodin  and  phosphorus.  The  amount  of  iodin  in  the  thyroid 
varies  from  0.33  to  i  milligram  for  each  gram  of  tissue.  To  this 
compound  the  term  thyroiodin  has  been  given.  The  administration 
of  this  compound  produces  efl'ects  similar  to  those  which  follow  the 
therapeutic  administration  of  the  fresh  thyroid  itself:  viz.,  a  diminu- 
tion of  all  myxedematous  symptoms.  In  normal  states  of  the  body, 
thyroiodin  influences  very  actively  the  general  metabohsm.  It  gives 
rise  to  a  decomposition  of  fats  and  proteids  and  to  a  decline  in  body- 
weight.  In  large  doses  it  may  produce  toxic  symptoms:  e.  g.,  in- 
creased cardiac  action,  vertigo,  and  glycosuria. 

The  Pituitary  Body. — This  is  a  small  body  lodged  in  the 
sella  turcica  of  the  sphenoid  bone.  It  consists  of  an  anterior  lobe, 
somewhat  red  in  color,  and  a  posterior  lobe,  yellowish-gray  in  color. 
The  former  is  much  the  larger  and  partly  embraces  the  latter.  The 
anterior  lobe  is  developed  from  an  invagination  of  the  epiblast  of  the 
mouth  cavity,  and  consists  of  distinct  gland  tissue.  The  posterior 
lobe  is  an  outgrowth  from  the  brain,  and  is  connected  with  the 
infundibulum  by  a  short  stalk.  It  has  been  suggested  that  the  term 
infundibular  body  be  reserved  for  the  posterior  lobe,  and  the  term 
hypophysis  cerebri  for  the  anterior  lobe.  This  distinction  appears 
to  be  desirable,  inasmuch  as  in  their  origin  and  structure  they  are 
separate  and  distinct  bodies. 

Removal  of  the  hypophysis  cerebri,  or  the  pituitary  body,  is  always 
followed  by  a  fatal  result,  preceded  by  symptoms  not  unlike  those 
which  follow  removal  of  the  thyroid:  viz.,  anorexia,  tremors,  spasms, 
etc.  Degeneration  of  the  pituitar}^  body  has  been  found  in  connection 
with  a  hypertrophic  condition  of  the  bones  of  the  face  and  extremities, 
to  which  the  term  acromegalia  has  been  given. 

Intravenous  injection  of  an  extract  of  the  pituitary  increases 
the  force  of  the  heart-beat  without  any  change  in  its  frequency,  and 
causes  a  rise  of  blood-pressure  from  a  stimulation  of  the  arterioles 
(Schafer  and  Ohver).     The  material  secreted  by  the  pituitary  has 


SECRETION. 


431 


not  been  isolated,  hence  its  chemic  features  are  unknown.  After  its 
formation  it  probably  passes  through  a  system  of  ducts  into  the 
cerebrospinal  fluid,  after  which  it  influences  the  metabolism  of  the 
nerve  and  osseous  tissues  as  well  as  the  force  of  the  heart  muscle. 

An  extract  of  the  anterior  lobe  itself  exerts  no  appreciable  effect 
on  the  blood-pressure  or  on  the  rate  of  the  heart-beat,  nor  does  it  in- 
fluence the  circulatory  and  respiratory  organs.     An  extract  of  the 
infundibular  body  intravenously  injected,  how- 
ever, gives   rise  to  increased  blood-pressure  and 
to  a  slowing  of  the  heart-beat  (Howell). 

Adrenal  Bodies,  or  Suprarenal  Capsules. — 
These  are  two  flattened  bodies,  somewhat  cres- 
centic  or  triangular  in  shape,  situated  each  upon 
the  upper  extremity  of  the  corresponding  kidney, 
and  held  in  place  by  connective  tissue.  They 
measure  about  40  mm.  in  height,  30  mm.  in 
breadth,  and  from  6  to  8  mm.  in  thickness.  The 
weight  of  each  is  about  4  gm. 

Function  of  the  Adrenal  Bodies. — It  was 
observed  by  Addison  that  a  profound  disturbance 
of  the  nutrition,  characterized  by  a  bronze-Hke 
discoloration  of  the  skin  and  of  the  mucous 
membranes  of  the  mouth,  extreme  muscular 
weakness,  and  profound  anemia,  was  associated 
with,  if  not  dependent  on,  pathologic  conditions 
of  the  suprarenal  glands.  In  the  progress  of  the 
disease  the  asthenia  gradually  increases,  the  heart 
becomes  weak,  the  pulse  small,  soft,  and  feeble, 
indicating  a  general  loss  of  tone  of  the  muscular 
and  vascular  apparatus.  Death  ensues  from 
paralysis  of  the  respiratory  muscles.  The  essen- 
tial nature  of  the  lesion  which  gives  rise  to  these 
symptoms  has  not  been  determined. 

Removal  of  these  bodies  from  various  animals 
is  invariably  and  in  a  short  time  followed  by 
death,  preceded  by  some  of  the  symptoms  char- 
acteristic of  Addison's  disease.  Their  develop- 
ment, however,  is  more  acute.  From  the  fact  that  animals  so 
promptly  die  after  extirpation  of  these  bodies,  and  the  further  fact 
that  the  blood  of  such  animals  is  toxic  to  the  subjects  of  recent 
extirpation,  but  not  to  normal  animals,  the  conclusion  was  drawn 
that  the  function  of  the  adrenal  bodies  is  to  remove  from  the  blood 
some  toxic  product  of  muscle  metabolism.  Its  accumulation  after 
extirpation  gives  rise  to  death  through  auto-intoxication. 

On  the  supposition  that  the  adrenals  might  secrete  and  pour  into 


Fig.  189. — Sagittal 
Section  of  the 
Pituitary  Body 
AND       Infundi- 

BULUM  with 

Adjoining 
Part  of  Third 
Ventricle.  a. 
Anterior  lobe. 
a'.  A  projection 
from  it  toward 
the  front  of  the 
infundibulum. 
b.  Posterior  lobe 
connected  by  a 
stalk  with  the 
infundibulum,  i. 
I.e.  Lamina  cin- 
erea.  o.  Right 
optic  nerve,  ch. 
Section  of  optic 
chiasm,  r.o.  Re- 
cess of  ventricle 
above  the  chias- 
ma.  cm.  Corpus 
mammillare.  — 
(Schwalbe,  from 
Qiiain.) 


432  TEXT-BOOK  OF  PHYSIOLOGY. 

the  blood  a  specific  material  which  favorably  influences  general 
metabolism,  Schafer  and  Oliver  injected  hypodermically  glycerin 
and  water  extracts,  and  observed  at  once  an  increased  activity  of  the 
heart-beats  and  of  the  respiratory  movements.  The  effects,  however, 
were  only  transitory.  When  these  extracts  are  injected  into  the  veins 
directly,  there  follows  in  a  short  time  a  cessation  of  the  auricular 
contraction  of  the  heart,  though  the  ventricular  contraction  continues 
with  an  independent  rhythm.  If  the  vagi  are  cut  previous  to  the 
injection  or  if  the  inhibition  is  removed  by  atropin,  the  rapidity  and 
vigor  of  both  auricles  and  ventricles  are  increased.  Whether  the 
inhibitory  influence  is  removed  or  not,  there  is  a  marked  increase  in 
the  blood-pressure,  though  it  is  greater  in  the  former  instance.  This 
is  attributed  to  a  direct  stimulation  and  contraction  of  the  muscle- 
fibers  of  the  arterioles  themselves,  and  not  to  vasomotor  influences, 
as  it  occurs  also  after  division  of  the  cord  and  destruction  of  the  bulb. 
The  contraction  of  the  arterioles  is  quite  general,  as  shown  by 
plethysmographic  studies  of  the  limbs,  spleen,  kidney,  etc.  Applied 
locally  to  the  mucous  membranes,  adrenal  extract  produces  contrac- 
tion of  the  blood-vessels  and  pallor.  The  skeletal  muscles  are  affected 
by  the  extract  very  much  as  they  are  by  veratrin.  The  duration 
of  a  single  contraction  is  very  much  prolonged,  especially  in  the 
phase  of  relaxation  or  of  decreasing  energy. 

It  is  evident  from  these  experiments  that  the  adrenal  bodies  are 
engaged  in  elaborating  and  pouring  into  the  blood  a  specific  material 
which  stimulates  to  increased  activity  the  muscle-fibers  of  the  heart 
and  arteries,  and  thus  assists  in  maintaining  the  normal  blood-pres- 
sure as  well  as  the  tonicity  of  the  skeletal  muscles.  An  alkaloidal 
substance  was  isolated  by  Abel  from  extracts  of  this  gland,  to  which 
the  term  epinephrin  was  given.  A  crystallizable  substance  was  iso- 
lated first  by  Takamine  and  later  by  Aldrich,  to  which  the 
term  adrenalin  was  given.  Both  substances  are  apparently  equally 
efficacious  in  causing  contraction  of  the  blood-vessels  and  in  raising 
the  blood  pressure.  The  question  as  to  which  of  these  twO' 
substances  represents  the  active  principle  of  the  gland  is  as  yet  a 
subject  of  discussion. 

The  Spleen. — The  spleen  is  a  soft  bluish-red  organ,  oval  in 
shape,  from  twelve  to  fifteen  centimeters  long  by  eight  broad  and 
four  thick.  It  is  situated  in  the  left  hypochondrium  between  the 
stomach  and  the  diaphragm.  In  this  situation  it  is  held  in  position 
by  a  fold  of  the  peritoneum  which  passes  from  the  upper  border 
to  the  diaphragm. 

Structure. — A  section  of  the  spleen  shows  that  it  consists  of 
connective  tissue,  blood-vessels,  lymph  corpuscles,  and  lymphoid 
tissue.  The  surface  of  the  spleen  is  covered  by  a  capsule  composed 
of  dense  fibrous  tissue,  from  the  inner  surface  of  which  septa  or 


SECRETION. 


433 


trabeculae  pass  inward  toward  the  center  of  the  organ.  In  their 
course  they  give  off  a  series  of  processes  which  unite  freely,  forming 
a  spongy  connective-tissue  framework.  The  capsule  and  the  main 
trabeculae  in  some  animals  contain  numerous  non-striated  muscle- 
fibers.  In  man  they  are  relatively  few  in  number.  The  blood- 
vessels which  enter  the  spleen  are  supported  by  the  connective- 
tissue  septa.  As  they  pass  toward  the  center  of  the  organ  they 
divide  very  rapidly  and  soon  diminish  in  size.  In  their  course  small 
branches  are  given  off,  which  penetrate  the  intertrabecular  tissue 
and  become  encased  with  spheric  or  cylindric  masses  of  adenoid 
tissue  known  as  Malpighian  corpuscles.  These  corpuscles  are 
composed  largely  of  leukocytes.  In  some  animals  the  leukocytes, 
instead  of  being  arranged 
in  masses,  are  distributed 
along  the  walls  of  the 
artery  as  a  continuous  layer. 
Within  the  corpuscles  the 
arteries  pass  into  capil- 
laries, whether  the  artery 
passes  directly  to  the  splenic 
pulp  or  indirectly  by  way  of 
the  corpuscles,  its  ultimate 
branches  terminate  in  capil- 
laries v^hich  open  into  the 
spaces  of  the  splenic  pulp. 
From  these  spaces  a  net- 
work of  venules  gathers  the 
blood  and  transmits  it  to 
the  veins.  It  is  a  disputed 
question  as  to  whether  the 
spaces  are  lined  by  epithe- 
Uum,  thus  forming  a  con- 
tinuous blood  channel,  or  whether  they  are  wanting  in  this  histologic 
element. 

The  Splenic  Pulp. — The  spaces  of  the  connective-tissue  frame- 
work are  filled  with  a  dark  red  semifluid  mass  known  as  the  splenic 
pulp.  When  microscopically  examined,  the  pulp  presents  a  fine 
loose  network  of  adenoid  tissue,  large  numbers  of  leukocytes  or 
lymph  corpuscles,  red  corpuscles  in  various  stages  of  disintegra- 
tion, and  pigment  granules.  Chemic  analysis  reveals  the  presence 
of  a  number  of  nitrogen-holding  bodies,  e.  g.,  leucin,  tyrosin,  xanthin, 
uric  acid;  organic  acids,  e.  g.,  acetic,  lactic,  succinic  acids;  pigments 
containing  iron,  and  inorganic  salts. 

The  Functions  of    the    Spleen. — Notwithstanding  all  the  ex- 
periments which  have  been  made  to  determine  the  functions  of  the 
28 


Fin.  I  go. — Malpighian  Corpuscle  of  a  Cat's 
Spleen  Injected,  a.  Artery.  6.  Meshes' 
of  the  pulp  injected,  c.  The  artery  of  the 
corpuscle  ramifying  in  the  lymphatic  tis- 
sue composing  it. 


434 


TEXT-BOOK  OF  PHYSIOLOGY. 


spleen,  it  can  not  be  said  that  any  very  definite  results  have  been 
obtained.  The  fact  that  the  spleen  can  be  removed  from  the  body 
of  an  animal  without  appreciably  interfering  with  the  normal  metabo- 
lism would  indicate  that  its  function  is  not  very  important.  The 
chief  changes  observed  after  such  a  procedure  are  an  enlargement 
of  the  lymphatic  glands  and  an  increase  in  the  activity  of  the  red 
marrow  of  the  bones.  The  presence  of  large  numbers  of  leukocytes 
in  the  splenic  pulp  and  in  the  blood  of  the  splenic  vein  suggested 
the  idea  that  the  spleen  is  engaged  in  the  production  of  leukocytes, 
and  to  this  extent  contributes  to  the  formation  of  blood.  The 
presence  of  disintegrated  red  blood-corpuscles  has  suggested  the 
view  that  the  spleen  exerts  a  destructive  action  on  functionally 
useless  red  corpuscles.  These  and  other  theories  as  to  splenic  func- 
tions have  been  offered 
by  different  observers, 
but  all  are  lacking  posi- 
tive confirmation. 

Volume  Variations 
of  the  Spleen. — It  was 
shown  some  years  since 
by  Roy,  with  the  aid  of 
the  plethysmograph,  that 
the  spleen  undergoes 
rhythmic  variations  in 
volume  from  moment  to 
moment.  In  the  cat  and 
in  the  dog  the  diminu- 
tion in  the  volume  (the 
systole)  and  the  increase 
in  volume  (the  diastole) 
together  occupied  about 
one  minute. 

This  fact  was  determined  by  withdrawing  the  spleen  through 
an  opening  in  the  abdominal  wall  and  enclosing  it  in  a  box  with 
rigid  walls,  the  interior  of  which  was  connected  with  a  piston  record- 
ing apparatus.  The  system  being  filled  with  oil,  each  variation 
in  volume  was  attended  by  a  to-and-fro  displacement  and  a  cor- 
responding movement  of  the  recording  lever.  The  special  form 
of  plethysmograph  used  for  this  purpose  is  known  as  the  oncometer 
or  bulk  measurer,  and  the  recording  apparatus  as  the  oncograph 
(Fig.  191  and  Fig.  196). 

The  cause  of  these  variations  in  volume  Roy  attributed  to  a 
rhythmic  contractility  of  the  non-striated  muscle-fibers  in  the  capsule 
and  trabecular,  and  not  to  changes  in  the  arterial  blood-pressure, 
as  the  curve  of  the  pressure  taken  simultaneously  remained  prac- 


FiG.   iQi. — Spleen  Oncometer  Laid  Open. 


SECRETION.  435 

tically  uniform.  The  effect  of  the  rhythmic  contractions  of  the 
splenic  muscle  tissue  is  to  force  the  blood  through  the  organ,  a 
condition  necessitated  perhaps  by  the  pressure  relations  within, 
though  what  function  is  thereby  fulfilled  is  not  apparent. 

It  was  subsequently  shown  by  Schafer  and  Moore  that  the  splenic 
volume  is  extremely  responsive  to  all  fluctuations  of  the  arterial 
blood-pressure;  that  though  the  spleen  may  passively  expand  and 
recoil  in  response  to  the  rise  and  fall  of  the  blood-pressure,  never- 
theless the  reverse  conditions  may  obtain:  viz.,  that  the  splenic 
volume  may  diminish  as  the  pressure  rises,  if  the  splenic  arterioles 
contract  simultaneously  with  the  contraction  of  the  arterioles  gener- 
ally. On  the  contrary,  the  splenic  volume  may  increase  coincident 
with  a  dilatation  of  the  splenic  and  systemic  arterioles.  In  addition 
to  the  rhythmic  variations,  the  spleen  steadily  increases  in  volume 
for  a  period  of  five  hours  after  digestion,  and  then  gradually  returns 
to  its  former  condition. 

Influence  of  the  Nerve  System. — The  nerves  which  supply  the 
vascular  and  visceral  muscles  in  the  spleen  are  derived  directly 
from  the  semilunar  ganglion  (post-ganglionic  fibers)  and  pass  to  it 
in  company  with  the  splenic  artery.  The  nerve-cells  from  which 
they  arise  are  in  physiologic  relation  with  nerve-fibers  (pre-ganglionic 
fibers)  which  emerge  from  the  spinal  cord  in  the  anterior  roots  of 
the  third  thoracic  to  the  first  lumbar  nerves  inclusive,  though  they 
are  found  most  abundantly  in  the  sixth,  seventh,  and  eighth  thoracic 
nerves.     Their  center  of  origin  is  in  the  medulla  oblongata. 

Stimulation  of  the  nerves  in  any  part  of  their  course  gives  rise 
to  a  diminution  in  splenic  volume;  division  of  the  nerves  is  followed 
by  an  increase  in  the  volume.  In  asphyxia  the  spleen  is  small  and 
contracted,  a  condition  attributed  to  a  stimulation  of  the  centers 
in  the  medulla  by  the  venosity  of  the  blood. 

The  musculature  of  the  spleen  may  also  be  excited  to  contraction 
by  reflex  influences,  as  show^n  by  the  fact  that  stimulation  of  the 
central  end  of  a  sensory  nerve  is  attended  by  a  diminution  of  volume. 

Inasmuch  as  the  excised  spleen  will  continue  to  exhibit  variations 
in  volume  when  perfused  with  blood,  it  would  appear  that  it  possess 
some  mechanism  independent  to  some  extent  of  the  nerve  system. 


CHAPTER  XVI. 

EXCRETION. 

As  stated  in  the  preceding  chapter,  the  term  excretion  is  limited  to 
the  process  by  which  the  end-products  of  tissue  metabohsm  are  re- 
moved from  the  body,  the  nature  of  the  process,  however,  differing 
in  no  essential  particulars  from  that  underlying  the  process  of  secre- 
tion. The  histologic  structures  involved  and  the  forces  at  work 
being  of  the  same  general  character,  it  is  impossible  to  draw  any 
sharp  hne  of  distinction  between  them.  As  a  general  fact  it  may 
be  stated  that  in  their  composition  all  the  characteristic  ingredients 
of  the  excretions  are  incapable  either  of  entering  into  the  formation 
of  tissue  or  of  undergoing  oxidation  for  the  purpose  of  heat-production. 
As  the  retention  of  these  end-products  in  the  body  would  exert  a 
deleterious  influence  on  normal  metabolism,  their  prompt  removal 
becomes  essential  to  the  maintenance  of  physiologic  activity.  The 
principal  excretions  of  the  body — urine,  perspiration,  and  bile — are 
complex  fluids  in  which,  with  the  exception  of  those  given  off  in  the 
lungs,  are  to  be  found  in  varying  proportions  the  chief  end-products 
of  metabohsm. 

THE  URINE. 

Normal  urine  has  a  pale  yellow  or  amber  color,  an  aromatic 
odor,  an  acid  reaction,  and  a  specific  gravity  of  1.020.  As  a  rule,  it 
is  perfectly  transparent,  though  its  transparency  may  be  diminished 
from  the  presence  of  mucus,  calcium  and  magnesium  phosphates,  and 
mixed  urates. 

The  color,  which  varies  within  physiologic  hmits  from  a  pale 
yellow  to  a  reddish-brown,  is  due  to  the  presence  of  the  coloring- 
matters  urobilin,  urochrome,  and  uroerythrin,  all  of  which  are  de- 
rivatives from  the  bile  pigments  absorbed  from  the  liver  or  the 
alimentary  canal. 

The  reaction  of  the  urine  is  acid,  owing  to  the  presence  of  the  acid 
phosphates  of  sodium  and  calcium.  The  degree  of  acidity,  however, 
varies  at  different  periods  of  the  day.  Urine  passed  in  the  morning 
is  strongly  acid,  while  that  passed  during  and  after  digestion,  espe- 
cially if  the  food  be  largely  vegetable  in  character  and  rich  in  alkaline 
salts,  is  either  neutral  or  alkahne  in  reaction.  The  diminished  acidity 
after  meals  is  attributed  to  the  formation  of  hydrochloric  acid  by  the 
gastric  glands  and  the  consequent  liberation  of  bases  which  are  ex- 
creted in  the  urine.     The  phosphoric  acid  which  enters  into  com- 

436 


EXCRETION.  437 

bination  with  sodium  and  potassium  bases  is  a  product  of  tissue 
metabolism. 

The  specific  gravity  is  about  1.020,  though  it  varies  from  1.015 
to  1.025.  It  will  diminish,  other  things  being  equal,  with  increased 
consumption  of  water  and  diminished  activity  of  the  skin;  it  will  be 
increased  of  course  by  the  opposite  conditions. 

The  quantity  of  urine  excreted  in  twenty-four  hours  varies  from 
1200  to  1700  c.c.  Amounts  both  above  and  below  these  are  fre- 
quently passed  from  a  variety  of  causes. 

The  odor  of  the  urine  is  characteristic  and  due  to  the  presence  of 
aromatic  compounds. 

COMPOSITION  OF  URINE. 

Water, 1500.00  c.c. 

Total  solids, 72.00  grams. 

Urea, 33-i8 

Uric  acid,  (urates), 0.55        " 

Hippuric  acid,  ( hippurates.) 0.40        " 

Kreatinin,  .xanthin,  hypoxanthin,  guanin,  ammo- \  ,< 

nium  salts,  pigment,  etc.  j 

Inorganic  salts:  sodium  and  potassium  sulphates,  ] 

phosphates,  and  chlorids;  magnesium  and  cal-  j 

cium  phosphates,  |-        27.00        " 

Organic  salts:  lactates,  acetates,  formates  in  small  j 

amounts,  J 

Sugar,   a  trace 

Gases    nitrogen,  and  carbonic  acid. 

The  estimation  of  total  urinary  solids  in  any  given  sample  of 
urine  is  frequently  a  matter  of  chnical  interest.  This  may  approxi- 
mately be  attained  by  multiplying  the  last  two  figures  of  the  specific 
gravity  by  the  coefficient  of  Haeser  or  Christison,  2.33.  The  result 
expresses  the  total  sohds  in  looo  parts:  e.g.,  urine  with  a  specific 
gravity  of  1.020  would  contain  20X2.33,  or  46.60  grams  of  sohd 
matter  per  looo  c.c.  If  the  amount  passed  in  twenty-four  hours  be 
1500  c.c,  the  total  solids  would  amount  to  69.9  grams. 

The  Water  of  the  Urine. — The  amount  of  urinary  water  and  its 
ratio  to  the  solid  constituents  will  vary  with  the  amount  consumed 
and  the  activity  of  the  skin  and  lungs.  In  summer  the  foods,  liquid 
and  solid,  remaining  the  same,  the  quantity  of  water  in  the  urine  is 
diminished  in  consequence  of  increased  activity  of  skin  and  lungs 
and  the  ratio  of  water  to  solids  decreased.  In  winter  the  reverse 
conditions  obtain.  The  food  remaining  the  same,  the  consumption 
of  large  quantities  of  water  hastens  at  least  the  removal  of  end- 
products  from  the  tissues  and  thus  increases  the  urinary  solids. 

Urea  is  the  most  abundant  of  the  organic  constituents  of  the 
urine  and  is  present  to  the  extent  of  from  2  to  3  per  cent.  It  is  a 
colorless  neutral  substance,  crystallizing  under  varying  conditions 
in  long  silky  needles  or  in  rhombic  prisms.  It  is  soluble  in  water 
and  alcohol.  It  is  composed  of  CONjH^.  When  subjected  to  pro- 
longed boihng,  it  combines  with  water,  giving  rise  to  ammonium 


438  TEXT-BOOK  OF  PHYSIOLOl^Y. 

carbonate.  The  presence  of  Micrococcus  urece  in  urine  will  also 
convert  the  urea,  by  combining  it  with  two  molecules  of  water,  into 
ammonium  carbonate,  C0N2H^  +  2H20  =  (NHJ2C03. 

The  average  amount  of  urea  excreted  daily  varies  from  30  to  34 
grams.  As  urea  is  now  known  to  be  the  principal  end-product  of 
proteid  metabolism  within  the  body,  it  is  evident  that  the  quantity 
produced  and  eliminated  in  the  twenty-four  hours  will  depend  on  the 
quantity  of  proteid  food  consumed  and  on  the  extent  to  which  the 
proteid  constituents  of  the  tissues  are  metabolized.  In  the  condition  of 
nutritive  equilibrium,  when  the  proteid  ingested  is  100  grams  and  the 
urea  egested  31.5  grams,  it  is  difficult  to  state  the  percentage  of  urea 
which  is  derived  from  the  metabolism  of  the  proteid  food  (circulating 
proteid)  and  that  derived  from  the  metabolism  of  the  proteids  of  the 
tissues  (organ  proteid).  In  this  condition,  however,  it  is  found  that 
if  the  proteid  consumed  is  varied  within  hmits  above  or  below  the 
standard  amount  of  100  grams,  the  quantity  of  urea  excreted  rises  and 
falls  in  practically  the  same  ratio,  indicating  apparently  that  the 
production  of  urea  is  directly  dependent  on  the  proteid  supply.  On 
the  contrary,  it  has  been  observed  in  human  beings  in  the  fasting 
condition  that  for  a  period  of  ten  days  there  is  a  daily  excretion  of 
about  21  grams  of  urea,  equivalent  to  about  70  grams  of  proteid. 
Again,  contrary  to  former  views,  the  metabolism  of  proteid  and  the 
production  of  urea  are  practically  independent  of  muscular  work. 
Even  after  severe  labor  extending  over  a  period  of  some  hours  there 
is  no  noticeable  increase  in  the  urea  eliminated. 

Seat  of  Urea  Formation. — It  is  quite  certain  in  the  light  of 
present  knowledge  that  urea  is  partly  formed  in  the  liver  by  the  action 
of  the  cells  out  of  cleavage  products  of  proteid  metabolism.  The  par- 
ticular compounds  out  of  which  the  cells  synthetize  urea  are  the 
ammonium  salts,  especially  the  carbamate  and  carbonate.  The 
experimental  reasons  for  this  view  have  already  been  stated  on  page 
426. 

Uric  acid  is  one  of  the  constant  ingredients  of  the  urine.  It  is  a 
crystalhne  nitrogen-holding  body  closely  resembhng  urea,  its  formula 
being  CgH^N^Og.  The  total  quantity  excreted  daily  varies  from  0.2 
to  I  gram.  It  is  doubtful  if  uric  acid  exists  in  a  free  state  in  the 
urine,  the  indications  being  that  it  is  combined  with  sodium  and 
potassium  in  the  form  of  a  quadriurate.  The  urates  are  frequently 
deposited  when  in  excess  from  the  urine  as  a  brick-red  sediment, 
the  color  being  due  to  their  combination  with  the  coloring-matter 
uroerythrin.  When  pure,  uric  acid  crystalhzes  in  the  rhombic  form, 
though  it  assumes  a  variety  of  forms.  Uric  acid  was  long  regarded 
as  a  product  of  general  proteid  metaboHsm  and  for  chemic  reasons 
an  antecedent  of  urea.  This  view  has  been  abandoned.  At  present 
it  is  believed  that  it  is  a  cleavage  product  of  nuclein,  a  constituent 
of  all  cell  nuclei.    In  the  metabolism  of  nuclein  a  proteid  and  nucleic 


EXCRETION.  439 

acid  are  formed,  from  the  latter  of  which  uric  acid  is  derived.  Nu- 
cleic acid  when  decomposed  yields  a  series  of  bases,  such  as  xanthin, 
hypoxanthin,  adenin,  guanin,  etc.  Because  of  the  fact  that  these 
bodies  can  also  be  obtained  from  a  s}nthetized  body  termed  purin 
they  are  known  collectively  as  the  purin  bases.  Though  there  is  a 
close  relationship  between  uric  acid  and  the  purin  bases,  it  has  been 
impossible  to  experimentally  derive  one  from  the  other.  When  hy- 
poxanthin, however,  is  given  internally  it  is  oxidized  and  converted 
into  uric  acid.  It  is  extremely  probable,  therefore,  that  uric  acid  is 
an  oxidation  product  of  one  or  more  of  the  purin  bases. 

It  is  probable,  however,  that  not  all  of  the  uric  acid  eliminated  is 
derived  from  the  nuclein  of  tissue-cells  and  their  decomposition 
products,  the  purin  bases.  Some  of  it  is  undoubtedly  derived  from 
the  nucleins  contained  in  foods.  The  uric  acid  eliminated  is  there- 
fore partly  endogenous  and  partly  exogenous  in  origin. 

Xanthin,  hypoxanthin,  guanin,  etc.,  are  also  found  in  urine 
in  small  but  variable  amounts.  They  are  nitrogenized  compounds 
derived  mainly  from  the  metabolism  of  the  nuclein  bodies. 

Kreatinin  is  a  crystalline  nitrogenous  compound  closely  resem- 
bling kreatin,  one  of  the  constituents  of  muscular  tissue.  The  amount 
excreted  daily  is  about  i  gram.  Though  kreatinin  may  arise  in  conse- 
quence of  proteid  metabohsm,  it  is  probable  that  it  is  largely  derived 
from  a  transformation  of  the  kreatin  contained  in  the  meat  consumed 
as  food. 

Hippuric  acid  in  combination  with  sodium  and  potassium  is 
very  generally  present  in  urine,  though  in  small  amounts.  It  is  more 
abundant  in  the  urine  of  the  herbivora  than  the  carnivora.  In  man 
the  amount  excreted  daily  is  about  0.7  gram,  though  the  amount  may 
be  raised  by  a  diet  of  asparagus,  plums,  cranberries,  etc.,  and  by  the 
administration  of  benzoic  and  cinnamic  acids.  There  is  evidence  that 
hippuric  acid  is  formed  in  the  kidney  from  benzoic  acid,  its  pre- 
cursors, or  related  bodies.  Various  compounds  of  this  class  are  found 
in  vegetable  foods,  a  fact  which  may  account  for  the  increase  in  the 
excretion  of  hippuric  acid  on  a  vegetable  diet. 

Leucin,  tyrosin,  phenol,  cystin,  indoxyl,  skatoxyl,  are  found 
in  small  amounts  even  under  normal  conditions.  They  arise  from 
putrefactive  change  in  the  intestine. 

Inorganic  Salts. — Sodium  and  potassium  phosphates,  known 
as  the  alkaline  phosphates,  are  found  in  both  blood  and  urine. 
The  total  quantity  excreted  daily  is  about  4  grams.  Calcium 
and  magnesium  phosphates,  known  as  the  earthy  phosphates,  are 
present  to  the  extent  of  i  gram.  Though  insoluble  in  water,  they 
are  held  in  solution  in  the  urine  by  its  acid  constituents.  If  the 
urine  be  rendered  alkaline,  they  are  at  once  precipitated.  Sodium 
and  potassium  sulphates  are  also  present  to  I  he  extent  of   about 


440  TEXT-BOOK  OF  PHYSIOLOGY. 

2  grams.  The  phosphoric  and  sulphuric  acids  which  are  combined 
with  these  bases  enter  the  body  for  the  most  part  in  the  foods,  though 
there  is  evidence  that  they  also  arise  by  oxidation  in  consequence  of 
the  metabolism  of  protcids  which  contain  phosphorus  and  sulphur. 
Sodium  chlorid  is  the  most  abundant  of  the  inorganic  salts.  It  is 
derived  mainly  from  the  food.  The  amount  excreted  is  about  15 
grams  in  twenty-four  hours. 

THE  KIDNEYS. 

The  kidneys  are  the  organs  engaged  in  the  excretion  of  the 
urinary  constituents  from  the  blood.  They  resemble  a  bean  in  shape, 
are  from  10  to  12  centimeters  in  length,  2  in  breadth,  and  weigh  from 
144  to  170  grams.  They  are  situated  in  the  lumbar  region,  one  on 
each  side  of  the  vertebral  column  behind  the  peritoneum,  and  extend 
from  the  eleventh  rib  to  the  crest  of  the  ihum.  The  anterior  surface 
is  convex,  the  posterior  surface  concave.  The  latter  presents  a  deep 
notch — the  hilum.  The  kidney  is  surrounded  by  a  thin  smooth 
membrane  composed  of  white  fibrous  and  yellow  elastic  tissue; 
though  it  is  attached  to  the  surface  of  the  kidney  by  minute  processes 
of  connective  tissue,  it  can  very  readily  be  torn  away.  The  sub- 
stance of  the  kidney  is  dense  but  friable. 

Upon  making  a  longitudinal  section  of  the  kidney  it  will  be  ob- 
served that  the  hilum  extends  into  the  interior  of  the  organ  and 
expands  to  form  a  cavity  known  as  the  sinus,  in  which  are  found  the 
blood-vessels,  nerves,  and  duct  (Fig.  192).  This  cavity  is  mainly 
occupied  by  the  upper  part  of  the  renal  duct,  the  ureter,  the  interior 
of  which  is  termed  the  pelvis.  The  ureter  divides  into  several  por- 
tions which  terminate  in  small  caps  or  calyces  which  receive  the 
apices  of  the  pyramids.  The  parenchyma  of  the  kidney  consists  of 
two  portions :  viz. — 

1.  An  internal  or  medullary  portion,  consisting  of  a  series  of  pyramids 

or  cones,  some  twelve  or  fifteen  in  number,  which  present  a  dis- 
tinctly striated  appearance. 

2.  An  external  or  cortical  portion,  half  an  inch  in  thickness  and  dis- 

tinctly friable  in  character. 
The  Histology  of  the  Kidney. — The  kidney  is  composed  of  a 
connective-tissue  framework  supporting  secreting  tubules,  blood- 
vessels, lymphatics,  and  nerves,  all  of  which  are  directly  connected 
with  the  removal  of  the  urinary  constituents  from  the  blood.  The 
kidney  is  structurally  a  compound  tubular  gland.  If  the  apex  of 
each  pyramid  be  examined  with  a  lens,  it  will  present  a  number  of 
small  orifices  which  may  be  regarded  as  the  beginnings  of  the  urinifer- 
ous  tubules.  From  this  point  the  tubules  pass  outward  in  a  straight 
but  somewhat  diverging  manner  toward  the  cortex,  giving  off   at 


EXCRETION. 


441 


acute  angles  a  number  of  branches  (Fig.  193).  From  the  apex  to  the 
base  of  the  pyramids  they  are  known  as  the  tubules  of  Bellini.  In 
the  cortical  portion  of  the  kidney  the  tubule  becomes  enlarged  and 
twisted,  and,  after  pursuing  an  extremely  convoluted  course,  turns 
backward  into  the 
medullary  portion  for 
some  distance,  form- 
ing the  ascending  limb 
of  Henle's  loop;  it 
then  turns  upon  itself, 
forming  the  descend- 
ing limb  of  the  loop,  re- 
enters the  cortex,  again 
expands  and  becomes 
convoluted,  and  finally 
terminates  in  an  ovoid 
enlargement  known  as 
Miiller's  or  Bowman's 
capsule,  in  which  is 
contained  a  small  tuft 
of  blood-vessels — the 
glomerulus.  Each 
tubule  consists  of  a 
basement  membrane 
hned  throughout  its 
entire  extent  by  epi- 
thelial cells.  The  epi- 
thelium as  well  as  the 
tubule  vary  in  shape 
and  size  in  different 
parts  of  its  course.  In 
the  capsule  the  epi- 
thelium is  flattened, 
lining  not  only  the 
inner  surface  of  the 
capsule  but  reflected 
over  the  blood-vessels 
as  well.  This  is 
knowai  as  the  glomer- 
ular epithelium.  In 
the  convoluted  por- 
tions of  the  tubules  the  epithehum  is  cuboidal,  granular,  and  some- 
what striated ;  in  Henle's  loop  it  is  more  or  less  flattened. 

The  Blood-vessels  of  the  Kidney. — The  renal  artery  enters  the 
kidney  at  the  hilum  behind  the  ureter;  it  soon  divides  into  several 


Fig. 


192. — Longitudinal  Section  through  the 
Kidney,  the  Pelvis  of  the  Kidney,  and  a 
Number  of  Renal  Calyces.  A.  Branch  of  the 
renal  artery.  U.  Ureter.  C.  Renal  calyx.  i. 
Cortex,  i'.  Medullary  rays.  i".  Labyrinth,  or 
cortex  proper.  2.  Medulla.  2'.  Papillary  por- 
tion of  medulla,  or  medulla  proper.  2".  Border 
layer  of  the  medulla.  3,  3.  Transverse  section 
through  the  axes  of  the  tubules  of  the  border  layer. 
4.  Fat  of  the  renal  sinus.  5,  5.  Arterial  branches. 
*.  Transversely  coursing  medulla  rays. — {Tyson, 
after  Henle.) 


442 


TEXT-BOOK  OF  PHYSIOLOGY. 


large  branches  which  penetrate  the  substance  of  the  kidney  between 
the  pyramids  and  pass  outward  into  the  cortex.     At  the  base  of  the 


Lobule. 


Lobule. 


Renal  corpuscle 


Thin 
division  of  tlu- 
loop  of  Hen 


Collectin.:; 
tubule. 


runica  albuginea. 


.    Stellate  vein. 


.  Interlobular 
artery. 

-  Interlobular 
vein. 


.  Arciform  artery. 


,.  Arciform  vein. 


Interlobar  artery. 
»  Interlobar  vein. 


Papillary  duct    -f-.t; 


Fig.  IQ1!.— Scheme  of  the  Course  of  the  Uriniferous  Tubules  and  the  Renal 


Vessels. 


EXCRETION. 


443 


pyramids  branches  of  the  arteries  form  an  anastomosing  plexus. 
From  this  plexus  vessels  are  given  off,  some  of  which  follow  the  straight 
tubules  toward  the  apex  of  the  pyramids,  vasa  recta,  while  others 
enter  the  cortex  and  pass  to  its  surface  (Fig.  193).  In  the  course  of 
the  latter  small  branches  are  given  off,  each  of  which  soon  divides  and 
subdivides  to  form  a  ball  of  capillary  vessels  known  as  the  glomer- 
ulus. These  capillaries,  however,  do  not  anastomose,  but  soon  re- 
unite to  form  an  efferent  vessel  the  caliber  of  which  is  less  than  that 
of  the  afferent  artery.  In  consequence  of  this,  there  is  a  greater  re- 
sistance to  the  outflow  of  blood  than  to  the  inflow,  and  therefore  a 
higher  blood-pressure  in  the  glomerulus  than  in  capillaries  generally. 
The  relation  of  the  glomer- 
ulus to  the  tubule  is  im- 
portant from  a  physiologic 
point  of  view.  As  stated 
above,  the  glomerulus  is 
received  into  and  sur- 
rounded by  the  terminal 
expansion  or  capsule  of 
the  tubule.  This  capsule, 
formed  by  an  indentation 
of  the  terminal  portion  of 
the  tubule,  consists  of  two 
walls,  an  outer  one  consist- 
ing of  an  extremely  thin 
basement  membrane, 
covered  by  flattened  epi- 
thehal  cells,  and  an  inner 
one  consisting  apparently 
only  of  flattened  epithelium 
which  is  reflected  over  and 
closely  invests  the  glomer- 
ular blood  -  vessels  (Fig. 
194).  The  blood  is  thus  separated  from  the  interior  of  the  capsule 
by  the  epithehal  wall  of  the  capillary  and  the  epithehum  of  the  re- 
flected wall  of  the  capsule.  During  the  periods  of  secretory  activity 
the  blood-vessels  of  the  glomerulus  are  filled  with  blood  to  such  an 
extent  that  the  sac  cavity  is  almost  obhterated.  After  its  exit  from 
the  capsule  the  efferent  vessel  of  the  glomerulus  soon  again  divides 
and  subdivides  to  form  an  elaborate  capillary  plexus  which  surrounds 
and  closely  invests  the  convoluted  tubules.  From  this  plexus  as 
well  as  from  the  plexus  wiiich  surrounds  the  straight  tubules  veins 
arise  which  pass  toward  and  empty  into  veins  at  the  base  of  the  pyra- 
mids. The  renal  vein  formed  by  the  union  of  these  latter  veins 
emerges  from  the  kidney  at  the  hilum  and  finally  empties  into  the 
vena  cava  inferior. 


Fig  194. — Scheme  of  the  Rexal  or  Mal- 
PiGHiAN  Corpuscle,  i.  Interlobular  ar- 
tery.  2.  Afferent  vessel.  3.  Efferent  vessel. 
4.  Outer  wall.  5.  Inner  wall.  6.  Glom- 
erulus.    7.  Neck  of  tubule. — {Stohr.) 


444  TEXT-BOOK  OF  PHYSIOLOGY. 

The  nerves  of  the  kidney  are  derived  from  the  renal  plexus  and 
follow  the  course  of  the  blood-vessels  to  their  termination. 

The  Renal  Duct. — The  excretory  duct  of  the  kidney,  the  ureter, 
is  a  musculo-mcmbranous  tube  about  5  mm.  in  diameter  when  dis- 
tended, 30  cm.  in  length,  and  extends  from  the  hilum  to  the  base  of 
the  bladder.  The  upper  extremity  is  expanded  and  within  the  renal 
sinus  becomes  irregularly  branched,  giving  rise  to  a  number  of  short 
tubes,  called  calyces,  each  of  which  embraces  the  apex  of  a  Malpighian 
pyramid.  The  interior  of  the  expanded  portion  of  the  ureter  is 
known  as  the  pelvis.  The  wall  of  the  ureter  consists  of  a  mucous 
membrane,  a  muscle  coat,  and  an  external  fibrous  investment. 


MECHANISM  OF  URINE  SECRETION. 

The  secretion  of  urine  is  a  complex  process  and  susceptible  of 
several  interpretations.  It  was  originally  inferred  by  Bowman  that, 
as  the  kidney  presents  anatomically  an  apparatus  for  filtration,  the 
capsule  with  its  enclosed  glomerulus,  and  an  apparatus  for  secretion, 
the  epithelium  of  the  urinary  tubules,  the  elimination  of  the  urinary 
constituents  from  the  blood  is  accomplished  by  the  two  processes  of 
filtration  and  secretion;  that  the  water  and  highly  diffusible  inorganic 
salts  simply  pass  by  diffusion,  under  pressure,  through  the  walls  of 
the  glomerular  capillaries,  while  the  organic  constituents  are  removed 
by  the  epithelium  lining  the  tubules. 

Influenced  largely  by  the  facts  of  blood-pressure  Ludwig  advanced 
the  view  that  the  factors  concerned  in  the  secretion  of  urine  were 
purely  physical;  that  in  consequence  of  the  high  pressure  in  the 
vessels  of  the  glomeruh,  due  to  the  resistance  offered  by  the  smaller 
efferent  vessel,  all  the  urinary  constituents  were  filtered  off  in  a 
state  of  extreme  dilution.  In  order  to  account  for  the  higher  per- 
centage of  the  organic  constituents  in  the  urine,  it  was  assumed  that 
as  the  dilute  urine  passed  through  the  tubules  the  water  was  partly 
reabsorbed,  passing  by  diffusion  into  the  lymph  and  blood  until  the 
urine  acquired  its  normal  characteristics.  In  support  of  this  view, 
a  large  number  of  facts  relating  to  the  influence  of  an  increase  and  a 
decrease  of  pressure  in  the  blood-vessels  of  the  glomeruh,  the  velocity 
of  the  blood-stream,  etc.,  in  determining  the  rate  of  urinary  flow 
were  adduced,  all  of  which  apparently  indicated  that  the  former 
stood  to  the  latter  in  the  relation  of  cause  and  effect,  and  that  the 
formation  of  urine  was  accomplished  entirely  by  physical  forces. 

The  progress  of  physiologic  investigation,  however,  has  thrown 
some  doubt  on  the  vaHdity  of  this  physical  interpretation,  and  has 
rather  served  to  support  the  view  of  Bowman  that  the  organic  con- 
stituents at  least  are  removed  from  the  blood  by  a  process  of  selection 
on  the  part  of  the  epithelium  of  the  convoluted  part  of  the  urinary 


EXCRETION.  445 

tubules;  in  other  words,  that  the  secretion  of  urine  is  physiologic 
rather  than  physical.  Heidenhain  has  brought  forward  a  series  of 
facts  which  support  this  view.  As  evidence  that  the  cells  possess  a 
selective  power,  he  presents  the  following  experiment :  The  spinal  cord 
of  an  animal  is  divided  in  the  neck  for  the  purpose  of  lowering  the 
blood-pressure  in  the  kidney  below  the  pressure  at  which  the  urine  is 
secreted;  a  solution  of  indigo-carmine  is  injected  into  the  blood- 
vessels; after  the  lapse  of  ten  minutes  the  animal  is  killed,  the  blood- 
vessels washed  out  with  alcohol  for  the  purpose  of  precipitating  the 
indigo-carmine  in  situ.  Section  of  the  kidney  shows  a  uniform  blue 
stain  of  the  cortex  alone.  Microscopic  examination  reveals  the  fact 
that  the  blue  stain  is  due  to  the  deposition  of  the  pigment  in  the  lumen 
and  in  the  lumen  border  of  the  cells  of  the  convoluted  tubules  and 
the  ascending  Hmb  of  Henle's  loop;  while  the  epithehum  of  Bow- 
man's capsule  as  well  as  the  glomerular  epithelium  present  no  evi- 
dence of  pigmentation. 

Nussbaum  attempted  to  establish  the  secretory  power  of  the  epi- 
thehum in  another  way.  In  the  frog  the  kidney  receives  blood  from 
two  sources:  the  glomeruli  receive  their  blood  from  the  renal  artery, 
the  tubules  from  the  capillaries  formed  by  the  anastomosis  of  branches 
of  the  efferent  vessel  of  the  glomerulus  and  the  branches  of  the  renal 
portal  vein.  Nussbaum  believed  that  by  ligating  the  renal  artery 
all  glomerular  activity  could  be  aboHshed  and  the  part  played  by  the 
epithelium  could  be  estabhshed.  After  so  doing  the  flow  of  urine  was 
at  once  checked;  the  injection  of  urea  at  once  reestablished  it.  This 
fact  was  taken  as  a  proof  that  the  tubular  epithehum  not  only  ex- 
creted urea,  but  water  and  perhaps  other  constituents  as  well.  It 
was  also  found  that  sugar,  peptones,  carmine,  etc.,  which  are  always 
ehminated  from  the  blood  under  normal  conditions,  are  not  removed 
after  ligation  of  the  renal  artery.  It  was  concluded  from  these  ex- 
periments that  the  secreting  structures  of  the  kidney  consist  of  two 
distinct  systems,  the  glomerular  and  the  tubular;  the  former  secreting 
water,  salts,  sugar,  peptone,  etc.;  the  latter  urea,  uric  acid,  etc. 
These  and  similar  facts  indicate  that  the  renal  epithehum  possesses 
a  secretory  rather  than  an  absorptive  function.  Heidenhain  and 
those  who  agree  with  him  assert  that  even  the  water  and  inorganic 
salts  which  pass  through  the  glomerular  epithelium  do  so  in  conse- 
quence of  cell  selection  and  cell  activity;  that  the  entire  process  is  one  of 
secretion,  though  conditioned  by  blood-pressure,  blood  velocity,  etc. 

Influence  of  Blood-pressure. — Whether  the  ehmination  of  the 
urinary  constituents  is  entirely  secretory  (physiologic)  in  character 
or  not  there  can  be  no  doubt  that  the  whole  process  is  largely  deter- 
mined by  the  pressure  and  velocity  of  the  blood  in  the  glomerular 
capillaries,  or,  to  state  it  more  accurately,  on  the  dilTerence  of  pres- 
sure between  the  blood  in  the  capillaries  and  the  urine  in  the  capsules. 


446 


TEXT-BOOK  OF  PHYSIOLOGY. 


As  a  rule,  this  latter  pressure  is  at  a  minimum.  If  the  urine  should 
accumulate  in  the  ureter  and  tubules  cither  from  ligation  or  mechan- 
ical obstruction  until  its  pressure  approximates  that  of  the  blood,  the 
secretion  would  be  diminished  if  not  abolished.  It  is  difficult  to 
determine  the  average  pressure  or  velocity  of  the  blood  in  the 
glomerular  capillaries,  though  they  both  must  be  greater  than  in 
capillaries  in  other  parts  of  the  body,  from  the  fact  that  the 
efferent  vessel  is   narrower   than   the   afferent,  and  therefore  offers 

great  resistance  to  the  outflow 
of  blood,  a  condition  most  favor- 
able to  the  production  of  a 
high  pressure  in  the  glomerulus. 
The  pressure  of  the  blood  in 
the  glomeruli  may  be  raised  and 
the  velocity  increased: 

1.  By  an  increase  in  blood-pres- 
sure generally. 

2.  By  an  increase  in  the  pressure 
of  the  renal  artery  alone. 

The  first  condition  may  be 
brought  about  by  an  increase  in 
either  the  force  or  frequency  of 
the  heart's  action  or  by  a  con- 
traction of  the  arterioles  of  vas- 
cular areas  in  any  or  all  parts  of 
the  body,  excepting,  of  course, 
the  renal  vascular  area.  The 
second  condition  is  brought  about 
by  a  dilatation  of  the  renal  artery 
alone  and  possibly  by  a  contrac- 
tion of  the  efferent  vessels  of  the 
glomeruli. 

The  pressure  of  the  blood  in 
the  glomeruli  may  be  diminished 
and  the  velocity  decreased — 

1.  By  a  decrease  in  the  blood-pressure  generally. 

2.  By  a  decrease  in  the  pressure  of  the  renal  artery  alone. 

The  first  condition  is  brought  about  by  a  decrease  in  either  the 
force  or  frequency  of  the  heart's  action  or  by  a  dilatation  of  the  arteri- 
oles of  large  vascular  areas  in  any  or  all  parts  of  the  body.  The 
second  condition  is  brought  about  by  contraction  of  the  renal  arter\' 
alone  and  possibly  by  a  dilatation  of  the  efferent  vessels  of  the  glom- 
eruli. The  effect  of  the  contraction  and  relaxation  of  either  the 
afferent  or  efferent  vessels  on  the  pressure  within  the  glomerulus 
is  shown  in  figure  195. 


Fig.  195. — To  Illustrate  the  Effect 
OF  Active  Changes  in  the  Yasa 
Afferentia  and  Efferentia  on 
THE  Pressure  in  the  Glomerul.ar 
Capillaries.  A.  Renal  arteries. 
G.  Glomerular     capillaries.  C. 

Tubular  capillaries.  V.  Vein.  The 
short  thick  lines  represent  the  vasa 
afferentia  and  efferentia.  The  con- 
tinuous heavy  line  represents  the 
mean  average  pressure.  If  the  vas 
afferens  dilates  and  the  vas  efferens 
contracts  separately  or  conjointly, 
the  pressure  will  rise,  as  indicated  by 
the  upper  dotted  Una.  If  the  vas 
afferens  contracts  and  the  vas 
efferens  dilates  separately  or  con- 
jointly, the  pressure  wfill  fall,  as  in- 
dicated by  the  lower  dotted  line. — 
{Ajter  Moral  and  Starling.) 


EXCRETION. 


447 


Coincident  with  the  rise  and  fall  of  pressure  in  the  glomerular 
capillaries  there  is  a  rise  and  fall  in  the  rate  of  urinary  flow.  Thus 
it  has  been  found  that  an  increase  in  the  aortic  pressure  from  127  to 
142  mm.  of  mercury,  by  ligation  of  the  carotid,  femoral,  and  vertebral 
arteries,  increased  the  rate  of  urinary  flow  from  8.7  grams  in  thirty 
minutes  to  21.2  grams.  On  the  contrary,  a  decrease  in  aortic  pres- 
sure below  40  mm.  of  mercury  caused  by  division  of  the  spinal  cord 
is  followed  by  a  total  abolition  of  the  urinary  flow.  These  facts 
serve  to  indicate  the  dependence  of  the  secretion  on  blood-pressure. 

That  there  is  an  increase  in  the  volume  of  the  blood  flowing: 
through  the  kidney  during  its  functional  activity  is  apparent  from 
inspection.  It  is  enlarged,  swollen,  and  red  in  color.  The  blood  in 
the  renal  vein  is  bright  red  in  color  and  contains  more  oxygen  and 
less  carbon  dioxid  than  venous  blood  generally.     During  the  intervals 


^1 -*'-         ^^^^ 


T(i:iz 


[• 


a 


Fig.    196. — Oncometer.     K.   Kidney;   the   thick   line   is   the   metallic   capsule,     h. 

Hinge.     I.  Tube  for  filling  apparatus.     T.  Tube  to  connect  with  T,.     a,  v,  u. 

Artery,  vein,  ureter. — {Stirling,  after  Roy.) 
Fig.    197. — Oncograph.     C.  Chamber  filled   with  oil,  communicating  by  T,   with 
T.     p    Piston.     /.  Writing-lever. — {Stirling,  after  Roy.) 


of  activity  the  kidney  diminishes  in  size,  is  pale  in  color  and  the 
blood  of  the  renal  vein  dark  and  venous  in  character.  These  varia- 
tions in  the  volume  of  the  kidney  have  also  been  experimentally  deter- 
mined and  registered  by  means  of  the  oncometer  and  oncograph 
devised  by  Roy  (Figs.  196  and  197). 

The  oncometer  consists  of  a  metalhc  box  (Fig.  196)  composed 
of  halves  which  open  and  close  by  means  of  a  hinge.  It  is  con- 
nected with  a  recording  apparatus,  the  oncograph  (Fig.  197),  through 
the  tube  T.  The  kidney,  withdrawn  from  the  body,  is  placed 
within  the  oncometer.  Through  an  opening  in  the  side  pass  the 
arter}%  vein,  and  ureter.  Between  the  kidney  and  the  wall  of 
the  capsule  there  is  placed  a  thin  membrane.  Oil  is  then  poured 
through  the  side  tube  I   until  the  space  between  the  capsule  and 


448  TEXT-BOOK  OF  PHYSIOLOGY. 

the  kidney,  as  well  as  the  tube  leading  to  the  chamber  of  the  onco- 
graph, are  completely  filled.  When  the  tube  I  is  closed,  the  condi- 
tions are  such  that  all  variations  in  the  volume  of  the  kidney  are 
taken  up  and  reproduced  by  the  recording  lever  attached  to  the 
piston  of  the  oncograph.  A  curve  of  the  variations  in  the  volume 
of  the  kidney  is  shown  in  figure  198,  taken  simultaneously  with  the 
curve  of  the  blood-pressure.  An  examination  of  this  curve  shows 
that  the  volume-changes  coincide  with  changes  in  the  blood-pressure, 
exhibiting  not  only  the  respiratory  but  also  the  cardiac  undulations. 
Influence  of  the  Nerve  System. — The  influence  of  the  nerve 
system  in  regulating  the  blood-supply  to  the  kidney  is  evident 
from  the  results  of  experimentation.  If  the  nerves  which  accom- 
pany the  renal  artery  into  the  kidney  are  divided,  the  artery 
at  once  dilates,  the  kidney  enlarges,  and  a  copious  flow  of  urine 
takes  place.  If  the  peripheral  ends  of  these  nerves  be  stimulated 
with  the  induced  electric  current,  the  artery  contracts,  the  kidney 


B.P. 


BLOOD     PRESSURE    CURVE 


KIDNEY     CURVE 


Fig.  198. — B.  P.  Blood-pressure  curve.  K.  Curve  of  the  volume  of  the  kidney.  T 
Time  curve;  intervals  indicate  a  quarter  of  a  minute.  A.  Abscissa. — {Stirling, 
after  Roy.) 

diminishes  in  size,  and  the  flow  of  urine  ceases.  In  addition  to 
these  vaso-constrictor  nerves,  there  is  evidence  that  the  kidney  also 
receives  vaso-dilator  nerves  which  emerge  from  the  spinal  cord  and 
are  found  in  the  anterior  roots  of  the  eleventh,  twelfth,  and  thirteenth 
dorsal  nerves,  in  the  dog.  Direct  and  reflex  stimulation  of  these 
nerves  gives  rise  to  a  dilatation  of  the  artery,  a  swelling  of  the 
kidney,  and  an  increase  in  secretion,  independent  of  any  variation 
in  general  blood-pressure. 

The  route  of  the  vaso-constrictor  nerves  is,  in  the  dog  at  least, 
through  the  splanchnics.  Section  of  these  nerves  is  followed  by  a 
dilatation  of  the  renal  vessels  and  an  increase  in  the  flow  of  urine. 
Stimulation  of  the  peripheral  ends  is  followed  by  a  constriction  of 
the  vessels  and  a  cessation  of  the  flow  of  urine. 

The  vasomotor  center  for  the  blood-vessels  of  the  kidney  is  in  all 
probability  situated  in  the  medulla  oblongata  in  close  proximity  to 
the  general  vasomotor  centers,  though  subordinate  centers  are  doubt- 


EXCRETION. 


449 


less  present  in  the  spinal  cord.  It  was  found  by  Bernard  that  punc- 
ture of  the  medulla  was  occasionally  followed  by  a  profuse  secretion 
of  urine  without  the  presence  of  sugar.  The  route  of  the  vaso-motor 
impulses  which  influence  the  renal  blood-supply  is  down  the  cord 
through  the  splanchnics  and  through  the  renal  plexus. 

Influence  of  Variations  in  the  Composition  of  the  Blood. — 
As  it  is  the  function  of  the  kidneys  to  excrete  water,  inorganic  salts, 
and  various  end-products  from  the  blood  and  thus  maintain  a  gen- 
eral average  composition,  it  is  highly  probable  that  as  soon  as  they 
accumulate  beyond  a  certain  percentage  they  themselves  act  as  stimu- 
lants to  renal  activity,  either  by  acting  directly  on  the  renal  epithelium 
or  by  increasing  the  glomerular  pressure.  There  is  evidence  at  least 
that  urea  acts  in  the  former  manner.  An  excess  of  water  in  the  blood 
that  from  copious  drinking  or  from  a  sudden  checking  of  the  skin  from 
a  fall  of  temperature  will  act  in  the  latter  way.  The  introduction  into 
the  blood  of  inorganic  salts,  such  as  potassium  nitrate,  sodium 
acetate,  etc.,  will  in  a  short  time  lead  to  increased  activity  of  the 
kidneys,  as  shown  by  an  increase  in  the  quantity  of  urine  excreted. 
The  manner  in  which  these  agents  and  other  members  of  their  class, 
the  so-called  saline  diuretics,  increase  renal  activity  is  yet  a  subject 
of  discussion.  On  the  one  hand,  it  is  stated  that  they  promote  an 
absorption  of  water  from  the  tissues  to  such  an  extent  that  a  condition 
of  hydremic  plethora  is  produced,  which  in  itself  increases  not  only 
the  general  blood-pressure  but  the  local  renal  pressure  as  well,  and 
that  it  is  this  factor  which  is  the  cause  of  the  increased  flow  of  urine. 
On  the  other  hand,  it  is  asserted  that  though  the  salts  increase 
the  local  pressure  and  the  volume  of  the  kidney,  they  nevertheless 
act  specifically  on  the  renal  epithelium,  and  therefore  may  be  re- 
garded as  secreto-motor  agents.  An  increase  in  the  percentage  of 
sugar  or  urea  in  the  blood  has  a  similar  influence  on  the  kidney. 

The  Storage  and  Discharge  of  Urine. — Urination. — The  uri- 
nary constituents,  as  soon  as  they  are  eliminated  from  the  blood, 
pass  into  and  through  the  uriniferous  tubules  and  by  them  are  dis- 
charged into  the  pelvis  of  the  kidney.  They  then  enter  the  ureter  bv 
which  they  are  conducted  to  the  bladder.  The  immediate  cause  of 
this  movement  is  undoubtedly  a  difference  of  pressure  between  the 
terminal  portions  of  the  tubules  and  the  terminal  portion  of  the 
ureter,  aided  by  the  peristaltic  contraction  of  the  muscle  wall  of 
the  ureter. 

The  bladder  is  a  reservoir  for  the  temporary  reception  of  the  urine 
prior  to  its  expulsion  from  the  body.  When  distended  it  is  ovoid  in 
shape  and  is  capable  of  holding  from  600  to  800  cu.  cm.  The  bladder 
is  composed  of  four  coats:  viz.,  serous,  muscle,  areolar,  and  mu- 
cous. The  muscle  coat  consists  of  external  longitudinal  and  inter- 
nal circular  and  obhque  layers  of  fibers  of  the  non-striated  variety 
29 


450 


TEXT-BOOK  OF  PHYSIOLOGY. 


which  collectively  encircle  the  entire  organ.  As  these  fibers  by  their 
contraction  expel  the  urine  from  the  bladder,  they  are  known  col- 
lectively as  the  detrusor  urince  muscle.  At  the  exit  of  the  bladder 
the  circular  fibers  are  somewhat  increased  in  number,  giving  rise  to 
the  appearance  of  a  distinct  muscle  which  has  been  termed  the 
sphincter  vesica  muscle.  The  presence  of  this  muscle  has,  however, 
been  denied  and  the  retention  of  the  urine  has  been  attributed  to 
mechanic  conditions  at  the  neck  of  the  bladder.  The  urethra  just 
beyond  the  bladder  is  provided  with  a  distinct  circular  muscle  com- 
posed of  striated  fibers,  the  sphincter  urethra  muscle.  When  the 
urine  passes  into  the  bladder  it  is  retained  there  and  prevented  from 
escaping  by  the  contraction  of  this  latter  muscle.  Under  normal 
conditions  the  urine  accumulates  to  a  considerable  extent  before  the 
intra-vesic  pressure  gives  rise  to  a  characteristic  sensation  and  the 
desire  for  urination. 

The  Nerve  Mechanism  of  Urination. — The  muscle  mechan- 
isms which  retain  as  well  as  expel  the  urine  are  under  the  control  of 
the  nerve  system.  The  sphincter  urethrae  muscle,  which  by  the  orifice 
of  the  bladder  is  closed,  is  kept  in  a  state  of  tonic  contraction  by  nerve 
impulses  coming  from  the  spinal  cord  through  the  anterior  roots  of 
the  third  and  fourth  sacral  nerves.  The  detrusor  urinae  muscle  is 
excited  to  contraction  by  impulses  coming  likewise  through  the  sacral 
nerves  and  through  the  upper  lumbar  nerves  irom  the  cord.  The 
centers  of  origin  for  these  two  sets  of  motor  nerves  are  located  in 
the  cord  in  the  neighborhood  of  the  fifth  lumbar  vertebra.  The 
expulsion  of  the  urine  is  largely  a  reflex  act,  though  under  the  con- 
trol of  the  will.  When  the  desire  to  urinate  is  experienced,  nerve 
impulses  are  coming  through  sensory  nerves  from  the  mucous  mem- 
brane of  the  bladder  which  are  reflected  to  the  centers  governing 
the  sphincter  urethrae  and  detrusor  urinae  muscles  and  to  the  brain. 
The  elfect  of  the  reflected  impulses  is  to  inhibit  the  sphincter  center 
and  to  stimulate  the  detrusor  center.  If  the  act  of  urination  is 
to  be  permitted,  vohtional  impulses  descend  through  the  spinal 
cord  which  have  the  effect  of  still  further  inhibiting  the  sphincter 
center  and  stimulating  the  detrusor  center,  the  result  being  a  re- 
laxation of  the  sphincter  muscle  and  a  contraction  of  the  detrusor 
muscle  and  the  expulsion  of  the  urine.  If  the  act  of  urination  is 
to  be  suppressed,  volitional  impulses  inhibit  the  detrusor  center  and 
stimulate  the  sphincter. 

PERSPIRATION;  SEBUM. 

The  perspiration  or  sweat,  the  chief  secretion  of  the  skin,  is  a 
clear  colorless  fluid,  sHghtly  acid  in  reaction  and  saline  to  the  taste. 
Its  specific  gravity  varies  from  1.003  to  1.006.  Unless  collected  from 
the  soles  of  the  feet  and  the  palms  of  the  hand,  it  is  apt  to  be  mixed 
with  epithehal  cells  and  sebum.     The  total  quantity  of  perspiration 


EXCRETION.  451 

secreted  daily  has  been  variously  estimated  at  from  700  to  1000  grams; 
the  exact  amount,  however,  is  difficult  of  determination,  for  the  reason 
that  the  rate  of  secretion  varies  readily  with  variations  in  tempera- 
ture, food,  drink,  season  of  the  year,  etc. 

Chemic  analysis  of  the  sweat  shows  that  it  contains  but  from  0.5 
to  2.5  per  cent,  of  solid  constituents,  the  variation  in  the  percentage 
depending  on  the  quantity  of  water  secreted.  The  solids  consist 
of  traces  of  urea,  neutral  fats,  lactic  and  sudoric  acids  in  combination 
with  alkaline  bases,  and  inorganic  salts  (Fovel).  Other  observers, 
however,  have  not  been  able  to  detect  the  presence  of  either  lactic 
or  sudoric  acid.  Urea  is  a  constant  ingredient,  though  its  percentage 
is  extremely  small,  possibly  not  more  than  o.i  per  cent.  The  amount, 
however,  may  be  very  much  increased  in  uremic  conditions,  the 
result  of  acute  or  chronic  disease  of  the  kidneys.  The  inorganic 
constituents  consist  mainly  of  sodium  chlorid  and  alkahne  and 
earthy  phosphates.  Carbonic  acid  is  also  present  in  the  free  state 
as  well  as  in  combination  with  alkaline  bases. 

The  very  small  quantity  of  the  sohd  constituents  in  the  sweat, 
taken  in  connection  with  the  fact  that  it  is  excreted  most  abundantly 
when  the  external  temperature  is  high,  indicates  that  it  is  not  so  im- 
portant as  an  excrementitious  fluid  as  it  is  as  a  means  for  the  regula- 
tion of  the  temperature  of  the  body. 

The  sweat  is  a  product  of  the  secretory  activity  of  speciahzed 
glands,  the  sweat-glands,  embedded  in  the  skin,  to  the  histologic 
structures  of  which  they  bear  a  special  relation. 

THE  SKIN. 

The  skin  is  a  complexly  organized  structure  investing  the  entire 
external  surface  of  the  body.  Its  total  area  varies  from  16  to  20  feet 
in  man  and  from  12  to  16  feet  in  woman.  It  varies  in  thickness  in 
different  localities  of  the  body  from  ^  to  yIt  of  an  inch.  The  skin 
consists  of  two  principal  layers:  viz.,  a  deep  layer,  the  derma  or 
corium,  and  a  superficial  layer,  the  epidermis. 

The  derma  on  corium  may  be  subdivided  into  a  reticulated  and  a 
papillary  layer.  The  reticulated  layer  consists  of  white  fibrous  and 
yellow  elastic  tissue,  non-striated  muscle  fibers,  woven  together  in 
every  direction  and  forming  an  areolar  network,  in  the  meshes  of 
which  are  deposited  masses  of  fat  and  a  structureless  amorphous 
matter;  the  papillary  layer  consists  mainly  of  club-shaped  elevations 
or  projections  of  the  amorphous  matter  constituting  the  papillae. 
The  reticulated  layer  serves  to  connect  the  skin  with  the  underlying 
structures  and  to  afl'ord  support  for  the  blood-vessels,  nerves,  and 
lymphatics  which  are  distributed  to  the  papillae  (Fig.  199). 

The  epidermis  is  an  extra- vascular  structure  consisting  entirely 
of  epithelial  cells.  It  may  also  be  subdi\dded  into  two  layers — 
the  Malpighian  or  pigmentary  layer,  and  the  corneous  or  horny  layer 


452 


TEXT-BOOK  OF  PHYSIOLOGY. 


,"3- 

3^-tc^ 


A:_ 


The  former  is  closely  applied  to  the  papillary  layer  of  the  true  skin 
and  is  composed  of  large  nucleated  cells,  the  lowest  layer  of  which,  the 
"prickle  cehs,"  contains  the  pigment  granules  which  give  to  the  skin 
its  varying  hues  in  different  individuals  and  in  different  races  of  men ; 
the  corneous  layer  is  composed  of  flattened  cells  which  from  their 
exposure  to  the  atmosphere,  etc.,  are  hard  and  horny  in  texture. 

The  Sweat-glands. — These  glands  are  tubular  in  shape,  the  inner 
extremity  of  each  being  coiled  upon  itself  a  number  of  times,  forming 

a  little  ball  situated 
in  the  derma  or 
the  subcutaneous 
connective  tissue. 
From  this  coil  the 
duct  passes  up  in  a 
straight  direction  to 
the  epidermis, 
where  it  makes  a 
few  spiral  turns, 
after  which  it  opens 
obliquely  on  the 
surface.  The  gland 
consists  of  a  base- 
ment membrane 
lined  with  epithelial 
cells.  It  is  sup- 
plied abundantly 
with  blood-vessels 
and  nerves.  The 
sweat  -  glands  are 
extremely  numer- 
ous all  over  the 
cutaneous  surface, 
though  they  are 
more  thickly  dis- 
posed in  some  situ- 
ations than  others. 
They  probably 
average  2500  to  the 
square  inch;  the  total  number  has  been  estimated  at  from  2,000,000 
to  2,500,000. 

The  Influence  of  the  Nerve  System  on  the  Production  of 
Sweat. — The  secretion  of  sweat,  though  a  product  of  the  activity 
of  epithehal  cells  and  dependent  on  a  variety  of  conditions,  is  reg- 
ulated to  a  large  extent  by  the  nerve  system.  Here  as  in  other 
secreting  glands  the  fluid  is  derived  from  materials  in  the  lymph- 
spaces,  furnished  by  the  blood.     Generally  the  two  conditions,  in- 


>=^,- 


^%. 


■<^^^'s/. 


■*^iiii^ 


Fig 


199. — Section  Perpendicularly  Through  the 
Healthy  Skin.  a.  Epidermis,  or  scarfskin.  b. 
Rete  mucosum,  or  rete  malpighii.  c.  Papillary 
layer,  d.  Derma,  corium,  or  true  skin.  e.  Pan- 
niculus  adiposus,  or  fatty  tissue.  /,  g,  h.  Sweat- 
gland  and  duct,  i,  k.  Hair,  with  its  follicle  and 
papilla.     /.  Sebaceous  gland. 


EXCRETION.  453 

creased  blood-flow  and  increased  glandular  action,  coexist.  At 
times,  however,  a  profuse  clammy  perspiration  is  secreted  with  dimin- 
ished blood-flow.  Two  sets  of  nerves  are  evidently  concerned  in  this 
process:  viz.,  vaso-motor  nerves,  which  regulate  the  blood-supply,  and 
secretor  nerves,  which  stimulate  the  gland-cells  to  activity. 

The  nerve-centers  which  control  the  sweat-glands  are  situated  in 
the  spinal  cord,  though  the  number  of  such  centers  and  their  exact 
location  for  the  different  regions  of  the  body  have  not  yet  been 
satisfactorily  determined.  In  a  general  way  it  may  be  stated  that  the 
centers  for  the  head  and  face  he  in  the  upper  cervical  portion  of  the 
cord;  for  the  upper  extremities,  in  the  lower  cervical  portion;  for  the 
lower  extremities,  in  the  lower  dorsal  and  upper  lumbar  portion. 
The  secretor  nerves  which  emerge  from  these  centers  reach  the  glands 
of  the  face  and  head  through  the  cervical  sympathetic;  of  the  arms 
and  legs,  through  the  brachial  plexus  and  the  sciatic  nerves.  It  is 
probable  that  there  is  also  a  general  dominating  sweat  center  located 
in  the  medulla  oblongata. 

That  the  sweat-glands  are  stimulated  to  activity  by  nerve  impulses 
is  shown  by  the  fact  that  stimulation  of  the  peripheral  end  of  the 
divided  cervical  sympathetic,  of  the  brachial  plexus,  or  of  the  sciatic 
nerve  is  followed  in  a  few  seconds  by  a  profuse  secretion.  Though 
under  physiologic  conditions  there  is  a  simultaneous  dilatation  of  the 
blood-vessels  and  an  increased  supply  of  blood,  this  is  merely  a 
condition  and  not  a  cause  of  the  secretion;  for  the  secretion  can  be 
excited  and  the  flow  maintained  for  a  period  of  from  ten  to  fifteen 
minutes  after  hgation  of  the  blood-vessels  of  the  limb  or  even  after 
its  amputation,  when  the  corresponding  nerve  is  stimulated. 

The  sweat-glands  may  be  excited  to  activity  by  their  related  nerve- 
centers,  either  by  central,  reflex,  or  peripheral  influences.  Among 
the  first  may  be  mentioned  mental  emotions,  venosity  of  the  blood, 
increased  temperature  of  the  blood,  hot  drinks,  violent  muscular 
exercise,  etc.  Among  the  second  may  be  mentioned  powerful 
stimulation  of  various  afferent  or  sensor  nerves,  heightened  external 
temperature,  etc.  Among  the  last  may  be  mentioned  various 
drugs.  Pilocarpin  injected  into  the  blood  causes  a  profuse  secretion 
even  when  the  nerves  have  been  divided.  Its  action  is  supposed  to 
be  exerted  on  the  terminal  branches  of  the  nerves  and  possibly  on 
the  cells  themselves.  As  in  the  case  of  the  sahvary  glands  atropin 
suspends  the  activity  of  the  terminal  branches  of  the  secretor 
nerves. 

Hairs. — Hairs  are  found  in  almost  all  portions  of  the  body,  and 
can  be  divided  into — 

1.  Long,  soft  hairs,  on  the  head. 

2.  Short,  stiff  hairs,  along  the  edges  of  the  eyelids  and  nostrils. 

3.  Soft,  downy  hairs  on  the  general  cutaneous  surface. 


454 


TEXT-BOOK  OF  PHYSIOLOGY. 


They  consist  of  a  root  and  a  shajt.  The  shaft  is  oval  in  shape 
and  about  ^^^^^  of  an  inch  in  diameter;  it  consists  of  fibrous  tissue, 
covered  externally  by  a  layer  of  imbricated  cells,  and  internally  by 
cells  containing  granular  and  pigment  material. 

The  root  of  the  hair  is  embedded  in  the  hair-follicle,  formed  by 
a  tubular  depression  of  the  skin,  extending  nearly  through  to  the 
subcutaneous  tissue;  its  walls  are  formed  by  the  layers  of  the  corium, 
covered  by  epidermic  cells.  At  the  bottom  of  the  folhcle  there  is  a 
papillary  projection  of  amorphous  matter,  corresponding  to  a  papilla 
of  the  true  skin,  containing  blood-vessels  and  nerves,  upon  v^hich 
the  hair-root  rests.  The  investments  of  the  hair-roots  are  formed  of 
epithelial  cells,  constituting  the  internal  and  external  root-sheaths. 

The  lower  portion  of  the  hair 
follicle  is  connected  with  the  upper 
surface  of  the  derma  by  bundles  of 
non-striated  muscle-fibers  which  are 
termed  arrectores  pilorum  muscles. 
Their  inclination  and  insertion  are 
such  that  their  contraction  is  fol- 
lowed by  erection  of  the  hair  follicle 
and  hair  shaft.  These  muscles  are 
excited  to  action  by  nerves  termed 
pilo-motor  nerves. 


THE  SEBUM. 

The  sebum  or  sebaceous 
matter  is  a  pecuhar  oily  material 
produced  by  specialized  glands  in 
the  skin.  It  consists  of  water,  epi- 
thelium, proteids,  fat,  cholesterin, 
and  inorganic  salts. 

The  sebaceous  glands  are 
simple  and  compound  racemose 
glands  opening  by  a  common  excretory  duct  on  the  surface  of  the 
epidermis  or  into  the  shaft  of  a  hair- folhcle  (Fig.  200).  These 
glands  are  extremely  numerous  and  found  in  all  portions  of  the 
body,  with  the  exception  of  the  palms  of  the  hands  and  soles 
of  the  feet,  and  most  abundantly  in  the  face.  They  are  formed 
by  a  dehcate  structureless  membrane  lined  by  polyhedral  epithe- 
hum. 

The  sebum  is  not  produced  by  an  act  of  true  secretion,  but  is 
formed  by  a  proliferation  and  degeneration  of  the  gland  epithehum. 
When  first  poured  on  the  surface,  the  sebum  is  oily  and  semi-hquid 
in  character,  but  soon  hardens  and  acquires  a  cheese-Hke  consistence. 


Fig.  200. — Large  Sebaceous  Gland. 
I.  Hair  in  its  follicle.  2,  3,  4,  5. 
Lobules  of  the  gland.  6.  Excre- 
tory duct  traversed  by  the  hair. 
— {Sappey.) 


EXCRETION.  455 

It  serves  to  lubricate  the  hair  and  skin  and  prevent  them  from  be- 
coming dry  and  harsh. 

The  surface  of  the  fetus  is  generally  covered  with  a  thick  layer  of 
sebaceous  matter,  the  vernix  caseosa,  which  possibly  keeps  the  skin 
in  a  normal  condition  by  protecting  it  from  the  effects  of  the  long- 
continued  action  of  the  amniotic  fluid  in  which  the  fetus  is  suspended. 


CHAPTER  XVII. 

THE   CENTRAL   ORGANS   OF  THE  NERVE  SYSTEM  AND 
THEIR  NERVES. 

The  central  organs  of  the  nerve  system  are  the  encephalon 
and  the  spinal  cord  lodged  within  the  cavity  of  the  cranium  and  the 
cavity  of  the  spinal  or  vertebral  column  respectively.  The  general 
shape  of  these  two  portions  of  the  nerve  system  corresponds  with 
that  of  the  cavities  in  which  they  are  contained.  The  encephalon  is 
broad  and  ovoid,  the  spinal  cord  is  narrow  and  elongated. 

The  encephalon  is  subdivided  by  deep  fissures  into  four  distinct, 
though  closely  related  portions:  viz.,  (i)  the  cerebrum,  the  large 
ovoid  mass,  occupying  the  entire  upper  part  of  the  cranial  cavity; 
(2)  the  cerebellum,  the  wedge-shaped  portion  placed  beneath  the 
posterior  part  of  the  cerebrum  and  lodged  within  the  cerebellar  fossae 
of  the  cranium;  (3)  the  isthmus  of  the  encephalon,  the  more  or  less 
pyramidal-shaped  portion  connecting  the  cerebrum  and  cerebellum 
with  each  other  and  both  with  (4)  the  medulla  oblongata.  (Fig. 
201.) 

The  spinal  cord  is  narrow  and  cylindric  in  shape.  It  occupies 
the  spinal  canal  as  far  as  the  second  or  third  lumbar  vertebra.  The 
central  nerve  system  is  bilaterally  symmetric,  consisting  of  distinct 
halves  united  in  the  median  line.  The  cerebrum  is  subdivided  by  a 
deep  fissure,  running  antero-posteriorly,  into  two  ovoid  masses  termed 
cerebral  hemispheres;  the  cerebellum  is  also  partially  subdivided  into 
hemispheres ;  the  isthmus  likewise  presents  in  the  median  line  a  partial 
division  into  halves;  the  medulla  oblongata  and  spinal  cord  are 
subdivided  by  an  anterior  or  ventral  and  a  posterior  or  dorsal  fissure 
into  halves,  a  right  and  a  left. 

The  nerves  in  anatomic  and  physiologic  relation  with  the  central 
organs  of  the  nerve  system  are  the  encephalic  and  the  spinal  nerves. 
The  encephahc  nerves,  twelve  in  number  on  each  side  of  the  median 
line,  are  in  relation  with  the  base  of  the  encephalon,  and  because  of 
the  fact  that  they  pass  through  foramina  in  the  walls  of  the  cranium 
they  are  usually  termed  cranial  nerves.  The  spinal  nerves,  thirty-one 
in  number  on  each  side,  are  in  relation  with  the  spinal  cord,  and 
because  of  the  fact  that  they  pass  through  foramina  in  the  walls  of 
the  spinal  column  they  are  termed  spinal  nerves.  As  both  cranial 
and  spinal  nerves  are  ultimately  distributed  to  the  structures  of  the 
body, — i.  e.,  the  general  periphery, — they  collectively  constitute  the 
peripheral  organs  of  the  nerve  system. 

456 


THE  ENCEPHALO-SPINAL  MEMBRANES. 


457 


The  central  organs  of  the  nerve  system  are  supported  and  protected 
by  three  membranes  named,  in  their  order  from  without  inward,  the 
dura  mater,  the  arachnoid,  and  the  pia  mater. 

The  dura  mater  is  a  tough  membrane 
composed  of  fibrous  tissue.  It  consists  of 
two  layers,  the  outer  of  which  lines  the 
cranial  cavity  and  forms  an  internal  peri- 
osteum; the  inner  layer  is  closely  attached 
to  the  outer  except  at  certain  regions  where 
it  separates  and  forms  supporting  structures, 
such  as  the  falx  cerebri,  falx  cerebelli, 
tentorium  cerebelH,  etc.;  at  the  margin  of 
the  foramen  magnum  the  outer  layer  be- 
comes continuous  with  the  periosteal  tissue, 
while  the  inner  layer  invests  the  cord  down 
to  its  ultimate  termination.     (Fig.  202.) 

The  arachnoid  is  a  delicate  serous 
membrane.  The  external  surface  is  smooth 
and  well  defined  and  separated  from  the 
dura  by  a  narrow  space,  the  subdural  space. 
The  inner  surface  sends  inward  fine  con- 
nective-tissue processes  which  interlace  in 
every  direction,  constituting  the  subarach- 
noid tissue.  This  tissue  is  abundant  in  the 
cranium,  much  less  so  in  the  spinal  canal. 
The  spaces  between  the  connective  tissue, 
taken  collectively,  constitute  the  general  sub- 
arachnoid space.  Around  the  spinal  cord 
this  space  is  well  defined,  and  at  the  base  of 
the  encephalon  expands  to  form  large  cavi- 
ties known  as  the  cisterna  magna,  cisterna 
pontis,  etc. 

The  pia  mater  is  a  delicate  membrane 
composed  of  areolar  tissue.  It  closely 
invests  the  encephalon  and  spinal  cord, 
dipping  into  the  various  fissures.  It  is 
exceedingly  vascular  and  sends  small  blood- 
vessels for  some  distance  into  the  brain  and 
spinal  cord. 

The  Encephalo-spinal  Fluid.  —  The 
general  subarachnoid  space,  as  well  as  cer- 
tain cavities  within  the  encephalon,  contain 
a  clear  transparent  fluid,  termed  the  en- 
cephalo-spinal. This  fluid  has  an  alkaline  reaction  and  a  specific 
gravity  of  1.007  o^  1.008.     It  is  composed  of  water,  proteids  (pro- 


FlG, 


,  201. — The  Central 
Organs  of  the  Nerve 
System,  f.  t.  o.  Fron- 
tal, temporal,  and  oc- 
cipital lobes  of  the 
cerebrum,  c.  Cerebel- 
lum, p.  Pons.  mo. 
Medulla  oblongata. 
ms.,  ms.  The  upper 
and  lower  limits  of  the 
spinal  cord.  The  re- 
maining letters  indicate 
the  region  and  number 
of  the  spinal  nerves. — 
{Quain,  after  Bourgery.) 


458 


TEXT-BOOK  OF  PHYSIOLOGY. 


teoses  and  serum-globulin),  and  a  compound  pyrocatechin,  capable 
of  reducing  copper  salts,  though  not  exhibiting  any  other  of  the 
properties  of  sugar.  In  many  respects  this  fluid  resembles  lymph. 
The  subarachnoid  space  and  the  general  encephalic  cavities,  termed 
ventricles,  communicate  with  one  another  by  an  opening  in  the  pia 
mater  (the  foramen  of  Magendie)  as  it  passes  over  the  lower  part 
ofjthe  fourth  ventricle. 

The  Functions  of  the  Nerve  System. — The  functions  of  the 
nerve  system  are  twofold:    (i)    It  unites  and  coordinates  the  organs 

and  tissues  of  the  body  in  such  a  man- 
ner that  they  are  enabled  to  cooperate 
for  the  accomplishment  of  a  definite 
object.  (2)  It  serves  to  arouse  in  the 
individual  a  consciousness  of  the  ex- 
istence of  an  external  world,  by  virtue 
of  the  impressions  which  it  makes  on 
his  sense  organs,  and  to  enable  him 
therefore  to  adjust  himself  to  his  en- 
vironment. 

By  virtue  of  the  coordination,  a 
stimulus,  if  of  sufficient  intensity,  ap- 
plied to  one  organ  or  tissue  will  call 
forth  activity  in  one  or  more  organs 
near  or  remote  from  the  part  stimula- 
ted .  This  coordination  is  accomphshed 
mainly  by  the  spinal  cord  and  the  me- 
dulla oblongata.  All  such  actions  tak- 
ing place  independently  of  voHtion 
are  termed  reflex  actions.  The  reflex 
activities  connected  with  digestion, 
the  circulation  of  the  blood,  with  res- 
piration, excretion,  etc.,  are  illustra- 
tions of  the  coordinating  capabilities 
of  the  nerve-centers  located  in  these 
portions  of  the  central  nerve  system. 

Consciousness  of  the  existence  of 
the  external  world  and  of  the  relation 
existing  betw^een  it  and  the  individual 
is  associated  with  the  physiologic  activities  of  the  encephalon,  and 
more  particularly  of  the  cerebral  hemispheres.  This  portion  of  the 
nerve  system  is  the  chief,  though  perhaps  not  the  sole,  organ  of  the 
mind,  and  its  main  functions  are  for  the  most  part  mental. 

The  function  of  a  part  at  least  of  the  peripheral  nerve  system  is  to 
afford  a  means  of  communication  between  the  central  nerve  system 
and  the  remaining  structures  of  the  body.  The  nerve-trunks  con- 
stituting this  part  may  be  divided  into  two  groups,  as  follows: 


Fig.  202. — The  Membranes  of 
THE  Spinal  Cord.  i.  Dura 
mater.  2.  Arachnoid.  3. 
Posterior  root  of  spinal  nerve. 
4.  Anterior  root  of  spinal 
nerve.  5.  Ligamentum  den- 
tatum.  6.  Linea  splendens. — 
{Morris,  after  Ellis.) 


THE  SPINAL  CORD.  459 

1.  The  first  group  comprises  nerves  in  connection  with  the  special 

sense-organs,  e.  g.,  eye,  ear,  nose,  tongue,  skin,  as  well  as  nerves 
in  connection  with  the  general  or  organic  sense-organs,  e.  g.,  mu- 
cous membranes,  viscera,  etc.,  which  transmit  nerve  impulses 
to  certain  localized  areas  in  the  cerebral  cortex,  where  they  are 
translated  into  conscious  sensations.  These  sensations,  both 
special  and  general,  by  their  grouping  and  combinations  are  the 
primary  elements  of  intelHgence. 

2.  The  second  group  comprises  those  nerves  which  terminate  in  the 

muscle  apparatus  and  which  transmit  nerve  impulses,  by  way  of 

the  medulla  and  spinal  cord,  from  locahzed  areas  in  the  cerebral 

cortex  to  the  muscles  of  the  face,  trunk  and  extremities,  which  are 

in  consequence  excited  to  activity.     The  muscle  movements  thus 

become  physical  expressions  of  mental  states,  and  if  directed  in  a 

definite  manner  to  the  overcoming  of  the  resistances  offered  by 

the  external  world  become  capable  of  modifying  it  in  accordance 

with  the  mental  states. 

The  first  group  of  nerves,  the  afferent,  especially  those  connected 

with  the  special  sense-organs,  are  excited  to  activity  by  impressions 

made  on  their  peripheral  terminations  by  agencies  in  the  external 

world,  and  thus  become  a  means  of  communication  between  the 

physical  and  the  mental  worlds. 

The  second  group  of  nerves,  the  efferent,  are  excited  to  activity 
by  those  molecular  disturbances  in  their  related  nerve-cells  which 
accompany  vohtional  efforts,  and  thus  they  become  a  means  of  com- 
munication between  the  mental  and  the  physical  worlds. 

The  central  nerve  system  is  thus  composed  of  a  number  of 
separate  though  closely  related  parts,  to  each  of  which  a  separate 
function  has  been  assigned.  In  the  study  of  the  structure  and  func- 
tion of  these  separate  parts  it  will  be  found  convenient,  and  con- 
ducive to  clearness,  to  consider  them  in  the  order  of  their  complexity, 
beginning  with  the  spinal  cord  and  ending  with  the  cerebrum. 

THE  SPINAL  CORD. 

The  spinal  cord  is  the  narrow  elongated  portion  of  the  central 
nerve  system  contained  within  the  spinal  canal.  It  is  cyhndric  in 
shape  though  presenting  an  enlargement  in  both  the  lower  cervical 
and  lower  lumbar  regions  corresponding  to  the  origins  of  the  nerves 
distributed  to  the  upper  and  lower  extremities.  The  cord  varies  in 
length  from  40  to  45  cm.,  measures  1 2  mm.  in  diameter,  weighs  42  gms, 
and  extends  from  the  atlas  to  the  second  lumbar  vertebra,  beyond 
which  it  is  continued  as  a  narrow  thread,  the  filum  terminale.  (Fig. 
203.)  It  is  divided  by  the  anterior  and  posterior  longitudinal  fissures 
into  halves,  and  is  therefore  bilaterally  symmetric.  A  transverse  sec- 
tion of  the  cord  shows  that  it  is  composed  of  both  white  and  gray  mat- 
ter, the  former  covering  the  surface,  the  latter  occupying  the  center. 


460 


TEXT-BOOK  OF  PHYSIOLOGY. 


Structure  of  the  Gray  Matter. — The  gray  matter  is  arranged 
in^the  form  of  two  crescents,  united  in  the  median  hne  by  a  trans- 
verse band  or  commissure  forming  a  figure  resembhng  the  letter  H. 
Though  varying  in  shape  in  different  regions  of  the  cord,  the  gray 
matter  in  all  situations  presents  on  either  side  an  anterior  or  ventral 


Superior  or  Cervical  Segment 
of  Spinal  Cord. 


Middle  or  Dorsal  Portion 
of  Cord. 


Inferior  Portion  of  Cord  and 
Cauda  Equina. 


Fig.  203. — Superior,  Middle,  and  Inferior  Portions  of  Spinal  Cord. — 
I.  Floor  of  fourth  ventricle.  2.  Superior  cerebellar  peduncle.  3.  Middle  cerebellar 
peduncle.  4.  Inferior  cerebellar  peduncle.  5.  Enlargement  at  upper  extremity 
of  postero-median  column.  6.  Glosso-pharyngeal  nerve.  7.  Vagus.  8.  Spinal 
accessory,  g,  9,  9,  q  Ligamentum  denticulatum.  10,  10,  10,  10.  Posterior  roots 
of  spinal  nerves.  11,  11,  11,  11.  Postero-lateral  fissure.  12,  12,  12,  12.  Ganglia 
of  posterior  roots.  13,  13.  Anterior  roots.  14.  Division  of  united  roots  into 
anterior  and  posterior  nerves.  15.  Terminal  extremity  of  cord.  16,  16.  Filum 
terminale.  17,  17.  Cauda  equina.  I,  VIII.  Cervical  nerves.  I,  XII.  Dorsal 
nerves.     I,   V.  Lumbar  nerves.     I,  V.  Sacral  nerves. — {Sappey.) 


and  a  posterior  or  dorsal  horn.  Between  the  two  horns  there  is  a 
portion  termed  the  intermediate  gray  substance.  The  commissure 
presents  in  its  center  a  narrow  canal  which  extends  throughout  the 
entire  length  of  the  cord.  This  canal  is  lined  by  cylindric  epithelium 
and  surrounded  by  gelatinous  material.     (Fig.  204.) 


THE  SPINAL  CORD. 


461 


The  anterior  horn  is  short  and  broad  and  entirely  surrounded 
by  white  matter.  The  posterior  horn  is  narrow  and  elongated  and 
extends  quite  up  to  the  surface  of  the 
cord,  where  it  is  capped  by  gelatinous 
matter,  the  substantia  gelatinosa.  in 
the  lower  cervical  and  thoracic  regions 
a  portion  of  the  intermediate  gray 
substance  projects  outward  and  forms 
the  so-called  lateral  horn.  The  gray 
matter  fundamentally  consists  of  a 
framework  of  fine  neuroglia  supporting 
blood-vessels,  lymphatics,  medullated 
and  non-medullated  nerves,  and  groups 
of  nerve-cells. 

The  Nerve-cells. — The  nerve-cells 
of  the  cord  are  very  numerous  and 
present  a  variety  of  shapes  and  sizes  in 
different  regions.  They  are  usually  ar- 
ranged in  groups  which  extend  for  some 
distance  up  and  down  the  cord,  forming 
columns  more  or  less  continuous. 

In  the  anterior  horn  two  well- 
marked  groups  are  found,  one  situated 
at  the  anterior  and  inner  angle,  known 
as  the  antero-median  group,  the  other 
situated  at  the  posterior  and  lateral 
angle  and  known  as  the  postero-lateral 
group.  In  the  lower  cervical  and  upper 
thoracic  regions,  in  the  region  of  the 
lateral  horn,  another  group  of  cells 
is  found,  known  as  the  intermediate 
group.  In  the  central  portion  of  the 
horn  there  is  also  a  central  group. 

The  cells  of  the  anterior  horns  are 
of  large  size,  nucleated  and  multipolar. 
They  are  the  modified  descendants 
of  pear-shaped  cells,  the  neuroblasts, 
which  migrated  from  the  medullary 
tube  (see  page  114).  In  the  course  of 
their  migration  they  developed  den- 
drites which  form  an  intricate  felt- 
work  throughout  the  anterior  horn. 
One    of     the    processes,    the    axon, 

approached  the  surface  of  the  cord,  penetrated  it,  grew  outward, 
became  covered  with  myelin  and  neurilemma,  and  developed   into 


D 


Fig.     204 


Sections  Through 
Different  Regions  of  the 
Spinal  Cord.  A.  At  the 
level  of  the  sixth  cervical 
nerve.  B.  At  the  mid-dorsal 
region.  C.  At  the  center  of 
the  lumbar  enlargement.  D. 
At  the  upper  part  of  the 
conus  medullaris.  i.  Poste- 
rior roots.  2.  Anterior  roots. 
3.  Posterior  fissure.  4.  Ante- 
rior fissure.  5.  Central  canal. 
— {Morris'  "Anatomy,''  after 
Schwalbe.) 


462  TEXT-BOOK  OF  PHYSIOLOGY. 

an  anterior  root-fiber.  These  nerve-cells,  with  their  dendrites, 
axons,  and  terminal  branches,  form  efferent  neurons  of  the  first 
order.  The  intimate  histologic  and  physiologic  relationship  existing 
between  the  nerve-cell  and  the  axon  is  revealed  by  the  degenerative 
changes  which  arise  in  the  latter  when  separated  from  the  former. 
The  cell  apparently  determines  the  nutrition  of  the  axon  and  may 
be  regarded  as  trophic  in  function.  Some  of  the  cells  of  the  ante- 
rior horn  send  their  axons  into  the  white  matter  of  the  same  side. 


Fig.  205.— Diagram  Illustrating  the  Chief  Cellular  Elements  of  the  Spinal 
Cord,  and  the  Probable  Relations  between  the  Cells  and  the  Fibers 
and  the  Principal  Tracts;  the  Left  Half  of  the  Figure  Exhibits  the 
Communications  OF  the  Several  Varieties  of  Nerve-cells.  A,  P.  Ventral 
or  anterior  and  dorsal  or  posterior  horns.  PR.  Posterior  root  bundles.  DP. 
Direct  pyramidal  tract.  CP.  Crossed  pyramidal  tract.  DC.  Direct  cerebellar 
tract.  GB.  Gowers's  tract,  a.  Alotor  cells  passing  directly  into  fibers  of  ventral 
roots,  b.  Various  cells  of  the  antero-lateral  column.  Some  give  off  collateral 
branches  of  remarkable  size.  c.  Commissural  (heteromeral)  cells,  d.  Cells  to 
dorsal  column  (tautomeral).  e.  Golgi  cells  of  dorsal  horn.  The  right  half  of  the 
diagram  shows  the  communications  established  by  means  of  the  collateral  fibers. 
— {Piersol,  after  Lenhossek.) 

after  which  they  divide  into  two  branches,  one  passing  up,  the  other 
down,  the  cord,  to  re-enter  the  gray  matter  at  different  levels.  They 
are  probably  associative  in  function.  Other  cells  send  their  axons 
into  that  portion  of  the  white  matter  on  the  same  and  opposite  sides 
known  as  Gowers's  antero-lateral  tract.     (Fig.  205.) 

In  the  posterior  horn  nerve-cells  are  also  present,  though  they 
are  not  so  numerous  as  in  the  anterior  horn.     At  the  base  of  the  horn 


THE  SPINAL  CORD.  463 

and  on  its  inner  side  there  is  a  well-marked  group  of  cells  which  ex- 
tends from  the  seventh  or  eighth  cervical  nerves  downward  to  the 
second  or  third  lumbar  nerves,  being  most  prominent  in  the  thoracic 
region.  This  column  is  known  as  Clarke's  vesicular  column.  From  the 
nerve-cells  constituting  this  column  axons  pass  obhquely  outward  into 
that  portion  of  the  white  matter  known  as  the  direct  cerebellar  tract. 

Classification  of  Nerve-cells. — The  cells  of  the  gray  matter  may 
be  divided  into  three  main  groups:  viz.,  intrinsic,  efferent,  and  afferent. 

The  intrinsic  cells  are  associative  in  function.  The  axons  to  which 
these  cells  give  origin  pass  more  or  less  horizontally  into  the  white 
matter,  where  they  divide  into  two  branches,  one  of  which  passes 
upward,  the  other  downward.  At  various  levels  they  reenter  the 
gray  matter  and  arborize  around  other  intrinsic  cells. 

The  efferent  cells,  independently  of  their  trophic  influence,  are 
also  motor  in  function,  inasmuch  as  the  excitation  arising  in  them 
is  transmitted  outwardly  through  their  axons  to  muscles,  blood- 
vessels, glands  and  viscera,  imparting  to  them  motion,  either  molar  or 
molecular.  As  the  efferent  fibers  in  the  ventral  roots  of  the  spinal 
nerves  are  classified  (see  page  116)  in  accordance  with  their  physio- 
logic action  into  motor,  vaso-motor,  secretor,  inhibitor  and  accelerator 
nerves,  so  the  nerve-cells  of  which  the  nerves  are  integral  parts  may 
be  classified  physiologically  as  motor,  vaso-motor,  secretor,  inhibitor, 
and  accelerator.  Collections  or  groups  of  such  cells  are  termed 
"centers." 

The  afferent  cells  are  largely  sentient  or  receptive  in  function, 
inasmuch  as  the  excitations  brought  to  the  spinal  cord  by  the  afferent 
nerves  in  the  dorsal  roots  from  the  general  periphery  are  received 
by  them  and  transmitted  through  their  axons  toward  the  cortex  of  the 
cerebrum,  where  they  are  translated  into  conscious  sensations.  As 
the  nerve-fibers  in  the  dorsal  roots  of  the  spinal  nerves  are  classified, 
in  accordance  with  the  sensations  to  which  they  give  rise,  as  sensor, 
thermal,  tactile,  etc.,  so  these  nerve-cells  may  be  similarly  classified 
according  as  they  transmit  their  excitations  to  those  specialized  areas 
in  the  cerebral  cortex  in  which  these  different  sensations  arise. 

Structure  of  the  White  Matter. — A  transverse  section  of  the 
cord  shows  that  the  white  matter  completely  covers  the  gray  matter 
except  where  the  posterior  horns  reach  the  surface.  Anteriorly  the 
white  matter  of  each  lateral  half  is  connected  by  a  narrow  strip  or 
bridge  of  white  matter,  the  anterior  commissure.  Microscopic 
examination  shows  that  the  white  matter  is  composed  of  vertically 
disposed  medullated  nerve-fibers  which  are  devoid  of  a  neurilemma. 
These  fibers  are  supported  partly  by  a  framework  of  connective 
tissue,  and  partly  by  neuroglia.  The  white  matter  of  each  side  of 
the  cord  is  anatomically  divided  into  an  anterior,  a  lateral,  and  a 
posterior  column  by  the  anterior  and  posterior  roots  of  the  spinal 
nerves. 


464 


TEXT-BOOK  OF  PHYSIOLOGY. 


Classification    of    the   Nerve-fibers. — From    a    study    of   the 

embryologic  development  of  the  white  matter  and  of  the  degenerative 
changes  which  follow  its  pathologic  and  experimental  destruction,  it 
has  been  differentiated  into  a  number  of  specialized  tracts  which  have 
different  origins,  destinations,  and  functions.  They  may  be  divided, 
however,  into  efferent,  afferent,  and  associative  fibers.     (Fig.  206.) 

I.  The  anterior  column,  comprising  that  portion  between  the 
anterior  longitudinal  fissure  and  the  anterior  roots,  has  been  sub- 
divided into: 

(a)  The  direct  pyramidal  tract,  or  column  of  Tiirck.  This  tract 
borders  the  longitudinal  fissure  and  extends  from  the  upper  extremity 


Cnhimn  of  Lissauer. 


Fig.  206. — Transection  of  the  Cervical  Spinal  Cord  Showing  Its  Chief  Sub- 
divisions.— {From  Mills'  ^^  Diseases  of  the  Nervous  System.") 


of  the  cord  as  far  down  as  the  mid-thoracic  region.  From  above 
downward  this  tract  diminishes  in  size,  for  the  reason  that  its  fibers 
or  their  collaterals  cross  at  successive  levels  to  the  opposite  side  of  the 
cord  by  way  of  the  anterior  commissure  to  enter  the  gray  matter  of 
the  anterior  horn.  The  cells  of  these  axons  are  located  in  the  cortex  of 
the  cerebral  hemisphere  of  the  same  side.  The  terminal  filaments  of 
these  fibers  or  axons  are  in  physiologic  relation  with  the  dendrites  of 
the  cornual  cells.  When  divided  in  any  part  of  their  course,  these 
fibers  undergo  descending  degeneraion.  They  are  therefore  efferent 
neurons  and  of  the  second  order. 

(h)  The  anterior  root  zone.    This  tract  lies  external  to  the  pyram- 


THE  SPINAL  CORD  465 

idal  tract,  surrounds  the  anterior  horn  of  the  gray  matter  and  ex- 
tends throughout  the  length  of  the  cord.  It  is  composed  of  short  com- 
missural fibers  which  come  from  nerve-cells  in  the  gray  matter  from 
the  same  and  opposite  sides  of  the  cord.  After  entering  the  white 
matter  they  divide  into  two  branches,  pursue  opposite  directions, 
then  re-enter  the  gray  matter  at  higher  and  lower  levels  and  come 
into  relation  with  other  nerve-cells. 

(c)  The  antero -lateral  tract  of  Marchi  and  Lowenthal.  This  tract 
is  situated  at  the  inner  and  anterior  angle  of  the  anterior  column.  After 
removal  of  the  one-half  of  the  cerebellum  it  degenerates  downward. 

2.  The  lateral  column,  comprising  that  portion  between  the 
ventral  and  dorsal  roots,  has  been  divided  into: 

{a)  The  antero-lateral  tract  of  Gowers.  This  tract  is  somewhat 
crescentic  in  shape  and  situated  on  the  lateral  aspect  of  the  cord 
external  to  the  anterior  root  zone.  It  extends  throughout  the  entire 
length  of  the  cord.  When  divided  it  undergoes  ascending  degenera- 
tion, which  would  indicate  that  the  axons  originate  in  nerve-cells 
in  the  gray  matter.     This  tract  is  therefore  afferent  in  function. 

{h)  The  lateral  limiting  tract.  This  tract,  which  is  quite  narrow, 
lies  close  to  the  external  border  of  the  gray  matter.  It  is  composed 
of  fibers  which  do  not  degenerate  to  any  considerable  extent  and  are 
in  all  probability  associative  fibers  which  come  from  nerve-cells  in 
the  gray  matter  to  re-enter  at  lower  and  higher  levels. 

(c)  The  crossed  pyramidal  tract.  This  tract  occupies  the  posterior 
portion  of  the  lateral  column,  though  its  exact  position  varies  some- 
what in  different  regions  of  the  cord.  In  the  cervical  and  thoracic 
regions  it  is  covered  by  a  layer  of  fibers.  In  the  lumbar  region, 
however,  it  comes  to  the  surface.  From  above  downward  this  tract 
gradually  diminishes  in  size,  for  the  reason  that  its  fibers  and  their 
collaterals  enter  the  gray  matter  at  successive  levels.  The  terminal 
branches  of  these  fibers  are  in  close  physiologic  relation  with  the 
dendrites  of  the  cornual  cells.  The  cells  of  these  axons  are  located 
in  the  cortex  of  the  cerebral  hemispheres  of  the  opposite  side.  When 
divided  in  any  part  of  their  course,  they  undergo  descending  de- 
generation. They  are  therefore  efferent  neurons  and  of  the  second 
order. 

{d)  The  direct  cerebellar  tract,  or  column  of  Flechsig.  This  tract 
is  situated  on  the  surface  of  the  lateral  column  external  to  the  crossed 
pyramidal  tract.  It  slightly  increases  in  size  from  below  upward. 
It  is  composed  of  fibers  the  cells  of  which  are  found  on  the  inner 
side  and  base  of  the  posterior  horn  (Clark's  vesicular  column).  From 
this  origin  the  fibers  pass  obliquely  outward  to  the  surface  and  then 
directly  upward  to  terminate,  as  its  name  implies,  in  the  cerebellum. 
Decussation  of  these  fibers  takes  place  in  the  superior  vermiform  lobe 
of  the  cerebellum.  When  divided  this  tract  degenerates  upward.  It 
30 


466  TEXT-BOOK  OF  PHYSIOLOGY. 

is  therefore  in  all  probability  an  afferent  tract  and  of  the  second  order. 

3.  The  posterior  column,  comprising  that  portion  between  the 
dorsal  roots  and  the  posterior  longitudinal  fissure,  has  been  sub- 
divided into : 

(a)  The  postero-external  tract  of  Burdach.  This  tract  hes  just 
within  the  posterior  horns.  A  portion  of  this  tract  is  composed 
of  ground  fibers  which,  though  vertically  disposed,  have  but  a  short 
course.  They  take  their  origin  in  cells  in  the  gray  matter,  and  after 
entering  this  tract  divide  into  ascending  and  descending  branches, 
which  with  their  collaterals  re-enter  the  gray  matter  at  different 
levels.  Another  portion  of  this  tract  is  made  up  of  nerve-fibers  de- 
rived from  the  dorsal  roots  of  the  spinal  nerves,  which  cross  this 
column  toward  the  median  line  in  an  obhque  or  horizontal  direction. 
The  fibers  of  the  upper  portion  of  this  tract  terminate  around  the 
nucleus  cuneatus  at  the  medulla  oblongata.  When  divided,  these 
fibers  degenerate  for  but  a  short  distance.  The  ground  fibers  are 
probably  associative  in  function. 

(6)  The  postero-median  internal  tract,  or  column  of  Goll.  This 
tract  is  separated  from  the  former  by  a  septum  of  connective  tissue 
which  is  most  marked  above  the  eleventh  thoracic  segment.  The 
fibers  which  compose  this  tract  are  long  and  derived  for  the  most 
part  from  the  dorsal  roots  of  the  spinal  nerves  of  the  same  side.  This 
is  shown  by  the  fact  that  division  of  these  roots  central  to  the  ganglion 
is  followed  by  ascending  degeneration  of  the  column  of  Goll  as  far 
as  the  nucleus  gracilis  in  the  medulla.  Fibers  derived  from  cells 
in  the  gray  matter  are  also  contained  in  this  column.  This  tract  is 
afferent  in  function. 

(c)  The  septo -marginal  tract.  This  is  an  oval-shaped  tract 
situated  along  the  margin  of  the  posterior  longitudinal  fissure. 

{d)  The  cornu-commissural  tract.  This  is  formed  along  the 
border  of  the  anterior  portion  of  the  posterior  column  as  far  forward 
as  the  posterior  commissure.  Both  of  these  tracts  are  best  developed 
in  the  lumbo-sacral  region.  They  arise  from  nerve-cells  in  the  gray 
matter.  They  undergo  descending  degeneration  when  divided,  but 
not  after  division  of  the  dorsal  roots. 

(e)  Lissauer^s  tract.  This  tract  embraces  the  tip  of  the  posterior 
horn  and  is  composed  principally  of  fibers  from  the  dorsal  roots  of  the 
spinal  nerves.  After  entering  the  tract  the  fibers  divide  into  ascend- 
ing and  descending  branches,  which  finally  terminate  around  cells 
in  the  posterior  horn. 

(/)  The  comma  tract.  This  is  a  narrow  tract  of  fibers  situated 
in  the  anterior  portion  of  the  column  of  Burdach.  When  divided, 
its  fibers  degenerate  downward. 

The  Relation  of  the  Spinal  Nerves  to  the  Spinal  Cord. — The 
spinal  nerves  present  near  the  spinal  cord  two  divisions  which  from 


THE  SPINAL  CORD.  467 

their  connection  with  the  anterior  or  ventral  and  the  posterior  or 
dorsal  surfaces  are  known  as  the  ventral  and  dorsal  roots.  The  ventral 
roots  are  the  axons  of  various  groups  of  cells  in  the  anterior  horns. 
From  their  origin  these  axons  pass  almost  horizontally  forward 
through  the  anterior  column  in  three  distinct  bundles.  After  emerging 
from  the  cord  they  curve  downward  and  backward  to  join  the  poste- 
rior root.  The  dorsal  roots  are  the  central  axons  of  nerve-cells  in  the 
spinal  gangha.  After  entering  the  cord  they  divide  into  two  main 
groups,  a  lateral  and  a  mesial.  A  portion  of  the  lateral  group  enters 
the  posterior  horn  directly  through  the  caput  cornu;  the  other  portion 
turns  upward  and  runs  through  Lissauer's  tract  and  ultimately  enters 
the  posterior  horn.  The  mesial  group  passes  into  the  postero- 
external column  (Burdach),  where  the  fibers  divide  into  descending 
and  ascending  branches.  The  former  constitute  the  comma  tract, 
the  terminal  branches  of  which  surround  cells  in  the  gray  matter; 
the  latter  (ascending)  cross  the  column  obhquely  and  enter  the 
postero-internal  column  (Goll),  in  which  they  pass  upward  to  ter- 
minate around  the  cells  of  the  nucleus  gracilis  of  the  same  side.  As 
these  root  fibers  pass  up  and  down  the  cord,  collateral  branches  are 
given  off  which  enter  the  gray  matter  at  successive  levels  and  come 
into  physiologic  relation  with  the  cells  of  Clark's  vesicular  column 
on  the  same  and  opposite  sides  and  with  the  cells  of  the  anterior  horn. 

Experimentally,  it  has  been  determined  that  the  anterior  or  ventral 
roots  contain  all  the  efferent  fibers,  the  posterior  or  dorsal  roots  all  the 
afferent  fibers.     The  proofs  in  support  of  this  view  are  as  follows: 

Stimulation  of  the  ventral  roots  produces : 

1.  Convulsive  movements  of  muscles. 

2.  The  discharge  of  a  secretion  from  glands. 

3.  Changes  in  the  caliber  of  blood-vessels. 

4.  Inhibition  of  the  rhythmic  activity  of  certain  organs. 

Division  of  these  roots  is  followed  by: 

1.  Loss  of  muscle  movement  (paralysis  of  motion). 

2.  Cessation  of  normal  secretion. 

3.  Cessation  of  active  vascular  changes. 

Stimulation  of  the  dorsal  roots  causes : 

1.  Reflex  activities. 

2.  Conscious  sensations. 

3.  Inhibition  of  the  rhythmic  activity  of  certain  organs. 

Division  of  these  roots  is  followed  by: 

1.  Loss  of  reflex  activities,  and 

2.  Loss  of  sensation  in  all  parts  to  which  they  are  distributed. 

The  ventral  roots  are,  therefore,  efferent  in  function,  transmitting 
nerve  impulses  from  the  spinal  cord  to  the  periphery.  The  dorsal 
roots  are  afferent  in  function,  transmitting  nerve  impulses  from  the 
general  periphery  to  the  spinal  cord. 


468  TEXT-BOOK  OF  PHYSIOLOGY. 

The  classification  of  the  nerve-fibers  in  the  ventral  and  dorsal 
roots  of  the  spinal  nerves  in  accordance  with  the  functions  they  sub- 
serve will  be  found  on  page  ii6. 

Though  both  the  efferent  and  afferent  fibers  of  the  spinal  nerves 
are  directly  connected  with  nerve-cells  in  the  spinal  cord,  they  are 
also  indirectly  connected  by  efferent  and  afferent  nerve-tracts  with 
the  cerebral  cortex. 

FUNCTIONS  OF  THE  SPINAL  CORD. 

Physiologic  investigation  has  demonstrated  that  the  spinal  cord, 
by  virtue  of  the  presence  of  nerve-cells  and  nerve-fibers,  may  be  re- 
garded as  composed  of: 

1.  Independent  nerve-centers,  each  of  which  has  a  special  function; 

and — 

2.  Conducting  paths  by  which  these  centers  are  brought  into  relation 

with  one  another  and  with  the  cerebrum  and  its  subordinate 
or  underlying  parts. 

The  cord,  moreover,  may  be  considered  as  consisting  physiologically 
of  a  series  of  segments  placed  one  above  the  other,  the  number  of 
segments  corresponding  to  the  number  of  spinal  nerves.  In  other 
words,  a  spinal  segment  comprises  that  portion  of  the  cord  to  which 
is  attached  a  pair  of  spinal  nerves.  The  nerve-cells  in  each  segment 
are  in  histologic  and  physiologic  relation  with  definite  areas  of  the 
body,  embracing  muscles,  blood-vessels,  glands,  skin,  etc. 

The  Spinal  Cord  as  an  Independent  Center. — The  efferent 
cells  of  the  spinal  segments  are  the  immediate  sources  of  the  nerve 
energy  which  excites  activity  in  muscles,  blood-vessels,  glands.  The 
discharge  of  their  energy  may  be  caused: 

1.  By  variations  in  the  composition  of  the  blood  or  lymph  by  which 

they  are  surrounded.  The  activity  of  the  cell  thus  occasioned 
is  termed  automatic  or  autochthonic  (Gad). 

2.  By  the  arrival  of  nerve  energy  coming  through  afferent  nerves 

from  the  general  sentient  periphery,  skin,  mucous  membrane,  etc. 

3.  By  the  arrival  of  nerve  energy  descending  the  spinal  cord  from 

the  cerebrum  or  subordinate  structures.  The  peripheral  activity 
in  the  former  instance  is  said  to  be  reflex  or  peripheral  in  origin ; 
in  the  latter  instance,  direct  or  cerebral  in  origin.  In  this  latter 
instance,  also,  the  muscle  movements  are  due  to  volitional,  the 
vascular  variations  and  glandular  discharges  to  emotional, 
forms  of  cerebral  activity. 
Each  segment  of  the  spinal  cord  may  be  regarded,  therefore, 
because  of  its  contained  nerve-cells : 

1.  As  a  center  for  automatic  activity. 

2.  As  a  center  for  the  reception  of  excitations  arising  either  at  the 

periphery  or  in  the  cerebrum,  and  for  their  subsequent  trans- 
mission through  efferent  nerves  to  various  peripheral  organs. 


THE  SPINAL  CORD.  469 

Automatism. — The  growth,  the  nutrition  and  multiplication  of 
the  cells  of  various  tissues,  and  their  continuous  and  rhythmic  activity, 
have  been  attributed  to  an  automatic  action  of  the  spinal  nerve- 
cells.  By  this  expression  is  meant  a  discharge  of  energy  from  the 
cells  occasioned  by  a  change  in  their  environment,  i.  e.,  in  the  chemic 
composition  of  the  blood  or  lymph  by  which  they  are  surrounded, 
and  independent  of  any  excitation  coming  through  afferent  nerves. 
If  the  cell  activity  is  continuous,  though  variable  in  degree  from  time 
to  time,  it  gives  rise  to  what  is  termed  tonus,  e.  g.,  trophic  tonus,  vas- 
cular, muscle  tonus,  etc.  If  the  cell  activity  is  intermittent,  it  imparts 
to  muscles  a  certain  rhythmic  activity,  e.  g.,  the  respiratory  movements. 

As  no  eft'ect  arises  without  a  sufficient  cause,  the  term  automatic 
has  been  objected  to  and  the  term  autochthonic  has  been  suggested 
(Gad),  expressive  of  the  idea  that  the  energy  originates  in  the  nerve- 
cell  as  a  result  of  a  reaction  between  the  cell  and  its  ever-changing 
environment.  A  center  so  acting  could  not  be  regarded  as  primarily 
a  center  for  reflex  action,  however  much  it  might  be  influenced  or 
conditioned  secondarily  by  afferent  impulses.  Though  automatic 
activity  of  the  spinal  cord  centers  is  advocated  by  some  physiologists, 
the  fact  must  be  recognized  that  with  increasing  knowledge  of  reflex 
activities  some  of  the  phenomena  hitherto  regarded  as  automatic 
have  been  found  to  be  reflex  in  origin.  Whether  this  will  eventually 
be  found  true  for  all  forms  of  so-called  automatic  or  autochthonic 
activity  remains  to  be  seen. 

Trophic  Tonus. — The  normal  metabohsm  of  muscle,  gland,  and 
connective  tissue  which  underlies  the  assimilation  of  food,  the  storing 
of  energy,  and  the  production  of  new  compounds,  is  dependent,  in  the 
higher  animals  at  least,  on  the  connection  of  these  tissues  with  the 
central  nerve  system;  for  if  the  eft'erent  nerves  be  divided,  not  only 
will  they  undergo  degeneration  in  their  peripheral  portions,  but  the 
muscles,  glands,  and  connective  tissues  to  which  they  are  distributed 
will  also  undergo  similar  changes.  This  is  to  be  attributed  not 
merely  to  inactivity,  but  rather  to  a  loss  of  nerve  influence,  inasmuch 
as  inactivity  leads  merely  to  atrophy  and  not  to  degeneration.  It 
would  appear  from  facts  of  this  character  that  the  normal  metabolism 
is  dependent  for  its  continuance  on  nerve  influences.  There  is  no 
evidence,  however,  as  to  the  existence  of  special  trophic  nerves, 
separate  from  those  which  impart  to  glands  and  muscles  their  cus- 
tomary activities.  The  trophic  centers  and  the  motor  centers  are  iden- 
tical, though  the  two  modes  of  their  activity  are  separate  and  distinct. 

Vascular  Tonus. — The  state  of  moderate  contraction  of  the 
arterioles  throughout  the  body,  in  consequence  of  which  the  average 
arterial  pressure  is  maintained,  is  attributed  to  constant  activity  of 
the  vaso-motor  centers,  this  activity  being  conditioned  by  variations 
in  the  composition  of  blood,  either  an  increase  in  the  quantity  of 
carbon  dioxid  or  a  decrease  in  the  quantity  of  oxygen.     The  vaso- 


470 


TEXT-BOOK  OF  PHYSIOLOGY. 


motor  centers  are  regarded  as  primarily  automatic,  though  capable 
of  being  influenced  secondarily  by  reflected  excitations  from  the 
periphery  or  direct  excitations  from  the  cerebrum. 

Muscle  Tonus. — It  is  wefl  known  that  if  a  muscle  be  divided  in 
the  living  animal  the  two  portions  will  contract  and  separate  them- 
selves to  a  certain  distance.  This  indicates  that  the  muscle  when  in 
a  state  of  rest  is  in  a  sHght  degree  of  contraction.  This  condition  of 
the  muscle,  to  which  the  term  muscle  tonus  is  given,  was  formerly 
attributed  to  an  automatic  and  continuous  discharge  of  energy  from 
the  nerve-cells.  Brondgeest,  however,  showed  that  this  tonus  is 
entirely  reflex  in  origin  and  immediately  disappears  on  division  of 
the  posterior  roots  of  the  spinal  nerves,  which  would  not  be  the  case 
if  the  cells  in  the  cord  were  acting  automatically.  The  afferent  nerves 
in  this  reflex  arise  in  the  muscle  or  its  tendons,  and  the  stimulus  is 
the  slight  degree  of  extension  to  which  the  muscle  is  subjected  in  virtue 
of  its  attachments  and  the  ever- varying  position  of  the  Hmbs  and  trunk. 


Fig  207.— Diagram  of  a  Simple  Reflex  Arc.  i.  Sentient  surface.  2.  Afferent 
nerve.  3.  Emissive  or  motor  cell.  4.  Efferent  nerve.  5.  Muscle. — (After  Moral 
and  Dayoii.) 

The  tonic  contraction  of  the  visceral  muscles, — e.  g.,  the  pyloric, 
the  vesical,  the  anal  sphincters, — though  regarded  as  automatic  by 
some,  is  probably  reflex  in  origin,  dependent  on  the  arrival  of 
afferent  impulses  from  the  periphery.  It  is  probable  that  future  investi- 
gation will  disclose  the  existence  and  pathway  of  these  afferent  fibers. 

Reflex  Actions. — It  has  already  been  stated  that  the  nerve-cells 
in  the  spinal  cord  are  capable  of  receiving  and  transforming  afferent 
nerve  impulses  into  efferent  nerve  impulses,  which  are  transmitted 
outward  to  muscles,  exciting  contraction;  to  glands,  provoking  secre- 
tion; to  blood-vessels,  changing  their  caliber;  and  to  organs,  inhibit- 
ing or  accelerating  their  activity.  All  such  actions  taking  place 
through  the  spinal  cord  and  medulla  oblongata  independently  of 
sensation  or  volition  are  termed  reflex  actions.  The  mechanism  in- 
volved in  every  reflex  action  consists  of  at  least  the  following  struc- 
tures (Fig.  207) : 

I.  A  sentient  surface;    e.  g.,  skin,  mucous  membrane,  sense 
organ,  etc. 


THE  SPINAL  CORD. 


471 


2.  An  afferent  fiber  and  cell. 

3.  An  emissive  cell,  from  which  arises — 

4.  An  efferent  nerve,  distributed  to  a  responsive  organ,  as — 

5.  Muscle,  gland,  blood-vessel,  etc. 

In  this  connection  the  reflex  contractions  of  skeletal  muscles  only 
will  be  considered. 

If  a  stimulus  of  sufficient  intensity  be  applied  to  the  sentient 
surface,  there  will  be  developed  in  the  terminals  of  the  afferent 
nerve  a  series  of  nerve  impulses  which  will  be  transmitted  by  the 
afferent  nerv^e  to,  and  received  by,  the  dendrites  of  the  emissive  cell 
in  the  anterior  horn  of  the  gray  matter.  With  the  reception  of 
these  impulses  there  will  be  a  dis- 
turbance in  the  equilibrium  of  the 
molecules  of  the  cell,  a  liberation  of 
energy  and  a  transmission  of  nerve 
impulses  outward  through  the  efferent 
nerve  to  the  muscle. 

A  reflex  mechanism  or  arc  of  this 
simplicity  would  subserve  but  a  simple 
movement.  The  majority  of  the 
reflexes,  however,  are  extremely  com- 
plex and  involve  the  cooperation  and 
coordination  of  a  number  of  centers  at 
different  levels,  of  the  spinal  cord  and 
medulla,  on  the  same  and  opposite 
sides,  and  of  muscles  situated  at  dis- 
tances more  or  less  remote  from  one 
another.  The  transference  of  nerve 
impulses  coming  from  a  localized  area 
of  a  sentient  surface,  to  emissive  cells 
situated  at  different  levels  is  accom- 
plished by  the  intermediation  of  a  third 
neuron  situated  in  the  gray  matter 
which  is  in  connection,  on  the  one 
hand,  with  the  central  terminals  of 
the  afferent  nerve,  and,  on  the  other 

hand,  with  the  dendrites  of  the  emissive  or  motor  cells  (Fig.  208). 
A  histologic  and  physiologic  mechanism  of  this  character  readily 
explains  how  a  localized  stimulation  can  give  rise  to  reflex  actions 
extremely  complex  in  character. 

The  reflex  contractions  of  skeletal  muscles  are  best  studied  after 
division  of  the  central  nerve  system  at  the  upper  limit  of  the  spinal 
cord.  After  this  procedure  the  spinal  centers  can  act  independently 
of,  and  uninfluenced  by  either  sensation  or  volitional  efforts  on  the 
part  of  the  animal.  Though  it  is  possible  to  provoke  reflex  contrac- 
tions under  such  circumstances  in  w^arm-blooded  animals,  they  are. 


Fig.  208. — Diagram  Showing 
THE  Relation  of  the  Third 
Neuron  a,  to  the  Afferent 
Neuron  h,  and'  to  the  Ef- 
ferent Neurons  c,  c,  c. — 
{Ajter  Kdllikcr.) 


472  TEXT-BOOK  OF  PHYSIOLOGY. 

as  a  rule,  incomplete  and  of  short  duration,  owing  to  disturbances 
of  the  circulation  and  respiration  and  the  consequent  loss  of  tissue 
irritability.  In  frogs  and  in  cold-blooded  animals  generally,  the 
spinal  cord  retains  its  irritability  for  a  long  period  of  time  after  re- 
moval of  the  brain,  and  therefore  is  well  adapted  for  the  study  of 
reflex  actions. 

The  division  of  the  spinal  cord  can  be  readily  effected  by  inserting 
a  spear-shaped  knife  between  the  occipital  bone  and  the  atlas.  The 
skin,  occipito-atlantal  membrane,  and  medulla  can  be  divided  with 
one  plunge  of  the  knife.  The  brain  can  then  be  destroyed  by  the 
insertion  of  a  fine  wire  into  the  brain  cavity.  A  frog  so  prepared, 
and  placed  on  the  table  and  allowed  to  remain  at  rest  for  a  few 
moments  until  the  shock  of  the  operation  passes  away,  will  draw  the 
limbs  close  to  the  body  and  assume  a  position  not  unlike  that  of  a 
normal  frog.  If  then  the  posterior  limbs  be  extended,  they  will 
immediately  be  drawn  close  to  the  side  of  the  trunk  in  the  usual 
flexed  position.  If  the  toes  are  pinched  with  forceps,  the  foot  will 
execute  a  series  of  movements  as  if  it  were  trying  to  free  itself  from 
the  source  of  irritation. 

If  the  frog  be  suspended,  the  hmbs,  through  the  force  of  gravity, 
will  be  gradually  extended  and  hang  down  freely.  In  this,  as  in  the 
sitting  position,  the  animal  will  remain  perfectly  quiet  and  will  not 
exhibit  spontaneous  movements.  Any  stimulus  apphed  to  the  skin, 
however,  provided  it  is  of  sufficient  intensity,  will  be  followed  by  a 
more  or  less  pronounced  movement.  Mechanic,  chemic  and  electric 
stimuli  apphed  to  any  part  of  the  skin  will  call  forth  the  characteristic 
reflex  movements.  Chemic  stimuli  such  as  weak  solutions  of 
sulphuric  or  acetic  acid  placed  on  the  toes  will  be  followed  by  feeble 
flexion  of  the  corresponding  leg,  to  be  succeeded  in  a  short  time  by 
extension.  Stronger  solutions  will  produce  more  extensive  and 
vigorous  movements,  the  foot  at  the  same  time  being  rubbed  against 
the  thigh,  apparently  for  the  purpose  of  freeing  it  from  the  irritant. 
Similar  phenomena  follow  the  apphcation  of  the  acid  to  the  fingers 
or  the  trunk.  As  a  rule,  the  extent  and  complexity  of  the  movement 
is  within  limits  proportional  to  the  strength  of  the  stimulus.  By 
hmiting  the  sphere  of  action  of  the  stimulus  to  definite  but  different 
areas  of  the  skin  a  great  variety  of  movements,  more  or  less  complex 
and  coordinated  and  apparently  purposive  and  defensive  in  character, 
can  be  produced.  The  coordinated  and  purposive  character  of  the 
movements  exhibited  by  a  brainless  frog  led  Pfliiger  to  the  assump- 
tion that  the  spinal  cord  in  this  as  well  as  in  other  cold-blooded  ani- 
mals is  possessed  of  sensorial  functions,  is  endowed  with  rudimentary 
consciousness.  This  view,  however,  is  not  generally  accepted,  the 
movement  being  attributed  to  specialized  mechanisms  in  the  cord, 
partially  inherited,  which  permit  of  one  and  the  same  movement  with 


THE  SPINAL  CORD.  473 

mechanic  regularity  and  precision,  so  long  as  the  conditions  of  the 
experiment  remain  the  same. 

In  warm-blooded  animals  similar  results  may  be  obtained  for  a 
short  time  after  division  of  the  cord,  especially  if  artificial  respiration 
is  maintained  and  the  circulation  of  the  blood  continued.  The  cord 
will  then  retain  its  irritability  for  some  time.  If  the  conditions  of 
experimentation  were  favorable,  it  is  highly  probable  that  the  human 
spinal  cord  would  execute  similar  movements.  Thus  it  was  observed 
by  Robin  in  a  man  who  had  been  decapitated  that  reflex  muscle  con- 
tractions could  be  ehcited  by  stimulating  the  skin  after  the  lapse  of  an 
hour  after  execution.  "While  the  right  arm  was  lying  extended  by 
the  side,  with  the  hand  about  25  centimeters  distant  from  the  upper 
part  of  the  thigh,  I  scratched  with  the  point  of  a  scalpel  the  skin  of  the 
chest  at  the  areola  of  the  nipple,  for  a  space  of  10  or  11  centimeters 
in  extent,  without  making  any  pressure  on  the  subjacent  muscles. 
We  immediately  saw  a  rapid  and  successive  contraction  of  the  great 
pectoral  muscle,  the  biceps,  probably  the  brachialis  anticus,  and 
lastly  the  muscles  covering  the  internal  condyle.  The  result  was  a 
movement  by  which  the  whole  arm  was  made  to  approach  the  trunk, 
with  rotation  inward  and  half- flexion  of  the  forearm  upon  the  arm; 
a  true  defensive  movement,  which  brought  the  hand  toward  the 
chest  as  far  as  the  pit  of  the  stomach.  Neither  the  thumb,  which 
was  partially  bent  toward  the  palm  of  the  hand,  nor  the  fingers, 
which  were  half  bent  over  the  thumb,  presented  any  movements. 
The  arm  being  replaced  in  its  former  position,  we  saw  it  again 
execute  a  similar  movement  on  scratching  the  skin,  in  the  same  manner 
as  before,  a  little  below  the  clavicle.  This  experiment  succeeded 
four  times,  but  each  time  the  movement  was  less  extensive;  and  at 
last  scratching  the  skin  over  the  chest  produced  only  contractions  in 
the  great  pectoral  muscle  which  hardly  stirred  the  limb"   (Dalton). 

Laws  of  Reflex  Action  (Pfliiger). 

1.  Law  of  Unilaterality. — If  a  feeble  irritation  be  applied  to  one  or 

more  sensory  nerves,  movement  takes  place  usually  on  one  side 
only,  and  that  the  same  side  as  the  irritation. 

2.  Law  0}  Symmetry. — If  the  irritation  becomes  sufficiently  intense, 

motor   reaction   is   manifested,   in   addition,    in   corresponding 
muscles  of  the  opposite  side  of  the  body. 

3.  Law  of  Intensity. — Reflex  movements  are  usually  more  intense 

on  the  side  of  irritation;  at  times  the  movements  of  the  opposite 
side  equal  them  in  intensity;  but  they  are  usually  less  pronounced. 

4.  Law  of  Radiation. — If  the  excitation  still  continues  to  increase,  it 

is  propagated  upward,  and  motor  reaction  takes  place  through 
centrifugal  nerves  coming  from  segments  of  the  cord  higher  up. 

5.  Law  of  Generalization. — When  the  irritation  becomes  very  intense, 

it  is  propagated  in  the  medulla  oblongata;  motor  reaction  then 


474  TEXT-BOOK  OF  PHYSIOLOGY. 

becomes  general,  and  it  is  propagated  up  and  down  the  cord,  so 
that  all  the  muscles  of  the  body  are  thrown  into  action,  the 
medulla  oblongata  acting  as  a  focus  whence  radiate  all  reflex 
movements. 
Special   Reflex    Movements. — Among   the   reflexes   connected 
with  the  more  superficial  portions  of  the  body  there  are  some  which 
are    so    frequently  either    increased    or  diminished    in   pathologic 
conditions  of  the  spinal  cord  that  their  study  affords  valuable  indi- 
cations as  to  the  seat  and  character  of  the  lesions.     They  may  be 
divided  into: 

1.  The  skin  or  superficial,  and 

2.  The  tendon  or  deep  reflexes. 

The  skin  reflexes,  characterized  by  contraction  of  underlying  mus- 
cles, are  induced  by  stimulation  of  the  skin — e.  g.,  pricking,  pinching, 
scratching,  etc.     The  following  are  the  principal  skin  reflexes : 

1 .  Plantar  reflex,  consisting  of  contraction  of  the  muscles  of  the  foot, 

induced  by  stimulation  of  the  sole  of  the  foot;  it  involves  the 
integrity  of  the  reflex  arc  through  the  lower  end  of  the  cord. 

2.  Gluteal    reflex,   consisting   of  contraction  of   the  glutei   muscles 

when  the  skin  over  the  buttock  is  stimulated;  it  takes  place 
through  the  segments  giving  origin  to  the  fourth  and  fifth  lumbar 
nerves. 

3.  Cremasteric  reflex,  consisting  of  a  contraction  of    the  cremaster 

muscle  and  a  retraction  of  the  testicle  toward  the  abdominal 
ring  when  the  skin  on  the  inner  side  of  the  thigh  is  stimulated; 
it  depends  upon  the  integrity  of  the  segments  giving  origin  to 
the  first  and  second  lumbar  nerves. 

4.  Abdominal  reflex,  consisting  of  a  contraction  of  the  abdominal 

muscles  when  the  skin  upon  the  side  of  the  abdomen  is  gently 
scratched;  its  production  requires  the  integrity  of  the  spinal 
segments  from  the  eighth  to  the  twelfth  dorsal  nerves. 

5.  Epigastric  reflex,  consisting  of  a  slight  muscular  contraction  in 

the  neighborhood  of  the  epigastrium  when  the  skin  between  the 
fourth  and  sixth  ribs  is  stimulated;  it  requires  the  integrity  of 
the  cord  between  the  fourth  and  seventh  dorsal  nerves. 

6.  The  scapular  reflex  consists  of  a  contraction  of  the  scapular  muscles 

when  the  skin  between  the  scapulas  is  stimulated;  it  depends 
upon  the  integrity  of  the  cord  between  the  fifth  cervical  and  third 
dorsal  nerves. 
The  skin  or  superficial  reflexes,  though  variable,  are  generally 
present  in  health.     They  are  increased  or  exaggerated  when  the  gray 
matter  of  the  cord  is  abnormally  excited,  as  in  tetanus,  strychnin- 
poisoning,  and  disease  of  the  lateral  columns. 

The  so-called  ''tendon  reflexes,"  characterized  by  the  con- 
traction   of   a   muscle,  also    are    of   much    value    in    the  diagnosis 


THE  SPINAL  CORD  475 

of  lesions  of  the  cord  and  are  elicited  by  a  sharp  tap  on  a 
given  tendon.  The  term,  tendon  reflex,  is,  however,  somewhat  in- 
accurate. The  fundamental  condition  for  the  production  of  the 
tendon  reflex  is  the  normal  tone  of  the  muscle,  which  is  a  true  reflex, 
maintained  by  afferent  nerve  impulses  developed  in  the  muscle  itself 
in  consequence  of  its  extension  and  hence  compression  of  the  end- 
organs  of  the  afferent  nerves,  the  muscle  spindles.  When  the  muscle 
is  passively  extended,  as  it  is  when  the  reflex  is  to  be  elicited,  there  is 
an  exaltation  of  the  tonus  and  an  increase  in  the  irritability.  To  this 
condition  of  the  muscle  due  to  passive  tension,  the  term  m}Otatic 
irritability  has  been  given.  If  the  muscle  extension  be  now  suddenly 
increased,  as  it  is  when  the  tendon  is  sharply  tapped,  the  increased 
compression  of  the  muscle  spindles  will  develop  additional  afferent 
impulses  which  af  ler  transmission  to  the  spinal  cord  will  give  rise  to 
contraction  of  the  corresponding  muscle. 

The  following  are  the  principal  forms  of  the  tendon  reflexes : 

1.  Patellar  reflex  or  knee-jerk,  consisting  of  a  contraction  of  the  ex- 

tensor muscles  of  the  thigh  when  the  ligamentum  patellae  is 
struck  between  the  patella  and  tibia.  This  reflex  is  best  ob- 
served when  the  legs  are  freely  hanging  over  the  edge  of  a  table. 
The  patella  reflex  is  generally  present  in  health,  being  absent 
in  only  2  per  cent.;  it  is  greatly  exaggerated  in  lateral  sclerosis, 
in  descending  degeneration  of  the  cord;  it  is  absent  in  locomotor 
ataxia  and  in  atrophic  lesions  of  the  anterior  gray  cornua. 

2.  Ankle- jerk  or  ankle  reflex. — If  the  extensor  muscles  of  the  leg  be 

placed  on  the  stretch  and  the  tendo  Achillis  be  sharply  struck,  a 
quick  extension  of  the  foot  will  take  place. 

3.  Ankle  Clonus. — This  consists  of  a  series  of  rhythmic  reflex  con- 

tractions of  the  gastrocnemius  muscle,  varying  in  frequency  from 
six  to  ten  per  second.     To  ehcit  this  reflex,  pressure  is  made  upon 
the  sole  of  the  foot  so  as  to  suddenly  and  energetically  flex  the 
foot  at  the  ankle,  thus  putting  the  tendo  Achillis  and  the  gas- 
trocnemius muscle  upon  the  stretch.     The  rhythmic  movements 
thus  produced  continue  so  long  as  the  tension  within  limits 
is  maintained.     Ankle  clonus  is  never  present  in  health,  but  is 
very  marked  in  lateral  sclerosis  of  the  cord. 
The  toe  reflex,  peroneal  reflex,  and  wrist  reflex  are  also  present  in 
sclerosis  of  the  lateral  columns  and  in  the  late  rigidity  of  hemiplegia. 
Reflex  Irritability. — The  general  irritability  or  quickness  of 
response  of  the  mechanism  involved  in  reflex  action  can  be  approxi- 
mately determined  by  observation  of  the  length  of  time  that  elapses 
between  the  application  of  a  minimal  stimulus  and  the  appearance  of 
the  muscle  response.     The  method  of  Tiirck  is  sufficiently  accurate 
for  general  purposes.      This  consists   in    suspending  a  frog,  after 
removal  of  the  brain,  and  immersing  the  foot  in  a  0.2  per  cent,  solu- 


476  TEXT-BOOK  OF  PHYSIOLOGY. 

tion  of  sulphuric  acid.  The  time  is  determined  by  means  of  a  metro- 
nome beating  one  hundred  times  a  minute.  Stimulation  of  the  skin 
can  also  be  effected  by  the  induced  electric  current,  as  suggested  by 
Gaskell.  A  single  shock  is,  however,  ineffective.  When  the  shocks 
follow  each  other  with  sufficient  rapidity,  they  give  rise  to  a  summa- 
tion of  effects  in  the  nerve-centers  which  will  soon  be  followed  by  a 
muscle  response.  It  is  highly  probable  that  the  chemic  stimulation 
gives  rise  to  a  similar  summation  of  effects. 

The  period  of  time  thus  obtained  is  distributed  over  the  entire 
mechanism.  The  true  reflex  time,  however, — i.  e.,  the  time  occupied 
in  the  passage  of  the  nerve  impulses  across  the  spinal  mechanism, — 
is  shorter  and  is  obtained  by  subtracting  from  the  whole  period 
the  time  occupied  by  the  passage  of  the  impulses  through  the  afferent 
and  efferent  nerves  as  well  as  the  latent  period  of  muscle  contraction. 
This  corrected  period,  the  true  reflex  time,  has  been  found  to  be 
twelve  times  longer  than  the  time  occupied  by  the  passage  of  the 
nerve  impulse  through  the  nerves,  including  the  latent  period  of  the 
muscle. 

The  reflex  irritability  is  increased  by: 

1.  Separation  of  the  Brain  from  the  Cord. — This  is  at  once  followed 

by  an  increase  in  reflex  irritabihty,  and  is  taken  as  evidence  that 
the  brain  normally  exerts  an  inhibitory  influence  over  the  reflex 
centers  of  the  cord.  The  same  increase  is  observed  upon  hemi- 
section  of  the  cord,  though  the  increase  is  limited  to  the  same 
side. 

2.  The  Toxic  Action  of  Drugs. — Strychnin  even  in  small  doses  in- 

creases the  irritabihty  to  such  an  extent  that  a  minimal  stimulus 
is  sufficient  to  call  forth  spasmodic  contractions  of  all  the 
skeletal  muscles.  Under  its  influence  the  usual  coordinated 
reflexes  disappear  and  are  succeeded  by  incoordinated  reflexes. 
The  explanation  of  this  fact  is  believed  to  be  a  diminution  in  the 
resistance  offered  by  the  cord  to  the  passage  of  the  afferent  im- 
pulses rather  than  to  a  direct  stimulation  of  the  efferent  cells. 
So  much  is  this  resistance  decreased  that  the  nerve  impulses, 
instead  of  being  confined  to  their  accustomed  paths,  are  radiated 
in  all  directions.  Absolute  repose  of  the  animal  and  the  exclu- 
sion of  all  external  stimuli  greatly  diminish  the  tendency  to 
the  occurrence  of  spasms. 

3.  Degeneration  of  the  Pyramidal  Tracts. — In  primary  lateral  scle- 

rosis, a  pathologic  condition  characterized  primarily  by  a  degen- 
eration of  the  terminal  filaments  of  the  pyramidal  tract  fibers, 
the  reflex  activity  of  the  cord  becomes  exalted.  As  the  disease 
progresses  the  irritability  increases  to  such  an  extent  that  violent 
spasmodic  contractions  of  the  arms  and  legs  arise  when  the  skin 
or    tendons    are    mechanically    stimulated.     The    explanation 


THE  SPINAL  CORD.  477 

offered  is  practically  the  same  as  in  division  of  the  cord:  viz., 
withdrawal  of    the  inhibitor  and  controlhng   influence  of    the 
brain. 
The  reflex  excitability  may  be  decreased  by: 

1.  Stimulation  of  Certain  Regions  of  the  Brain. — It  was  discovered 

by  Setchenow  that  when  the  frog  brain  is  divided  just  anterior 
to  the  optic  lobes  and  the  reflex  time  subsequently  determined 
according  to  the  method  of  Tiirck,  that  the  time  can  be 
considerably  lengthened  by  stimulation  of  the  optic  lobes. 
This  is  readily  accomplished  by  placing  small  crystals  of  sodium 
chlorid  on  the  optic  lobes.  It  was  concluded  from  this  fact  that 
these  lobes  contain  centers  which  exert  an  inhibitory  influence  over 
centers  in  the  spinal  cord  through  descending  nerve-fibers 
This  conclusion  is  strengthened  by  the  fact  that  division  of  the 
brain  just  behind  the  optic  lobes  causes  a  temporary  inhibition 
of  the  reflexes  in  consequence  of  a  mechanical  irritation  of  these 
fibers.  It  is  quite  probable  that  the  volitional  inhibition  of 
certain  reflexes  is  accomplished  through  the  intermediation  of 
this  center  localized  by  Setchenow. 

2.  Stimulation  of  Sensor  Nerves. — If  during  the    application  of   a 

stimulus  sufficient  to  call  forth  a  characteristic  reaction  in  a 
definite  period  of  time,  a  sensor  nerve  in  a  distant  region  of  the 
body  be  simultaneously  stimulated,  it  will  be  found  that  the  reflex 
time  will  be  lengthened  or  the  reaction  completely  inhibited. 
The  explanation  of  this  phenomenon  is  not  apparent. 

3.  Lesions  of  the  spinal  cord;  e.  g.,  atrophy  of  the  multipolar  cells 

of  the  anterior  horns  of  the  gray  matter;  degeneration  of  the 
terminals  of  the  posterior  fibers. 

4.  The  toxic  action  of  various  drugs, — e.  g.,  chloroform,  chloral, — 

which  are  believed  to  exert  their  depressing  action  on  the  nerve- 
cells  themselves. 
The  Spinal  Cord  as  a  Conductor. — The  white  matter  of  the 
spinal  cord  consists  of  nerve-fibers  the  specific  function  of  which  is 

1 .  To  conduct  nerve  impulses  from  one  segment  of  the  cord  to  another. 

2.  To  conduct  ner^'e  impulses  coming  to  the  cord  through  afferent 

nerves,  directly  or  indirectly  to  various  areas  of  the  encephalon. 

3.  To   conduct  nerve  impulses  from   the   encephalon  to  the  spinal 

cord  segments. 

Intersegmental  or  Associative  Conduction. — The  spinal  cord 
consists  of  a  series  of  physiologic  segments  each  of  which  has  specific 
functions  and  is  associated  through  its  related  spinal  nerve  with  a 
definite  segment  of  the  body.  For  the  harmonious  cooperation  and 
coordination  of  all  the  spinal  segments  it  is  essential  that  they  should 
be  united  by  commissural  or  associative  fibers.  This  is,  in  fact, 
accomplished  by  the  axons  of  the  intrinsic  cells  of  the  gray  matter, 


478  TEXT-BOOK  OF  PHYSIOLOGY. 

which  constitute  such  a  large  part  of  the  anterior  and  posterior  root 
zones.  In  consequence  of  tliis  association,  the  cord  becomes  capable 
of  complex  coordinated  and  purposive  reflex  actions. 

Spino-encephalic  or  Sensor  Conduction. — The  nerve  impulses 
that  arise  in  consequence  of  impressions  made  on  the  terminals  of 
the  nerves  in  the  cutaneous  and  mucous  surfaces,  in  the  viscera  and 
in  the  muscles,  are  transmitted  through  the  dorsal  roots  of  the  spinal 
nerves  to  the  cord.  When  transmitted  through  the  cord  to  the  cere- 
bral hemispheres  directly  or  indirectly,  they  are  received  by  specialized 
nerve-cells  in  the  cortex  and  translated  into  conscious  sensations. 
The  sensations  thus  arising  may  be  divided  into  special  and  general 
sensations.  Of  the  former  may  be  mentioned  pain,  touch,  tem- 
perature; of  the  latter  may  be  mentioned  hunger,  thirst,  fatigue, 
well-being,  etc. 

The  pathways  through  the  spinal  cord  that  conduct  these  afferent 
impulses  to  the  brain  are  ill  defined  and  imperfectly  known,  and  only 
for  a  few  sensations  can  it  be  said  that  their  pathways  have  been 
determined.  The  reason  for  this  obscurity  hes  partly  in  the  diffi- 
culties of  experimentation,  partly  in  the  difficulties  of  interpretation. 
Clinical  observations  are  for  special  reasons  more  or  less  untrust- 
worthy. 

Section  of  one  lateral  half  of  the  cord,  or  a  lesion  involving  the 
one  lateral  half,  as  a  rule  abolishes  all  forms  of  cutaneous  sensibility 
on  the  opposite  side  below  the  injury.  This  would  seem  to  prove  that 
the  nerve  impulses  cross  the  median  line  of  the  cord  immediately  or 
very  shortly  after  entering.  At  the  same  time,  muscle  sensibihty  is 
abolished  on  the  corresponding  side  below  the  injury.  This  would 
seem  to  prove  that  the  fibers  of  the  posterior  roots  which  enter  and 
cross  the  column  of  Burdach  and  ascend  in  the  column  of  Goll  are 
derived  mainly  from  the  muscles.  It  is,  however,  believed  by  some 
investigators  that  those  fibers  which  subserve  the  sense  of  touch  do 
not  decussate  at  once,  but  ascend  in  the  column  of  Goll  as  far  as  the 
medulla  oblongata,  where  they,  in  common  with  the  fibers  coming 
from  the  muscles,  arborize  around  the  nerve-cells  in  the  gracile  and 
cuneate  nuclei.  The  afferent  path  is  then  continued  by  new  nerve- 
fibers  which  emerge  from  these  cells,  and  which,  after  crossing  the 
median  plane  and  decussating  with  the  fibers  coming  from  the  oppo- 
site side,  join  the  afferent  path  from  the  spinal  cord.  These  fibers 
are  known  as  the  internal  arcuate  fibers  and  assist  in  the  formation 
of  the  lemniscus  or  fillet.  (Fig.  209.)  The  sensor  pathway  decus- 
sates in  part  at  different  levels  of  the  spinal  cord  and  in  part  at  the 
level  of  the  gracile  and  cuneate  nuclei.  The  former  is  often  termed 
the  lower,  the  latter  the  upper  sensor  decussation. 

The  pathways  for  the  impulses  that  give  rise  to  the  different  sen- 
sation have  been  variously  located  by  different  observers,  e.  g.,  in  the 


THE  SPINAL  CORD. 


479 


gray  matter,  in  the  limiting  layer,  and  in  the  antero-lateral  tract  of 
Gowers ;  the  pathway  for  the  impulses  that  give  rise  to  temperature 
sensations  has  been  located  in  the  gray  matter;  the  pathway  for 
tactile  impressions  has  been  located  in  the  posterior  columns,  though 
this  is  not  beyond  dispute.  The  pathway  for  pain  sensations  has 
been  located  in  Gowers'  tract. 


Fig.  209. — Diagram  of  the  Sensor  Pathways  in  the  Spinal  Cord  Augmented 
ABOVE  BY  Fibers  of  the  Sensor  Cranial  Nerves  and  Nerves  of  Special 
Sense.  V.  The  trifacial  Nerve.  VIII.  The  vestibular  branch  of  the  acoustic 
nerve.  IX.  The  glosso -pharyngeal  nerve.  X.  The  pneumogastric  nerve. — {Van 
Gehiichten). 


Encephalo-spinal  or  Motor  Conduction. — At  birth  the  child  is 
capable  of  performing  all  the  functions  of  organic  life,  such  as  sucking, 
swallowing,  breathing,  etc.  It  is,  however,  deficient  in  psychic 
activity  and  in  volitional  control  of  its  muscles.  Its  movements  are 
therefore  largely,  if  not  entirely,  reflex  in  character. 


48o  TEXT-BOOK  OF  PHYSIOLOGY. 

Embryologic  and  histologic  examination  of  the  spinal  cord  and 
medulla  show  that  so  far  as  their  mechanisms  for  independent  phys- 
iologic activities  are  concerned  both  are  fully  developed.  Similar 
investigations  of  the  cerebral  hemispheres  and  of  the  nerve-fibers 
which  bring  their  nerve-cells  into  relation  with  the  spinal  segments 
show  that  the  cells  of  the  cortex  are  not  only  immature,  but  that  their 
descending  axons  are  incompletely  invested  with  myelin.  With  the 
growth  of  the  child,  psychic  life  unfolds  and  volitional  control  of  mus- 
cles is  acquired.  Coincidently  the  cells  of  the  cerebral  cortex  grow 
and  develop  and  the  fibers  become  covered  with  myelin. 

The  nerve-fibers  which  have  their  origin  in  the  cells  of  the  cerebral 
cortex,  and  which  terminate  in  tufts  around  the  cells  in  the  anterior 
horns  of  the  gray  matter  of  the  spinal  segments,  are  to  be  regarded  as 
long  commissural  tracts  uniting  and  associating  these  two  portions 
of  the  central  nerve  system. 

Experimental  investigations  and  observations  of  pathologic  lesions 
accord  with  the  view  that  physiologically  these  fibers  are  efferent 
pathways  for  the  transmission  of  motor  or  volitional  impulses  from 
the  cortex  to  the  spinal  segments.     The  nerve-cells  in  which  the 
motor  impulses  originate  are  located  for  the  most  part,  as  will  be  fully 
stated  later,  in  the  central  portion  of  the  cortex  of  the  cerebral  hemi- 
spheres in  the  neighborhood  of  the  central  or  Rolandic  fissure.     The 
axons  of  these  cells  from  each  hemisphere  descend  through  the 
corona  radiata  to  and  through  the  internal  capsule,  along  the  inferior 
surface  of  the  crura  cerebri,  behind  the  pons  to  the  medulla,  of  which 
they  constitute  the  anterior  pyramids.    (Fig.  210.)    At  this  point  the 
pyramidal  tract*  of  each  side  divides  into  two  portions,  viz.: 
r.  A  large  portion,  containing  from  80  to  90  per  cent,  of  the  fibers, 
which  decussates  at  the  lower  border  of  the  medulla  and  passes 
downward  in  the  posterior  part  of  the  lateral  column  of  the 
opposite  side,   constituting  the  crossed  pyramidal  tract;  as  it 
descends  it  gradually  diminishes  in  size  as  its  fibers  or  their 
collaterals  enter  the  gray  matter  of  each  successive  segment. 
2.  A  small  portion,  containing  from  20  to  10  per  cent,  of  the  fibers,, 
which  does  not  decussate  at  the  medulla  but  passes  downward 
on  the  inner  side  of  the  anterior  column  of  the  same  side,  con- 
stituting the  direct  pyramidal  tract  or  column  of  Tiirck.     This 
tract  can  be  traced  down,  as  a  rule,  only  as  far  as  the  mid-dorsal 
region.     As  it  descends  it  becomes  smaller  as  its  fibers  cross  the 
anterior  commissure  to  enter  the  gray  matter  of  the  opposite 


*  From  the  fact  that  the  region  included  between  the  origin  of  these  fibers  and 
the  internal  capsule  presents  somewhat  the  form  of  a  pyramid  with  four  sides, 
Charcot  designated  it  the  pyramidal  region  and  the  fibers  composing  it  the  pyram- 
idal tract.  The  base  of  the  pyramid  includes  the  cortex  of  the  convolutions 
around  the  Rolandic  fissure.  The  summit  of  the  pyramid  is  truncated  and  covers 
the  pyramidal  region  of  the  Internal  capsule. 


THE  SPINAL  CORD. 


481 


side.  Thus  all  the  fibers  of  the  pyramidal  tract  from  each 
cerebral  hemisphere  eventually  are  brought  into  relation  with 
the  cells  of  the  gray  matter  of  the  opposite  side  of  the  cord. 


Fig.  210. — Diagram  of  the  Pyramidal  Tract  or  Motor  Path.  III.  Common 
oculo-motor  nerve.  IV.  Pathetic  nerve.  V.  Motor  division  of  the  trigeminal  nerve. 
VI.  The  abducens  nerve.  VII.  Facial  nerve.  IX.  and  X.  Motor  divisions 
of  the  glosso-pharyngeal  and  pneumogastric  nerves.  XI.  Spinal  accessory  nerve. 
XII.  Hypoglossal  nerve. — -{Van  Gehiichten.) 

That    the    pyramidal    tracts    arc    the    conductors    of   volitional 
impulses   throughout   the   length    of    the   cord   to   its   various  seg- 
ments has  been  made  evident  by  the  results  of  section,  electric  stimu- 
lation, and  disease.     Division  of  the  anterior  and  lateral  columns 
31 


482  TEXT-BOOK  OF  PHYSIOLOGY. 

of  one  side  of  the  cord  in  any  part  of  its  extent  is  invariably  followed 
by  a  loss  of  motion  or  paralysis  of  the  muscles  below  the  section, 
while  electric  stimulation  of  the  peripheral  end  of  the  isolated  crossed 
pyramidal  tract  is  followed  by  marked  characteristic  movements  of 
the  muscles.  Similar  results  follow  division  of  the  pyramidal  tract 
in  any  part  of  its  course  from  the  cerebral  cortex  downward.  Electric 
stimulation  of  the  cortical  cells  which  give  origin  to  the  pyramidal 
tract  is  also  followed  by  contraction  of  the  muscles  of  the  opposite 
side,  while  their  destruction  is  attended  by  paralysis  of  the  same 
muscles.  As  the  nutrition  of  the  fibers  is  governed  by  the  cells,  it 
follows  that  when  the  axon  is  separated  from  its  cell  it  degenerates. 
It  has  been  found  that  a  lesion  of  the  pyramidal  tract  in  any  part  of 
its  course  is  followed  by  descending  degeneration,  which  is  taken  in 
evidence  that  it  conducts  nerve  impulses  from  above  downward. 
Thus  experimental  investigation  and  pathologic  observation  are  in 
accord  in  the  view  that  physiologically  these  nerve-fibers  are  the 
pathways  for  the  transmission  of  motor  or  vohtional  impulses  from 
the  encephalon  to  the  spinal  cord. 

The  relation  of  the  motor  and  sensor  pathways  to  each  other  in 
the  spinal  cord  and  brain  are  shown  in  Plate  II.  The  afferent  fibers 
which  decussate  at  various  levels  through  the  spinal  cord  are  not 
represented. 


Diagram  Indicating  the  Course  of  the  Motor  and  Sensory  Fibers  of  the 
Spinal  Cord  and  Medulla. — {Gordinier.) 

a,  a.  Motor  cells  of  the  cerebral  cortex,  b,  b.  Arborizations  of  the  fibers  of  the  sensory 
tract  in  the  cerebral  cortex,  c.  Nucleus  of  the  column  of  Burdach,  showing  ter- 
minal arborizations  of  the  long  sensory  fibers  of  the  cord.  d.  Nucleus  of  the 
column  of  GoU,  showing  terminal  arborizations  of  the  long  sensory  fibers  of  the 
cord.  e.  Section  of  the  medulla,  showing  sensory  decussation.  /.  Section  of 
medulla,  showing  motor  or  pyramidal  decussation,  g,  g.  Motorial  end  plates. 
h.  Section  through  the  cervical  region  of  the  cord,  showing  termination  in  the 
anterior  horn  of  the  motor  fibers  of  the  direct  pyramidal  tract  after  they  have 
crossed  in  the  anterior  commissure;  also  fiber  of  crossed  pyramidal  tract  ending 
about  anterior  horn  cell  of  same  side,  i,  i.  Posterior  spinal  ganglia,  j,  k.  Sensory 
fibers  of  short  course.  /.  Sensory  fibers  of  long  course,  terminating  in  medulla. 
m,  m,  m.  Sensory  end  organs,     n.  Section  through  lumbar  cord. 


Plate  II. 


CHAPTER  XVIII. 

THE  MEDULLA  OBLONGATA;  THE  ISTHMUS  OF  THE 
ENCEPHALON;  THE  BASAL  GANGLIA. 

THE  MEDULLA  OBLONGATA. 

The  medulla  oblongata  is  that  portion  of  the  central  nerve 
system  immediately  superior  to  and  continuous  with  the  spinal  cord. 
It  has  the  shape  of  a  truncated  cone,  the  base  of  which  is  directed 
upward,  the  truncated  apex  downward.  It  is  38  mm.  in  length, 
18  mm.  in  breadth,  and  12  mm.  in  thickness.  By  the  continuation 
upward  of  the  anterior  and  posterior  median  fissures,  the  medulla  is 
divided  into  symmetric  halves  (Figs.  211  and  212).  Like  the  cord, 
of  which  it  is  a  continuation,  it  is  composed  of  white  matter  externally 
and  gray  matter  internally. 

Structure  of  the  Gray  Matter. — The  gray  matter  of  the  medulla 
is  continuous  with  that  of  the  cord,  though  owing  to  the  shifting  of 
position  of  the  different  tracts  of  the  white  matter  it  is  arranged 
with  much  less  regularity.  The  appearance  which  the  gray  matter 
presents  on  transverse  section  varies  also  at  different  levels. 

At  the  level  of  the  first  cervical  nerve  the  posterior  horns  are 
narrow,  elongated,  and  directed  outward.  The  lateral  horns  are  well 
developed  and  present  a  collection  of  cells  near  their  bases  which 
can  be  traced  forward  and  backward  for  some  distance.  At  the 
level  of  the  decussation  of  the  pyramidal  tracts  the  head  of  the 
anterior  horn  becomes  completely  detached  from  the  rest  of  the 
gray  matter  and  is  pushed  backward  toward  the  posterior  horn;  the 
bases  of  the  anterior  horns  become  spread  out  to  form  a  layer  of 
gray  matter  near  the  dorsal  aspect  of  the  medulla.  Transverse  sec- 
tions of  the  medulla  at  all  levels  show  a  more  or  less  extensive  network 
of  nerve-fibers  known  as  the  reticular  formation.  In  its  meshes  are 
found  collections  of  nerve-cells  of  varying  size.  Toward  the  dorsal 
aspect  of  the  medulla  special  groups  of  cells  are  found  from  which 
axons  arise  to  become  the  fibers  of  various  efferent  cranial  nerves,  e.  g., 
the  hypoglossal,  the  efferent  fibers  of  the  vagus,  and  glosso-pharyngeal. 

Structure  of  the  White  Matter. — The  white  matter  is  com- 
posed of  nerve-fibers  supported  by  connective  tissue  and  neuroglia. 
It  is  subdivided  on  either  side  by  grooves  into  three  main  columns: 
viz.,  an  anterior  column  or  pyramid,  a  lateral  column,  and  a  posterior 
column. 

483 


484  TEXT-BOOK  OF  PHYSIOLOGY. 

The  anterior  column  or  pyramid  is  composed  partly  of  fibers 
continuous  with  those  of  the  anterior  column  of  the  spinal  cord  (the 
direct  pyramidal  tract),  and  partly  of  fibers  continuous  with  those  of 
the  lateral  column  of  the  cord  of  the  opposite  side  (the  crossed  pyram- 
idal tract),  which  decussate  at  the  anterior  portion  of  the  medulla. 
The  united  fibers  can  be  traced  upward  to  the  pons,  where  they 
disappear  from  view. 

The  lateral  column  is  composed  of  fibers  continuous  with  those 
of  the  lateral  column  of  the  cord.  As  the  fibers  pass  upward,  how- 
ever, they  diverge  in  several  directions.  The  fibers  of  the  crossed 
pyramidal  tract  cross  the  median  hne,  as  previously  stated,  to  enter 
into  the  formation  of  the  anterior  column;  the  fibers  of  the  direct 
cerebellar  tract  gradually  curve  backward,  and  in  so  doing  unite  with 
other  fibers  to  form  the  restiform  body,  after  which  they  enter  the 
cerebellum  by  way  of  the  inferior  peduncle.  Situated  between  the 
anterior  pyramid  and  the  restiform  body  is  a  small  oval  mass,  the 
olivary  body,  composed  of  both  white  and  gray  matter. 

The  posterior  column  is  composed  largely  of  fibers  continuous  with 
those  of  the  posterior  column  of  the  cord.  The  subdivision  of  this 
column  into  a  postero- external  (Burdach)  and  a  postero-internal 
(GoU)  is  more  marked  in  the  medulla  than  in  the  cord.  The  former 
is  here  known  as  the  funiculus  cuneatus,  the  latter  as  the  funiculus 
gracilis.  These  two  strands  of  fibers  are  apparently  continued  into 
the  restiform  body.  Owing  to  the  divergence  of  the  restiform  bodies 
a  V-shaped  space  is  formed,  the  floor  of  which  is  covered  with  epithe- 
lium resting  on  the  ependyma.  At  the  upper  extremity  of  the 
funiculus  cuneatus  and  funiculus  gracihs,  two  collections  of  gray 
matter  are  found,  known  respectively  as  the  nucleus  cuneatus  and 
nucleus  gracilis.  Around  the  cells  of  these  nuclei  many  of  the  fibers 
of  the  posterior  column  end  in  brush-like  expansions. 

The  Fillet  or  Lemniscus. — From  the  ventral  surface  of  the  cu- 
neate  and  gracile  nuclei  axons  emerge  which  pass  forward  and 
upward  through  the  gray  matter  and  decussate  with  corresponding 
fibers  coming  from  the  opposite  nuclei.  They  then  assume  a  position 
just  posterior  to  the  pyramids  and  between  the  ohvary  bodies.  These 
fibers  thus  form  a  new  distinct  tract,  termed  the  fillet  or  lemniscus. 
As  this  tract  ascends  toward  the  brain  it  receives  additional  axons  from 
the  sensory  end-nuclei  of  all  the  afferent  cranial  nerves  of  the  opposite 
side  with  the  exception  of  the  auditory.  From  the  end-nuclei  of  the 
auditory  nerve  new  axons  ascend  as  a  distinct  tract  situated  near 
the  lateral  aspect  of  the  pons.  From  their  position  these  two  separate 
tracts  have  been  termed  the  mesial  and  lateral  fillets  respectively. 

Before  proceeding  to  a  consideration  of  the  functions  of  the 
medulla  oblongata  it  will  be  found  conducive  to  clearness  to  sketch 
the  saHent  anatomic  features  of  the  parts  anterior  to  it  and  their 
relations  one  to  another. 


MEDULLA  OBLONGATA. 


485 


Fig.  211. — Anterior  or  Ventral 
View  of  the  Medulla  Ob- 
longata AND  Isthmus,  i.  In- 
fundibulum.  2.  Tuber  ciner- 
eum.  3.  Corpora  albicantia.  4. 
Cerebral  peduncle.  5.  Tuber 
annulare.  6.  Origin  of  the  mid- 
dle peduncle  of  the  cerebellum. 
7.  Anterior  pyramids  of  the 
medulla  oblongata.  8.  Decussa- 
tion of  the  anterior  pyramids.  9. 
Olivary  bodies.  10.  Restiform 
bodies.  11.  Arciform  fibers.  12. 
Upper  extremity  of  the  spinal 
cord.  13.  Ligamentum  denticu- 
latum.  14,  14.  Dura  mater  of 
the  cord.  15.  Optic  tracts.  16. 
Chiasm  of  the  optic  nerves.  17. 
Motor  oculi  communis.  18. 
Patheticus.  19.  Fifth  nerve.  20. 
Motor  oculi  e.xternus.  2 1 .  Facial 
nerve.  22.  .A.uditory  nerve.  23. 
Nerve  of  Wrisberg.  24.  Glosso- 
pharyngeal ner\'e.  25.  Pneumo- 
gastric.  26,  26.  Spinal  accessory. 
27.  Sublingual  nerve.  28,  29,  30. 
Cervical  nerves. — (Sappey.) 


Fig.  212. — Posterior  or  Dorsal  View  of 
the  Medulla  Oblongata,  Isthmus, 
AND  Basal  Ganglia.  i.  Corpora 
quadrigemina.  2.  Corpus  quadrige- 
minum  anterior  (pregeminum).  3.  Cor- 
pus quadrigeminum  posterior  (post- 
geminum).  4.  Tract  of  fibers  (bra- 
chium)  passing  to  the  corpus  genicula- 
tum  externum.  5.  Tract  of  fibers 
(brachium)  passing  to  6,  the  corpus 
geniculatum  internum.  7.  Posterior 
commissure.  8.  Pineal  gland.  9.  Su- 
perior cerebellar  peduncle.  10,  11,  12. 
The  valve  of  Vieussens.  13.  The  pa- 
thetic nerve.  14.  Lateral  groove  of  the 
isthmus.  15.  Triangular  bundle  of  the 
isthmus.  16.  Superior  cerebellar  pedun- 
cle. 17.  Middle  cerebellar  peduncle. 
18.  Inferior  cerebellar  peduncle.  19. 
Antero-inferior  wall  of  the  fourth  ven- 
tricle. 20.  Acoustic  nerve.  21.  Spinal 
cord.  22.  The  postero-median  column. 
23.  The  posterior  pyramids. — (Sappey.) 


486  TEXT-BOOK  OF  PHYSIOLOGY. 


THE  ISTHMUS  OF  THE  ENCEPHALON. 

The  isthmus  of  the  encephalon  comprises  that  portion  of  the 
central  nerve  system  connecting  the  cerebrum  above,  the  cerebellum 
behind,  and  the  medulla  below.  Its  ventral  surface  presents  below 
an  enlargement,  convex  from  side  to  side,  the  pons  Varolii.  On 
each  side  the  fibers  of  which  the  pons  consists  converge  to  form  a 
compact  bundle,  the  middle  peduncle,  which  enters  the  correspond- 
ing half  of  the  cerebellum.  Above  the  pons,  this  surface  presents 
two  large  columns  of  white  matter  which,  diverging  somewhat 
from  below  upward,  enter  the  base  of  the  cerebrum  and  are  known 
as  the  crura  cerebri.  Embracing  the  crura  above  are  two  large 
bands  of  white  matter,  the  optic  tracts  (Fig.  211). 

The  dorsal  surface  presents  below  two  diverging  columns  of  white 
matter,  the  inferior  peduncles;  above,  two  converging  columns,  the 
superior  peduncles  of  the  cerebellum  (Fig.  212).  At  the  extreme 
upper  part  of  this  surface  there  are  four  small  grayish  eminences, 
the  corpora  quadrigemina.  From  the  disposition  of  the  white  matter 
on  the  dorsal  surface  of  the  isthmus  and  medulla,  there  is  formed  a 
lozenge- shaped  space,  the  fourth  ventricle.  This  space  is  merely  an 
expansion  of  the  central  cavity  of  the  cord,  the  result  of  the  changed 
relations  of  the  white  and  gray  matter  in  this  region  of  the  central 
nerve  system.  Above,  this  ventricle  communicates  by  a  narrow  canal, 
the  aqueduct  of  Sylvius,  with  the  third  ventricle.  The  floor  of  the 
fourth  ventricle  is  covered  with  a  layer  of  epithehum  resting  on  the 
ependyma  continuous  with  that  lining  the  central  canal  of  the  cord. 
Beneath  this  is  a  layer  of  gray  matter. 

The  pons  Varolii  comprises  in  a  general  way  that  portion  of  the 
central  nerve  system  situated  between  the  medulla  oblongata  and 
the  crura  cerebri.  The  ventral  surface  is  convex  from  side  to  side; 
the  lateral  surface,  owing  to  the  convergence  of  the  fibers  of  which 
it  is  composed,  is  contracted  to  form  the  middle  peduncle  of  the 
cerebellum;  the  posterior  surface  is  flat  and  forms  the  upper  half  of 
the  floor  of  the  fourth  ventricle.  The  pons  consists  of  white  fibers 
and  gray  matter  supported  by  connective  tissue  and  neuroglia.  Trans- 
verse sections  of  the  pons  show  that  it  is  divided  into  an  anterior  or 
ventral,  and  a  posterior  or  dorsal  portion,  the  latter  being  usually 
termed  the  tegmentum. 

The  ventral  portion  consists  for  the  most  part  of  white  fibers,  ar- 
ranged longitudinally  and  transversely  (Fig.  213).  The  longitudinal 
fibers  are  largely  continuations  of  the  pyramidal  tracts,  or  the  fibers 
composing  the  anterior  pyramid  of  the  medulla.  In  the  lower  part  of 
the  pons  these  fibers  are  compactly  arranged,  but  at  higher  levels  they 
are  separated  into  a  number  of  bundles  by  the  interlacing  of  the  trans- 
verse fibers.     The  transverse  fibers  are  divided  into  a  superficial  and 


ISTHMUS  OF  THE    ENCEPHALON. 


487 


a  deep  set.  Among  these  fibers  are  groups  of  nerve-cells  which 
collectively  are  known  as  the  nucleus  pontis.  Some  of  the  transverse 
fibers,  especially  the  superficial  ones,  are  commissural  in  character — 
i.  e.,  they  connect  corresponding  parts  of  the  gray  matter  of  the  lateral 
halves  of  the  cerebellum;  others  coming  from  the  gray  matter  of  the 
cerebellum  cross  the  median  fine  and  terminate  around  the  cells  of 
the  nucleus  pontis;  others  again  are  connected  with  the  gray  cells  of 
the  same  side.  Through  the  intermediation  of  the  nucleus  pontis 
and  certain  of  the  longitudinal  fibers  of  the  pons,  the  cerebellum  is 
brought  into  relation  with  the  cerebrum. 

The  dorsal  or  tegmental  portion  consists  of:  (i)  The  fillet;  (2)  the 
formatio  reticularis;  (3)  the  posterior  longitudinal  bundle;  (4)  the 
substantia  ferruginosa;  (5)  groups  of  nerve-cells  from  which  arise 
various  cranial  nerves — e.  g.,  the  fifth,  sixth,  sev^enth,  and  eighth. 

The  fillet  or  lemniscus  in  this  region 
is  divided  into  a  mesial  and  a  lateral 
portion.  The  fibers  of  the  mesial  por- 
tion are  partly  the  axons  of  the  nerve- 
cells  of  the  gracile  and  cuneate  nuclei 
of  the  opposite  side  of  the  medulla, 
and  partly  of  the  axons  of  the  sensor 
nerve-cells  of  the  afferent  cranial  nerves 
with  the  exception  of  the  auditory. 
The  fibers  of  the  lateral  portion  are 
mainly  the  axons  of  the  cells  in  the 
floor  of  the  fourth  ventricle  around 
which  the  auditory  nerve-fibers  end. 
They  are  therefore  a  continuation  of 
the  auditory  tract. 

The  formatio  reticularis  is  a  con- 
tinuation of  that  of  the  medulla. 

The   posterior  longitudinal   bundle 
is  triangular  in  shape  and  situated  behind  the  formatio  reticularis 
and  close  to  the  median  fine.     The  fibers  composing  it  are  largely 
derived  from  the  ground  fibers  of  the  antero-lateral  column  of  the 
spinal  cord. 

The  superior  olive  is  a  cylindric  mass  of  gray  matter  situated  in 
the  pons  in  the  anterior  part  of  the  formatio  reticularis.  It  consists 
of  nerve-cells  the  axons  of  which  pass  dorso-laterally,  decussate  in 
the  median  line,  and  form  the  lateral  fillet  of  the  opposite  side.  Some 
few  axons  go  to  the  lateral  fillet  of  the  same  side. 

The  substantia  }errugi?tosa  is  composed  mainly  of  pigmented 
cells. 

The  groups  of  nerve-cells  lying  just  beneath  the  floor  of  the 
fourth  ventricle  give  origin  to  axons  of  the  motor  portion  of  the  fifth. 


Fig.  213. — Traxsection  of  the 
Pons  through  its  Middle 
Portion,  Showing  the 
Relation  of  the  Nerve 
Tracts  of  Which  it  is 
Composed.  D.  I.  f.  Dorsal 
longitudinal  fasciculus.  L.c. 
and  c.  Locus  ceruleus.  L.f. 
Lateral  fillet. 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  sixth,  the  seventh  cranial  nerves.     Some  of  the  groups  are  the 
sensor  end-nuclei  of  the  fifth  and  eighth  cranial  nerves. 

The  crura  cerebri  comprise  that  portion  of  the  central  nerve  sys- 
tem situated  between  the  pons  below  and  the  cerebrum  above.  They 
are  composed  of  strands  of  nerve-fibers  which  are  divided,  as  shown 
on  cross-section,  into  a  ventral  and  a  dorsal  portion  by  a  crescentic 
shaped  layer  of  gray  matter,  the  substantia  nigra  (Fig.  214).  Of  the 
fibers  which  compose  the  ventral  portion  of  each  crus,  the  crusta 
or  pes,  the  larger  part  is  continuous  below,  through  the  longitudinal 
fibers  of  the  pons,  with  the  pyramid  of  the  medulla  and  the  pyramidal 
tract;  above  they  assist  in  the  formation  of  the  internal  capsule. 

On  the  inner  and  on  the  outer 

.2Q .  side  of  each  crusta  there  is  a 

bundle  of  fibers  derived  from 
the  frontal,  and  from  the 
temporal  and  occipital  por- 
tions of  the  cerebrum,  respec- 
tively. These  fibers  are  con- 
nected directly  with  the  nuclei 
pontis  and  indirectly  with 
the  cerebellum  of  the  same 
and  opposite  sides.  The 
fibers  which  compose  the 
dorsal  portion,  the  tegmentum, 
are  continuous  with  those 
which  pass  upward  from  the 
medulla  and  pons,  e.  g.,  the 
fillet,  both  mesial  and  lateral, 
the  formatio  reticularis,  the 
posterior  longitudinal  bundle, 
and,  in  addition,  the  fibers  of 
the  superior  peduncles  of  the 
cerebellum.  Above,  the  fibers  terminate  largely  in  collections  of  gray 
matter  at  the  base  of  the  cerebrum. 

The  aqueduct  of  Sylvius  is  a  short  narrow  canal  which  connects 
the  cavity  of  the  fourth  with  the  cavity  of  the  third  ventricle.  It  is 
lined  by  the  ependyma  and  surrounded  by  a  layer  of  gray  matter 
continuous  with  that  forming  the  floor  of  the  fourth  ventricle.  In 
that  portion  of  the  gray  matter  lying  beneath  or  ventral  to  the  aqueduct 
there  are  groups  of  nerve-cells  which  give  origin  to  axons  which  unite 
to  form  the  third  and  fourth  cranial  nerves. 


Fig.  214. — Scheme  of  Transverse  Sec- 
tion OF  THE  Cerebral  Peduncles. 
CQ.  Corpora  quadrigemina.  Aq.  Aque- 
duct, p.l.b.  Posterior  longitudinal  bun- 
dle. F.  Fillet  or  lemniscus.  RN.  Red 
nucleus.  SN.  Substantia  nigra.  III. 
Third  nerve.  Py.  Pyramidal  tracts. 
FC.  Fronto-cerebellar;  and  TOC,  tem- 
poro-occipital  fibers  of  the  crusta.  CC. 
Caudate-cerebellar  fibers  in  upper  part 
of  crusta. — {After  Wernicke  and  Cowers.) 


CORPORA  QUADRIGEMINA. 


THE  CORPORA  QUADRIGEMINA. 

The  corpora  quadrigemina  are  four  small  grayish  eminences 
situated  beneath  the  posterior  border  of  the  corpus  callosum  and  be- 
hind the  third  ventricle.  They  rest  upon  the  lamina  quadrigemina, 
which  fonns  the  roof  of  the  aqueduct  of  Sylvius.  The  anterior  pair 
are  termed  the  nates,  or  the  pregemina,  the  posterior  pair  the  testes, 
or  the  postgemina. 

From  the  external  surface  of  each  body  there  pass  outward 
bundles  of  fibers  termed  hrachia.  The  fibers  which  compose  the 
brachium  of  the  pregeminum  pass  outward  and  enter  a  small  col- 
lection of  gray  matter,  the  corpus  geniculatum  extenmm,  and  the  optic 
tract.  The  fibers  which  compose  the  brachium  of  the  postgeminum 
are  divided  into  two  bundles,  one  of  which  enters  a  second  small 
collection  of  gray  matter,  the  corpus  geniculatum  internum,  while  the 
other  passes  forward  beneath  this  body  to  enter  the  internal  capsule, 
beyond  which  it  passes  to  the  cortex  of  the  temporal  region  of  the 
cerebrum  (Fig.  212). 

Though  these  bodies  are  closely  associated  anatomically,  they 
differ  in  origin,  in  their  relations  and  in  their  functions. 

Microscopic  examination  of  sections  of  the  quadrigeminal  bodies 
shows  that  they  are  composed  of  nerve-cells  and  nerve-fibers,  both  of 
which  are  so  intricately  arranged  that  it  is  difficult  to  trace  their 
relation  one  to  another  and  to  adjoining  structures.  Some  of  the 
cells  of  the  pregeminum  give  off  axons  which  course  outward  and 
forward,  enter  the  internal  capsule,  and  pass  through  the  optic 
radiation  to  the  cortex  of  the  occipital  region  of  the  cerebrum.  Many 
fibers  of  the  optic  tract,  axons  of  the  cells  of  the  retina,  end  in 
brush-like  expansions  around  these  same  cells.  There  is  thus  formed 
a  connected  pathway  between  the  retina  and  the  occipital  cortex. 

The  cells  of  the  occipital  cortex,  however,  send  axon  fibers  in  the 
reverse  direction  through  the  optic  radiation  to  terminate  around  the 
cells  of  the  pregeminum,  while  axons  of  pregeminal  cells  pass  for- 
ward to  the  retina  and  to  the  cells  of  origin  of  the  third  nerve. 

The  cells  of  the  postgeminum  give  origin  to  axons  which  pass 
upward,  forward,  and  outward,  enter  the  internal  capsule,  and  pass 
by  way  of  the  auditory  tract  to  the  cortex  of  the  temporo-sphenoidal 
region  of  the  cerebrum.  ]\Iany  of  the  fibers  of  the  lateral  fillet,  a 
portion  of  the  auditory  tract,  terminate  in  brush-like  expansions 
around  these  same  cells.  There  is  thus  established  a  connected 
pathway  between  the  cochlea  and  the  temporo-sphenoidal  cortex. 
The  cells  of  the  temporal  cortex,  however,  send  axons  in  the  re- 
verse direction  by  way  of  the  auditory  tract  to  the  cells  of  the 
postgeminum.  There  is  thus  established  a  double  communication 
between  the  occipital  and  temporal  region  of  the  cerebral  cortex, 
and  the  pregeminal  and  postgeminal  bodies  respectively. 


490 


TEXT-BOOK  OF  PHYSIOLOGY. 


THE   BASAL   GANGLIA;    THE    CORPORA   STRIATA   AND    OPTIC 

THALAMI. 

The  basal  ganglia  surmount  the  crura  cerebri,  but  are  only 
made  visible  by  removal  of  the  cerebrum  (Fig.  215). 


Fig.  215.— Dissection  of  Brain,  from  above,  Exposing  the  Lateral  Fourth 
AND  Fifth  Ventricles  with  the  Surrounding  Parts,  i. — a.  Anterior  part, 
or  genu  of  corpus  callosum.  b.  Corpus  striatum,  b'.  The  corpus  striatum  of 
left  side,  dissected  so  as  to  e.xpose  its  gray  substance,  c.  Points  by  a  line  to  the 
taenia  semicircularis.  d.  Optic  thalamus,  e.  Anterior  pillars  of  fornix  divided; 
below  they  are  seen  descending  in  front  of  the  third  ventricle,  and  between  them 
is  seen  part  of  the  anterior  commissure;  in  front  of  the  letters  is  seen  the  slit-like 
fifth  ventricle,  between  the  two  laminae  of  the  septum  lucidum.  /.  Soft  or  middle 
commissure;  g  is  placed  in  the  posterior  part  of  the  third  ventricle;  immediately 
behind  the  latter  are  the  posterior  commissure  (just  visible)  and  the  pineal 
gland,  the  two  crura  of  which  extend  forward  along  the  inner  and  upper  margins 
of  the  optic  thalami.  h  and  /.  The  corpora  quadrigemina.  k.  Superior  crus 
of  cerebellum.  Close  to  k  is  the  valve  of  Vieussens,  which  has  been  divided  so 
as  to  expose  the  fourth  ventricle.  /.  Hippocampus  major  and  corpus  fimbria- 
tum,  or  tasnia  hippocampi,  m.  Hippocampus  minor,  n.  Eminentia  collaterahs. 
o.  Fourth  ventricle,  p.  Posterior  surface  of  medulla  oblongata,  r.  Section 
of  cerebellum,  s.  Upper  part  of  left  hemisphere  of  cerebellum  exposed  by  the 
removal  of  part  of  the  posterior  cerebral  lobe. — {Hirschfeld  and  Leveille.) 


The  corpora  striata  are  two  large  ovoid  collections  of  gray  and 
white  matter  situated  at  the  base  of  the  cerebrum.  The  larger  portion 
of  each  body  is  embedded  in  the  cerebral  white  matter,  while  the 


BASAL  GANGLIA. 


491 


smaller  portion  projects  into  the  anterior  part  of  the  lateral  ventricle. 
A  transection  of  the  corpus  striatum  shows  that  it  is  divided  by  a 
band  of  white  matter  into  two  portions,  viz.: 

1.  The   caudate   nucleus,   the   intra- ventricular  portion,    convex   in 

shape  with  its  base  directed  forward,  its  apex  or  tail  directed 
backward  and  downward. 

2.  The  lenticular  nucleus,  the  extra-ventricular  portion,  somewhat 

biconvex  in  shape  and  embedded  largely  in  the  white  matter. 

Each    lenticular    nucleus     is     sub- 
divided   by    two    lamina    of    white 

matter  into  three  portions.    The  two 

inner,  from  their  pale  yellow  color, 

form  the  globus  pallidus;  the  outer, 

somewhat   darker    in    color,    is    the 

putamen. 
The  Internal  Capsule. — The  band 
of  white  matter  separating  the  caudate 
from  the  lenticular  nucleus  has  been 
termed  the  internal  capsule  from  the 
manner  in  which  it  embraces  the  inner 
surface  of  the  lenticular  nucleus.  It 
consists  of  nerve-fibers  which  associate 
histologically  and  physiologically  all  por- 
tions of  the  cerebral  cortex  with  the  optic 
thalamus,  pons,  medulla,  spinal  cord,  and 
cerebellum.  The  relation  of  the  capsule 
to  the  nuclei  through  which  it  passes  is 
readily  shown  on  cross-section  (Fig.  216). 
The  appearance  which  it  presents,  how- 
ever, varies  considerably  at  different  levels. 
At  a  given  level  it  may  be  said  to  con- 
sist of  two  segments  or  limbs,  an  anterior, 
situated  between  the  caudate  nucleus  and 
the  anterior  extremity  of  the  lenticular 
nucleus,  and  a  posterior,  situated  between 
the  optic  thalamus  and  the  posterior 
extremity  of  the  lenticular  nucleus.    The 

two  segments  unite  at  an  obtuse  angle,  termed  the  knee,  which  is 
directed  toward  the  median  hne. 

The  optic  thalami  are  two  oblong  masses  of  gray  matter  situated 
upon  the  crura  cerebri  and  behind  the  corpora  striata.  The  anterior 
and  posterior  extremities  of  each  thalamus  present  enlargements 
known  respectively  as  the  anterior  tubercle  and  the  posterior  tubercle 
or  pulvinar.  The  mesial  surface  of  the  thalamus  forms  the  lateral 
wall  of  the  third  ventricle  and  is  covered  by  epithehum  resting  on  a 
thin  layer  of  ependyma. 


Fig.  216. — Horizontal  Sec- 
tion OF  THE  Internal 
Capsule  showing  its 
Relations  to  the  Cau- 
date Nucleus,  Optic 
Thalamus,  and  the 
Lenticular  Nucleus,  i. 
Caudate  nucleus.  2.  An- 
terior segment  of  the  in- 
ternal capsule.  3.  Exter- 
nal capsule.  4.  Lenticu- 
lar nucleus.  5.  Claus- 
trum.  6.  Posterior  seg- 
ment of  internal  capsule. 
7.  Optic  thalamus. — 
{Modified  from  Landois.) 


492  TEXT-BOOK  OF  PHYSIOLOGY. 

A  transection  of  the  thalamus  shows  that  it  is  not  only  covered 
externally  but  penetrated  by  white  matter,  which  subdivides  its  con- 
tained gray  cells  into  four  more  or  less  distinct  masses  termed  nuclei, 
viz.,  an  anterior,  a  lateral,  occupying  the  external  part  of  the  thalamus, 
a  ventral,  close  to  the  entire  ventral  surface,  and  a  posterior,  situated 
beneath  the  pulvinar.  Beneath  and  somewhat  internal  to  each 
optic  thalamus  there  is  a  region,  the  subthalamic,  consisting  of  an 
intricate  network  of  nerve-fibers  and  several  nuclei  of  gray  matter, 
e.  g.,  the  red  or  tegmental  nucleus,  the  subthalamic  nucleus,  or  Luys' 
body,  and  the  substantia  nigra. 

Though  the  thalamus  has  extensive  connections  with  many  por- 
tions of  the  central  nerve  system,  the  most  important  are  with  the 
cortex,  the  tegmentum,  and  the  optic  tracts. 

From  the  cells  of  these  various  nuclei  axons  emerge  which  pass  into 
the  internal  capsule,  and  through  the  corona  radiata  to  all  portions 
of  the  cortex.  Those  axons  which  come  from  the  pulvinar  and  pass 
to  the  occipital  lobe  constitute  a  part  of  the  optic  radiation;  those 
from  the  lateral  and  ventral  nuclei  ultimately  reach  the  parietal  lobe; 
those  from  the  anterior  nucleus  pass  to  the  hippocampal  and  unci- 
nate convolutions.  In  a  similar  manner  all  portions  of  the  cortex  are 
brought  into  relation  with  the  thalam.us,  axons  from  the  cortical  cells 
passing  downward  to  terminate  in  tufts  around  the  thalamic  nuclei. 

The  tegmentum  is  intimately  related  to  the  thalamus,  though  the 
exact  distribution  of  various  strands  of  fibers  is  a  subject  of  much 
discussion.  Most  of  the  fibers  of  the  mesial  fillet  end  in  tufts  around 
the  cells  of  the  ventral  and  lateral  nuclei;  other  fibers  pass  directly 
to  the  cortex. 

The  optic  tract  sends  fibers  directly  into  the  pulvinar,  around  the 
cells  of  which  they  terminate  in  brush-like  expansions. 


SUMMARY  OF  THE  STRUCTURE   OF  THE  MEDULLA,  ISTHMUS, 
AND  BASAL  GANGLIA. 

Structure  of  the  Central  Gray  Matter. — Though  the  general 
arrangement  of  the  central  gray  matter  has  been  incidentally  alluded 
to  in  the  foregoing  presentation  of  the  anatomic  features  of  the 
medulla  and  isthmus,  it  will  be  convenient  to  summarize  its  arrange- 
ment and  structure  at  this  point. 

The  gray  matter  of  the  cord,  of  the  dorsal  aspect  of  the  medulla 
and  pons,  of  the  region  surrounding  the  aqueduct  of  Sylvius,  and  of  the 
fining  of  the  third  ventricle,  constitute  practically  a  continuous  system, 
though  presenting  modifications  in  various  parts  of  its  extent.  In 
the  transition  region  of  the  spinal  cord  and  medulla  the  gray  matter 
of  the  former  becomes  much  changed  in  shape  owing  to  the  shifting 
of  position  of  the  various  tracts  of  white  matter,  until  in  the  medulla 


MEDULLA  AND  BASAL  GANGLIA.  493 

and  pons  it  is  spread  out  in  the  form  of  a  thin  layer  near  tlieir  dorsal 
surfaces,  where,  together  with  the  ependyma,  it  forms  the  floor  of 
the  fourth  ventricle. 

In  the  region  of  the  aqueduct  of  Sylvius  the  gray  matter  again 
converges  and  ultimately  surrounds  the  canal,  to  again  expand  at  its 
anterior  extremity,  to  form  the  hning  of  the  third  ventricle 

The  Nerve-cells. — The  nerve-cells  in  these  different  regions  do 
not  differ  morphologically  from  those  in  the  gray  matter  of  the  spinal 
cord.  The  corpus,  or  body  of  the  cell,  presents  a  number  of  den- 
drites as  well  as  the  sharply  defined  axon.  As  a  rule,  the  cells  are 
arranged  in  groups,  or  clusters,  or  nests,  partially  surrounded  and 
enclosed  by  supporting  tissue,  and  situated  beneath  the  floor  of  the 
fourth  ventricle  and  the  floor  of  the  aqueduct  of  Sylvius.  From  some 
of  the  cell  groups  axons  pass  ventrally  through  the  white  matter  to 
emerge  on  the  ventral  and  lateral  surfaces  of  the  medulla,  pons,  and 
crura,  where  they  are  known  as  efferent  or  motor  cranial  nerves. 
From  other  groups  of  cells,  axons  cross  the  median  line,  and  after 
joining  the  mesial  fillet  ascend  toward  the  cerebrum.  Around  these 
latter  cells  the  terminal  filaments  of  the  afferent  or  sensor  cranial 
nerves  arborize.  The  collection  of  cells  formed  in  the  central  gray 
matter  may  be  divided  into  two  groups — efferent  and  afferent. 

The  efferent  cells,  like  those  of  the  cord  independent  of  a  trophic 
influence,  are  motor  in  function,  inasmuch  as  the  excitation  arising 
in  them  is  transmitted  outward  through  their  related  axons  to  mus- 
cles, glands,  or  blood-vessels,  imparting  to  them  motion,  either 
molar  or  molecular. 

The  afferent  cells  are  largely  sentient  or  receptive  in  function, 
inasmuch  as  the  excitations  brought  to  them  by  the  afferent  cranial 
nerves  from  skin  and  mucous  membranes  and  from  sense-organs, 
such  as  the  tongue  and  ear,  are  received  by  them  and  transmitted 
through  their  ascending  axons  to  the  cortex  of  the  cerebrum,  where 
they  are  translated  into  conscious  sensations. 

Structure  of  the  White  Matter.— The  white  matter  is  com- 
posed of  medullated  nerve-fibers,  and  though  arranged  in  a  very 
complex  manner  may  be  divided  into  longitudinal  and  transverse 
fibers. 

The  longitudinal  fibers  which  compose  the  main  portion  of  the 
isthmus  may  be  subdivided  into  (i)  a  ventral  or  pedal  portion  and  (2) 
a  dorsal  or  tegmental  portion.  The  fibers  constituting  the  ventral 
or  pedal  portion  may  for  convenience  be  said  to  extend  from  the 
cerebral  cortex  to  the  pons,  medulla,  and  spinal  cord.  They  may  be 
divided  into  three  distinct  tracts:  e.  g.,  the  pyramidal  tract,  the 
fronto-cerebellar  tract,  and  the  occipito-temporo-cerebellar  tract 
(Fig.  217). 

The  pyramidal  tract  descends  from  the  cortex  of  the  cerebrum 


494 


TEXT-BOOK  OF  PHYSIOLOGY. 


bordering  the  fissure  of  Rolando,  passes  through  the  posterior  one- 
third  of  the  anterior  segment  and  the  anterior  two-thirds  of  the 
posterior  segment  of  the  internal  capsule,  the  middle  two-fifths 
of  the  crusta,  behind  the  transverse  fibers  of  the  pons,  to  become 
the  anterior  pyramids  of  the  medulla,  beyond  which  it  divides  into  the 
direct  and  crossed  pyramidal  tracts  of  the  cord.  In  its  course  some 
of  the  fibers  and  their  collaterals  arborize  around  efferent  cells  from 
the  anterior  extremity  of  the  aqueduct  of  Sylvius  to  the  termination 
of  the  spinal  cord. 


Fig 


217. — Diagrammatic  Arrangement  of  the  Projection  Tracts  Connecting 
THE  Cerebral  Cortex  with  the  Lower  Nerve-centers.  A.  Fronto- 
cerebellar  tract.  B.  The  pyramidal  or  motor  tract.  C.  Sensory  tract.  D. 
Visual  tract  from  optic  thalamus  (O.T.)  to  the  occipital  lobe.  E.  Central  audi- 
tory tract.  F.  Superior  cerebellar  peduncle.  G.  Middle  cerebellar  peduncle. 
H.  Inferior  cerebellar  peduncle.  C.N.  Caudate  nucleus.  C.Q.  Corpora  quad- 
rigemina.  Vt.  Fourth  ventricle.  The  numerals  refer  to  cranial  nerves.  J. 
Eighth  nerve  nucleus. — (After  Starr.) 


The  jronto-cerehellar  tract  descends  from  the  cortex  of  the  frontal 
portion  of  the  anterior  lobe,  passes  through  the  anterior  portion  of  the 
anterior  segment  of  the  internal  capsule,  the  inner  fifth  of  the  crusta 
to  the  pons,  where  its  fibers  terminate  or  arborize  around  the  nucleus 
pontis  of  the  same  and  opposite  sides. 

The  occipito-temporo-cerebellar  tract  descends  from  the  occipital 
and  temporal  lobes,  passes  to  the  inner  side  of  the  lenticular  nucleus, 


MEDULLA  OBLONGATA  AND  ISTHMUS.  495 

and  continues  downward  on  the  outer  side  of  the  crusta,  occupying 
about  one-fifth  of  its  bulk,  to  the  pons,  wliere  its  fibers  also  arborize 
around  the  nucleus  pontis  of  the  same  and  opposite  sides.  By 
means  of  fibers  in  the  middle  peduncle  these  descending  fibers  are 
brought  into  relation  with  the  cerebellum. 

The  fibers  constituting  the  dorsal  or  tegmental  portion  of  the 
longitudinal  system  may  be  said  for  convenience  to  extend  from  the 
posterior  portion  of  the  medulla  and  pons  to  the  optic  thalamus 
and  cerebrum.  They  may  be  subdivided  into  several  tracts:  viz.,  the 
fillet,  the  posterior  longitudinal  bundle,  Gowers'  tract,  etc. 

The  fillet  or  lemniscus,  consisting  of  fibers  having  their  origin 
partly  from  the  cells  of  the  cuneate  and  gracile  nuclei  and  partly  from 
the  cells  of  the  sensor  end-nuclei  of  various  sensor  cranial  nerves, 
occupies  a  region  in  the  ventral  and  mesial  portion  of  the  tegmentum 
throughout  its  entire  extent.  Superiorly  this  mesial  fillet  divides 
into  two  portions,  one  of  which  passes  to  the  thalamus  and  pregem- 
inum  (anterior  corpus  quadrigeminum),  the  other  to  the  cortex  of 
the  parietal  and  limbic  lobes.  The  fibers  coming  from  the  sensor 
end-nucleus  of  the  auditory  nerve  (the  lateral  fillet)  lie  on  the  lateral 
aspect  of  the  pons  and  crus.  Superiorly  they  terminate  in  the  post- 
geminum  (the  posterior  corpus  quadrigeminum). 

The  posterior  longitudinal  bundle,  an  upward  extension  of  the 
fibers  composing  a  portion  of  the  ground  bundle  of  the  spinal  cord, 
is  located  on  either  side  of  the  median  line  just  beneath  the  floor 
of  the  fourth  ventricle  and  the  aqueduct  of  Sylvius.  As  it  passes 
upward  collateral  branches  are  given  off,  some  of  which  arborize 
around  the  cell  nuclei  of  the  third,  fourth,  and  sixth  cranial  nerves 
of  the  same  side,  while  others  cross  the  median  line  and  arborize 
around  the  corresponding  cell  nuclei  of  the  opposite  side.  Superi- 
orly some  of  the  fibers  become  related  to  cells  in  the  thalamus  and 
subthalamic  region.  This  bundle  of  fibers  appears  to  be  mainly 
commissural  in  character. 

Gowers'  tract,  the  antero-lateral  tract  of  the  spinal  cord,  occupies 
a  position  in  the  lateral  region  of  the  formatio  reticularis  both  in  the 
medulla  and  pons.  Continuing  upward,  it  enters  the  mesial  fillet, 
and  in  company  with  it  passes  through  the  posterior  division  of  the 
internal  capsule  and  finally  terminates  around  cells  in  the  cortex  of 
the  parietal  lobe. 

The  transverse  fibers  of  the  isthmus  are  found  in  the  pons.  The 
fibers  of  the  ventral  as  well  as  those  of  the  more  dorsal  regions  have 
their  origin  in  nerve-cells  in  the  cortex  of  the  cerebellum.  From 
their  origin  they  pass  through  the  cerebellar  white  matter,  and  through 
the  middle  peduncle  as  far  as  the  median  line,  where  they  decussate 
with  fibers  coming  from  the  opposite  side.  Beyond  this  point  they 
pass  to  the  cerebellar  cortex.     From  their  anatomic  relations  it  is  prob- 


496  TEXT-BOOK  OF  PHYSIOLOGY. 

able  that  these  transverse  fibers  are  commissural  in  character,  bring- 
ing into  relation  opposite  but  corresponding  regions  of  the  cerebellar 
cortex.  In  addition  to  the  commissural  fibers  other  transverse  fibers 
associate  the  cerebellar  cortex  with  the  gray  matter  in  the  pons  on 
both  the  same  and  opposite  sides.  In  this  way  the  cerebellum  is 
brought  into  relation  with  longitudinal  fibers  coming  from  and  going 
to  the  cerebrum. 

FUNCTIONS  OF  THE  MEDULLA  OBLONGATA,  ISTHMUS,  AND 
BASAL  GANGLIA. 

Microscopic  examination  of  the  white  and  gray  matter  of  these 
various  parts  of  the  central  nerve  system  shows  that  they  are  com- 
posed of  nerve-cells  and  nerve-fibers  which  morphologically  do  not 
differ  in  essential  respects  from  those  found  in  the  spinal  cord,  though 
their  arrangement  is  far  more  complicated  and  involved.  The  func- 
tions of  these  closely  related  structures  are  in  consequence  equally 
complex  and  involved  and  but  imperfectly  known. 

In  a  general  way  it  may  be  said  that  by  virtue  of  the  presence 
of  nerve-cells  and  definite  tracts  of  nerve-fibers  these  structures  col- 
lectively may  be  regarded  as  consisting: 

1.  Of  centers  for  reflex  actions;  and — 

2.  Of  conducting  paths  by  which  the  various  parts  are  brought  into 

relation  one  with  another  and  with  the  spinal  cord,  the  cerebel- 
lum, and  the  cerebrum. 

The  Medulla  Oblongata  and  Pons. — The  gray  matter  situated 
in  these  structures — i.  e.,  just  beneath  the  floor  of  the  fourth  ventricle 
— contains  nerve-cells  arranged  in  more  or  less  well-defined  groups 
which  may  be  divided  into  efjerent  and  afferent. 

The  efferent  cells  are  the  immediate  sources  of  energy  which 
is  transmitted  through  efferent  axons  to  various  peripheral  organs — 
muscles,  glands,  and  blood-vessels.  Their  activity  may  be  excited 
by  the  same  influences  which  excite  the  efferent  cells  of  the  spinal 
cord:  e.  g.,  variations  in  the  composition  of  the  blood  or  lymph;  the 
arrival  of  nerve  energy  coming  through  afferent  pathways  in  the 
spinal  cord  and  through  afferent  cranial  nerves;  the  arrival  of  nerve 
energy  coming  through  efferent  pathways  from  the  cerebrum.  The 
peripheral  activity  resulting  from  their  excitation  may  therefore  be 
automatic  or  autochthonic,  peripheral  (reflex)  or  cerebral  (volitional) 
in  origin. 

The  afferent  cells  are  sentient  or  receptive  in  function,  inasmuch 
as  they  receive  nerve  energies  coming  through  lower  afferent  pathways 
and  transmit  them  through  their  related  axons  to  the  cortex  of  the 
cerebrum,  where  they  are  translated  into  conscious  sensations.         N'- 

The  efferent  cells  give  origin  to  nerve-fibers  which  pass  ventrally 
and  become  the  efferent  or  motor  cranial  nerves. 


FUNCTIONS  OF  THE  MEDULLA  OBLONGATA.        497 

The  afferent  cells  give  origin  to  fibers  which  pass  to  the  cerebral 
cortex.  Around  both  groups  of  cells,  the  afferent  or  sensor  cranial 
nerves  terminate  in  tuft-like  expansions.  In  a  subsequent  section  the 
origin,  course,  and  distribution  of  the  various  cranial  nerves  will  be 
considered.  But  as  the  function  of  the  nerve  is  but  to  transmit 
energy  from  the  cell  of  which  it  constitutes  a  part,  the  function 
ascribed  to  it  may  without  impropriety  be  transferred  to  the  cell 
itself. 

Since  it  is  by  means  of  nerve-cells  and  their  associated  fibers  that 
many  important  functions  of  organic  life  are  initiated  and  maintained, 
it  would  naturally  be  expected  from  its  extensive  nerve  connections 
that  this  region  of  the  nerve  system  plays  an  extensive  role  in  this 
respect.  As  the  accomplishment  of  these  functions  requires  the 
cooperation  and  coordination  of  a  number  of  separate  but  related 
structures,  it  is  evident  that  there  must  exist  in  the  medulla  and  pons 
a  number  of  coordinating  mechanisms  consisting  of  nerve-cells  and 
nerve-fibers  which  are  associated  in  various  ways  for  the  accomplish- 
ment of  definite  functions.  To  such  a  coordinating  mechanism  the 
term  "center"  has  been  given:  e.  g.,  respiratory,  cardiac,  deglutitory, 
etc.* 

As  centers  for  reflex  activities.  Experimentation  has  shown 
that  the  medulla  and  pons  contain  a  number  of  such  centers,  the 
more  important  of  which  are  as  follows : 

1.  A  cardiac  center,  which  exerts  (i)  an  accelerator  influence  over  the 

heart's  pulsations  through  nerve-fibers  emerging  from  the  spinal 
cord  in  the  roots  of  the  first  and  second  dorsal  nerves  and  reach- 
ing the  heart  through  the  sympathetic  nerve;  (2)  an  inhihiior 
or  retarding  influence  on  the  action  of  the  heart  through  efferent 
fibers  in  the  trunk  of  the  pneumogastric  nerve.     (See  page  303.) 

2.  A  vaso- motor  center,  which  regulates  the  cahber  of  the  blood- 

vessels throughout  the  body  in  accordance  with  the  needs  of  the 
organs  and  tissues  for  blood,  through  nerve-fibers  passing  by 
way  of  the  spinal  nerves  to  the  walls  of  the  blood-vessels.  (See 
page  342.) 

3.  A  respiratory  center,  which  coordinates  the  muscles  concerned  in 

the    production    of    the    respiratory    movements,     (See    page 

397-) 

4.  A  mastication  center,  which  excites  to  activity  and  coordinates  the 

muscles  of  mastication.     (See  page  160.) 

5.  A  deglutition  center,  which  excites  and  coordinates  the  muscles 

*  By  the  term  center  as  here  employed  is  meant  a  collection  of  nerve-cells  and 
nerve-fibers  occupying  an  area  of  greater  or  less  extent,  though  its  exact  anatomic 
limits  may  not  be  accurately  defined.  That  an  area  may  merit  the  term  center,  it 
is  necessary  that  its  stimulation  should  increase,  its  destruction  should  abolish  or 
impair,  functional  activity. 
32 


498  TEXT-BOOK  OF  PHYSIOLOGY. 

concerned  in  the  transference  of  the  food  from  the  mouth  to 
the  stomach.     (See  page  179.) 
6.  An  articulation  center,  which  coordinates  the  muscles  necessary  to 
the  production  of  articulate  speech. 

In  addition,  the  gray  matter  contains  centers  which  influence  the 
secretion  of  saliva,  provoke  vomiting,  coordinate  the  muscles  of  the 
face  concerned  in  expression,  and  control  the  secretion  of  the  per- 
spiration. 

As  conducting  pathways.  The  anterior  pyramids  of  the  medulla 
and  their  continuations  through  the  more  ventral  portions  of  the 
pons,  being  portions  of  the  general  pyramidal  tract,  serve  to  conduct 
volitional  efferent  nerve  impulses  from  higher  portions  of  the  brain 
to  the  spinal  cord.  Division  of  either  of  these  pathways  is  at  once 
followed  by  a  loss  of  volitional  control  of  the  muscles  below  the 
section. 

The  dorsal  or  tegmental  portion,  containing  the  fillet  and  Gowers' 
tract,  serves  to  transmit  afferent  nerve  impulses  from  the  spinal 
cord  to  higher  portions  of  the  brain.  Transverse  division  of  one-half 
of  the  dorsal  portion  of  the  pons  is  followed  by  complete  anesthesia 
of  the  opposite  half  of  the  body  without  any  impairment  of  motion. 

The  restiform  bodies  constitute  a  pathway  between  the  spinal  cord 
and  the  cerebellum.  The  transverse  fibers  of  the  pons  associate 
opposite  but  corresponding  portions  of  the  cerebellar  hemispheres. 

The  Crura  Cerebri. — The  crura  cerebri  consist  ventrally  of  fibers 
which  are  largely  derived  from  the  pyramidal  tracts  and  are  con- 
tinuous with  the  longitudinal  fibers  of  the  ventral  portion  of  the  pons 
and  medulla;  and  dorsally  of  fibers  continuous  with  those  coming 
through  the  lower  portions  of  the  tegmentum.  Hence  they  are  con- 
ductors of  motor  impulses  in  the  former  and  of  sensor  impulses 
in  the  latter  region.  It  is  not  definitely  known  as  to  whether  reflex 
actions  take  place  through  the  gray  matter,  the  locus  niger,  or  not. 

The  gray  matter  beneath  the  aqueduct  of  Sylvius  contains  nerve- 
cell  groups  which  are  centers  for  reflex  actions  in  connection  with 
ocular  movements:  e.  g.,  closure  of  the  lids,  contraction  of  the  sphinc- 
ter pupillas,  convergence  of  the  eyes,  etc. 

The  Corpora  Quadrigemina. — From  the  anatomic  relation  of 
the  anterior  quadrigeminal  body  (the  pregeminum)  to  the  optic  tract, 
on  the  one  hand,  and  to  the  optic  radiation,  on  the  other,  the  in- 
ference can  be  drawn  that  it  is  in  some  way  essential  to  the  per- 
formance of  the  visual  process.  Experimental  investigations  and 
pathologic  changes  support  the  inference. 

Irritation  of  the  pregeminum  in  monkeys  on  one  side  is  followed 
by  dilatation  of  the  pupils  first  on  the  opposite  side  and  then  almost 
immediately  on  the  same  side.  The  eyes  at  the  same  time  are  also 
widely  opened  and  the  eyeballs  turned  upward  and  to  the  opposite 


FUNCTIONS  OF  THE  BASAL  GANGLIA. 


499 


side.  If  the  irritation  be  continued,  motor  reactions  are  exhibited 
in  various  parts  of  the  body.  Destruction  of  the  pregeminum  in  both 
monkeys  and  rabbits  is  followed  by  bhndness,  dilatation  and  immo- 
bility of  the  pupils,  with  marked  disturbance  of  equihbrium  and 
locomotion  (Ferrier). 

From  the  anatomic  relation  of  the  posterior  quadrigeminal  body 
(the  postgeminum)  to  the  lateral  fillet,  the  basal  tract  for  hearing, 
the  inference  may  be  drawn  that  it  is  in  some  way  connected  with  the 
auditory  process. 

Stimulation  of  the  postgeminum  gives  rise  to  cries  and  various 
forms  of  vocalization. 
Pathologic  states  of  this 
body  are  also  attended  by 
impairment  of  hearing 
and  disorders  of  the 
equilibrium. 

From  the  foregoing 
facts  it  is  probable  that 
the  corpora  quadrigem- 
ina  are  associated  with 
station  and  locomotion. 
Ferrier  assumes  that  in 
these  bodies  "sensory 
impressions,  retinal  and 
others,  are  coordinated 
with  adaptive  motor  re- 
actions such  as  are  in- 
volved in  equilibration 
and  locomotion." 

The  Corpora  Striata. 
— The  relation  of  these 
bodies  to  the  pyramidal 
motor  tract  would  indi- 
cate that  they  are  in 
some  way  connected  with 
motor  activities.  Their 
function,  however,  is  ob- 
scure. While  stimulation  of  one  corpus  produces  convulsion  of  the 
muscles  of  the  opposite  side  of  the  body,  and  destruction  gives  rise 
to  paralysis  of  the  corresponding  muscles,  it  is  difficult,  owing  to  the 
intimate  association  of  the  white  and  the  gray  matter,  to  state  to 
which  the  phenomena  are  to  be  attributed.  The  evidence  at  hand 
points  to  the  conclusion  that  if  a  lesion  is  limited  to  the  gray  matter 
the  paralysis  which  might  resuk  would  be  but  temporary  and  of  short 
duration.    The  pathologic  evidence  is  of  a  similar  character.    Gowers 


Fig.  218. — Horizontal  Section  of  the  Internal 
Capsule  Showing  the  Position  and  Rela- 
tion OF  THE  Motor  Tracts  for  the  Eye, 
Head  (Hd.),  Tongue  (Tg.),  Mouth  (Mth.), 
Shoulder  (Shi.),  Elbow  (Elb.),  Digits  of 
Hand  (Dig.),  Abdomen  (Abd.),  Hip,  Knee 
(Kn.),  Digits  of  Foot  (Dig.).  S.  Sensor 
tract.  O.  T.  Optic  tract.  A.  T.  Auditory 
tract.  I.  Caudate  nucleus.  2.  Anterior  seg- 
ment of  internal  capsule.  3.  E.xternal  capsule 
4.  Island  of  Reil.  5.  Lenticular  nucleus.  6. 
Claustrum.  7.  Posterior  segment  of  internal 
capsule. — {Modified  from  Landois.) 


500 


TEXT-BOOK  OF  PHYSIOLOGY. 


is  of  the  opinion,  that  if  the  lesion  is  small  and  at  a  sufficient  distance 
from  the  white  fibers  of  the  capsule,  there  may  even  be  no  initial 
hemiplegia;  neither  motor  nor  sensory  paralysis  will  arise  if  the  lesion 
is  confined  to  the  gray  matter. 

It  is  stated  by  some  experimenters  that  localized  injuries,  both 
experimental  and  pathologic,  are  followed  by  a  persistent  rise  of 
temperature,  varying  from  i°  to  2.6°  C. 

The  Optic  Thalami. — From  the  anatomic  relation  of  the  optic 
thalami  to  the  general  and  special  sense  nerve-tracts,  on  the  one 
hand,  and  to  the  cerebral  cortex,  on  the  other  hand,  it  is  assumed 

that  they  are  connected 
with  the  production  of 
sensations  both  general 
and  special,  and  act  as 
intermediates  between 
the  peripheral  sense- 
organs  and  the  cortex. 

The  results  of  ex- 
perimental stimulation 
and  destruction  of  the 
thalami  are  extremely 
contradictory  and  fail  to 
throw  much  light  on 
their  functions.  Ferrier 
states  that  destruction 
of  the  posterior  part  of 
one  thalamus  produced 
blindness  in  the  opposite 
eye  and  impairment  of 
the  sense  of  touch  and 
pain  in  the  opposite  side 
of  the  body.  In  a  pa- 
tient under  the  care  of 
Hughhngs-Jackson  there 
was  blindness  in  the 
right  half  of  each  eye, 
loss  of  hearing  in  the  left 
ear,  impairment  of  taste  on  the  left  side  of  the  tongue,  and  a  diminu- 
tion of  the  sense  of  touch  on  the  left  side  of  the  body.  Postmortem 
examination  showed  a  patch  of  softening  in  the  posterior  part  of  the 
right  thalamus,  the  remainder  of  the  organ  being  normal. 

It  is  probable  that  in  the  thalamus  visual,  tactile,  and  labyrinthine 
impressions  are  received,  coordinated,  and  reflected  outward,  with 
the  result  of  producing  various  adaptive  motor  reactions  connected 
with  station  and  equilibrium.    It  is  also  beheved  by  some  investigators 


Fig.  219. — Vertical  Section  Through  the 
Right  Cerebral  Hemisphere  in  Front  of 
the  Gray  Commissure,  i.  Caudate  nucleus. 
2.  Corpus  callosum.  3.  Pillars  of  the  fornix. 
4.  Internal  capsule.  5.  Optic  thalamus.  6. 
Gray  commissure.  7.  External  capsule.  8. 
Claustrum. — {Landois.) 


FUNCTIONS  OF  THE  INTERNAL  CAPSULE.  501 

to  act  as  an  intermediate  between  emotional  states  and  their  expres- 
sion in  the  muscles  of  the  face,  this  power  being  lost  in  certain  patho- 
logic conditions.  The  power  of  regulating  the  temperature  of  the 
body  has  also  been  assigned  to  the  thalamus,  as  destruction  of  its 
anterior  extremity  is  usually  followed  by  a  rise  in  temperature. 

The  Internal  Capsule. — The  internal  capsule  has  been  shown  by 
the  results  both  of  experiment  and  of  pathologic  processes  to  be,  first, 
a  pathway  for  the  transmission  of  nerve  impulses  from  the  cerebral 
cortex  to  the  pons,  medulla,  and  spinal  cord,  which  give  rise  to 
contraction  of  the  muscles  of  the  opposite  side  of  the  body;  and, 
second,  a  pathway  for  the  transmission  of  nerve  impulses  coming 
from  skin,  mucous  membrane,  muscles,  and  special  sense-organs  to 
the  cortex,  where  they  give  rise  to  sensations  general  and  special. 
It  is  therefore  the  common  motor  and  sensor  pathway.  For  the 
reason  that  it  transmits  both  motor  and  sensor  impulses,  and  for 
the  further  reason  that  it  is  frequently  the  seat  of  pathologic  lesions 
which  are  followed  by  either  a  loss  of  motion  or  sensation  or  both, 
the  internal  capsule  is  one  of  the  most  important  parts  of  the  central 
nerve  system.  As  shown  in  Fig.  218,  it  consists  of  two  segments  or 
hmbs  united  at  an  obtuse  angle,  the  knee  or  elbow,  which  is  directed 
toward  the  median  line.  The  motor  tract  is  confined  to  the  posterior 
one-third  of  the  anterior  segment  and  the  anterior  two-thirds  of  the 
posterior  segment.  The  sensor  tract  is  confined  to  the  posterior 
one-third  of  the  posterior  segment,  the  extreme  end  of  which  also 
contains  the  optic  and  auditory  tracts. 

The  regioh  of  the  anterior  segment  in  front  of  the  motor  tract 
contains  the  fibers  of  the  fronto-ccrebellar  tract,  the  function  of 
which  is  unknown. 

The  motor  region  contains  fibers  which  descend  from  the  cerebral 
cortex  to  nerve-centers  situated  in  the  gray  matter  beneath  the 
aqueduct  of  Sylvius,  in  the  gray  matter  beneath  the  floor  of  the 
fourth  ventricle,  and  in  the  anterior  horns  of  the  gray  matter  of  the 
spinal  cord,  and  which  in  turn  are  connected  by  the  cranial  and 
spinal  nerves  with  the  muscles  of  the  eye,  head,  face,  trunk,  and 
hmbs.  The  positions  occupied  by  these  dift'erent  tracts  are  shown  in 
Fig.  218. 

The  relation  of  the  internal  capsule  to  the  caudate  nucleus  and 
the  optic  thalamus  internally,  and  to  the  lenticular  nucleus  exter- 
nally, is  also  shown  in  a  vertical  section  of  the  cerebrum  made  in 
front  of  the  gray  commissure  (Fig.  219).  From  the  fact  that  the 
internal  capsule  contains  efferent  or  motor  tracts,  and  afferent  or 
sensor  tracts,  it  is  evident  that  a  destructive  lesion  of  the  motor  tract 
would  be  followed  by  a  loss  of  motion;  and  of  the  sensor  tract,  by 
a  loss  of  sensation  on  the  opposite  side  of  the  body. 


CHAPTER  XIX. 
THE  CEREBRUM. 

The  cerebrum  is  the  largest  portion  of  the  encephalon,  constitut- 
ing about  85  per  cent,  of  its  total  weight.  In  shape  it  is  ovate,  convex 
on  its  outer  surface,  narrow  in  front  and  broad  behind.  It  is  divided 
by  a  deep  longitudinal  cleft  or  fissure  into  halves,  known  as  the 
cerebral  hemispheres.  The  hemispheres  are  completely  separated 
anteriorly  and  posteriorly  by  this  fissure,  but  in  their  middle  portions 
are  united  by  a  broad  white  band,  the  corpus  callosum.  Each 
hemisphere  or  hemi-cerebrum  is  convex  on  its  outer  aspect,  and 
corresponds  in  a  general  way  with  the  cavity  of  the  skull;  the  inner 
or  mesial  surface  is  fiat  and  forms  the  lateral  boundary  of  the  longi- 
tudinal fissure. 

The  surface  of  each  hemi-cerebrum  presents  a  series  of  alternate 
indentations  and  elevations,  known  respectively  as  fissures  or  sulci, 
and  convolutions  or  gyres.  A  knowledge  of  the  situation  and  extent 
of  the  principal  fissures  and  convolutions,  as  well  as  of  their  relation 
one  to  another,  is  essential  to  a  clear  understanding  of  many  phys- 
iologic processes,  clinical  phenomena,  and  surgical  procedures. 
The  general  arrangement  of  the  primary  fissures  and  convolutions 
is  represented  in  Figs.  220  and  221. 

Fissures. — 

1.  The  fissure  0}  Sylvius,  one  of  the  most  important  of  the  primary 

fissures,  is  found  on  the  side  of  the  cerebrum.  It  begins  at  the 
base  and  extends  upward,  outward,  and  backward  to  a  point 
corresponding  to  the  eminence  of  the  parietal  bone,  where  it 
usually  terminates.  Anteriorly  a  short  branch  is  given  of? 
which  passes  upward  and  forward  into  the  frontal  lobe.  The 
Sylvian  fissure  is  the  first  to  appear  in  the  development 
of  the  fetal  brain,  becoming  visible  at  the  third  month.  In  the 
adult  it  is  deep  and  well  marked  and  divides  the  hemi-cerebrum 
into  a  frontal  and  a  temporo-sphenoidal  lobe. 

2.  The  fissure  0}  Rolando,  or  central  fissure,  equally  important,  is 

found  on  the  superior  and  lateral  aspects  of  the  cerebrum.  It 
runs  from  a  point  on  the  convexity  of  the  hemisphere  near  the 
median  line  transversely  outward  and  downward  toward  the 
fissure  of  Sylvius,  but  as  a  rule  does  not  pass  into  it.  It  divides 
the  frontal  from  the  parietal   lobe.      The   inchnation  of    the 

502 


CEREBRUM. 


503 


central  fissure  is  such  as  to  form  with  the  longitudinal  fissure 
an  angle  of  about  67  degrees. 
The  intra- parietal  fissure  arises  a  short  distance  behind  the  central 
fissure.  It  then  runs  upward,  backward,  and  downward  to 
terminate  near  the  posterior  extremity  of  the  hemisphere.  It 
divides  the  parietal  lobe  into  a  superior  and  an  inferior  portion. 


Fig.  220. — Diagram  Showing  Fissures  and  Convolutions  of  the  Left  Side  of 
THE  Human  Brain.  F.  Frontal.  P.  Parietal.  O.  Occipital.  T.  Temporo- 
sphenoidal  lobe.  S.  Fissure  of  Sylvius.  S'.  Horizontal.  S".  Ascending  ramus 
of  S.  c.  Sulcus  centralis,  or  fissure  of  Rolando.  A.  .A^scending  frontal,  and  B. 
Ascending  parietal,  convolution.  Fj.  Superior,  F,.  Middle,  and  F,.  Inferior 
frontal  convolutions,  f,.  Superior,  fj.  Inferior,  frontal  fissures,  fs.  Sulcus  prje- 
centralis.  P.  Superior  parietal  lobule.  P,.  Inferior  parietal  lobule,  consisting  of 
Pj.  Supramarginal  gyrus,  and  Pj'.  Angular  gyrus,  ip.  Sulcus  interparietalis. 
cm.  Termination  of  callosomarginal  fissure.  O,.  First,  O2.  Second,  O3.  Third, 
occipital  convolutions,  po.  Parieto-occipital  fissure,  o.  Transverse  occipital 
fissure.  02-  Inferior  longitudinal  occipital  fissure.  Tj.  First,  Tj.  Second,  T3. 
Temporo-sphenoidal,  convolutions.  /,.  First,  (2.  Second,  temporo-sphenoida] 
fissures. — {Landois'  ^'Physiology,"  after  Ecker.) 

4.  The  parieto-occipital  fissure,  situated  on  the  mesial  surface  of  the 
hemisphere,  divides  the  latter  into  a  parietal  and  an  occipital 
lobe.  It  begins  as  a  deep  notch  on  the  surface  of  the  hemisphere, 
and  is  then  continued  downward  and  forward  until  it  enters  the 
calcarine  fissure. 


504 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  calcarine  fissure  begins  on  the  posterior  extremity  of  the 
mesial  surface  of  the  occipital  lobe.  From  this  point  it  passes 
downward  and  forward  to  unite  with  the  parieto-occipital 
fissure. 

The  calloso-marginal  fissure  is  a  deep  cleft  on  the  mesial  surface 
of  the  hemisphere.  It  begins  below  the  anterior  extremity  of  the 
corpus  callosum  and  in  a  general  way  follows  the  course  of  this 
structure  as  far  as  its  posterior  extremity,  where  it  turns  upward 
to  terminate  at  the  margin  of  the  hemisphere  just  posterior  to 
the  fissure  of  Rolando. 


P'iG.  221. — Diagram  Showing  Fissures  and  Convolutions  on  Mesial  Aspect 
OF  THE  Right  Hemisphere.  Median  aspect  of  the  right  hemisphere.  CC. 
Corpus  callosum  divided  longitudinally.  Gf.  Gyrus  fornicatus.  H.  Gyrus  hip- 
pocampi, h.  Sulcus  hippocampi.  U.  Uncinate  gyrus,  cm.  Calloso-marginal 
fissure.  F.  First  frontal  convolution,  c.  Terminal  portion  of  fissure  of  Rolando. 
A.  Ascending  frontal,  B.  Ascending  parietal,  convolution  and  paracentral  lobule. 
P/.  Precuneus  or  quadrate  lobule.  Oz.  Cuneus.  Po.  Parieto-occipital  fissure. 
Op  Transverse  occipital  fissure,  oc.  Calcarine  fissure,  oc'.  Superior,  oc".  Inferior, 
ramus  of  the  same.  D.  Gyrus  descendens.  T4.  Gyrus  occipitotemporalis  lateralis 
(lobulus  fusiformis).  T5.  Gyrus  occipitotemporalis  medialis  (lobulus  lingualis). — 
(Ecker.) 


Secondary  fissures  of  more  or  less  importance  are  present  in  the 
different  lobes,  subdividing  the  surface  into  convolutions:  e.  g.,  in  the 
frontal  lobe  are  found  the  pre-central,  the  superior  and  middle  frontal 
fissures;  in  the  temporo-sphenoidal  lobe  the  superior  and  injerior 
or  the  first  and  second  temporo-sphenoidal  fissures;  in  the  occipital 
lobe,  the  transverse  and  inferior  longitudinal  fissures. 

Convolutions. — The  convolutions  or  gyres  are  the  portions  of 
the  cerebral  surface  comprised  between  the  fissures.     The  arrange- 


CEREBRUM.  505 

ment  of  the  surface  is  such  that  only  the  more  superficial  portions  are 
visible.  The  depth  of  the  convolution,  the  portion  bordering  the 
fissure,  is  concealed  from  view.  Each  lobe  presents  a  series  of  such 
convolutions,  which  differ  considerably  in  their  relative  physiologic 
importance. 

The  Frontal  Lobe. — The  frontal  lobe  presents  on  its  convex 
surface  four  convolutions:  viz.,  the  anterior  or  pre-central  convolution, 
and  the  superior,  middle,  and  inferior  frontal  convolutions. 

1.  The  anterior  or  pre-central  convohition  is  situated  just  in  front  of 

the  Rolandic  or  central  fissure,  with  which  it  corresponds  in 
direction.  It  is  continuous  above  with  the  superior  frontal  and 
below  with  the  inferior  frontal  convolution. 

2.  The  superior  jrontal  convolution  is  bounded  internally  by  the 

longitudinal  fissure  and  externally  by  the  superior  frontal  fissure. 
From  the  upper  end  of  the  pre-central  convolution,  with  which 
it  is  continuous,  it  runs  forward  and  downward  to  the  anterior 
extremity  of  the  frontal  lobe,  where  it  turns  backward  and  rests 
on  the  orbital  plate  of  the  frontal  bone. 

3.  The  middle  jrontal  convolution  is  situated  on  the  side  of  the  lobe, 

between  the  superior  frontal  fissure  above  and  the  middle  frontal 
fissure  below.     Its  general  direction  is  downward  and  forward. 

4.  The    inferior   jrontal   convohition   winds    around   the   ascending 

branch  of  the  fissure  of  Sylvius  in  the  anterior  and  inferior  por- 
tion of   the  cerebrum.     It  is  continuous  posteriorly  with  the 
lower  end  of  the  pre-central  convolution. 
The   Parietal   Lobe. — The  parietal  lobe   presents   three  well- 
marked  convolutions:  viz.,  the  posterior  or  post-central  convolution, 
and  the  superior  and  inferior  parietal. 

1.  The  posterior  or  post-central  convolution  is  situated  just  behind 

the  Rolandic  or  central  fissure,  with  which  it  corresponds  in 
direction.  Above,  it  is  continuous  with  the  superior  parietal 
convolution;  below,  with  the  inferior  parietal  and  the  pre-central 
convolutions. 

2.  The  superior  parietal  convolution  is  bounded  internally  by  the 

longitudinal  fissure  and  externally  by  the  intra-parietal  fissure. 
From  the  upper  end  of  the  post-central  convolution,  with  which 
it  is  connected,  it  runs  downward  and  backward  as  far  as  the 
parieto-occipital  fissure. 

3.  The  inferior  parietal  convolution  is  connected  anteriorly  with  the 

post-central  convolution.  Passing  backward,  it  winds  around 
the  superior  extremity  of  the  fissure  of  Sylvius,  in  which  situa- 
tion it  is  known  as  the  supra-marginal  convolution.  Beyond 
this  point  it  divides  into  two  portions,  one  of  which  runs  forward 
into  the  temporal  lobe  above  the  first  temporal  fissure,  while 
the  other  runs  downward  and  backward,  following  the  intra- 


5o6  TEXT-BOOK  OF  PHYSIOLOGY. 

parietal  fissure  to  its  termination.     At  this  point  it  makes  a  sharp 
bend  and  runs  forward  into  the  temporal  lobe  just  beneath  the 
first  temporal  fissure.     In  the  neighborhood  of  the  bend  it  is 
generally  known  as  the  angular  convolution  or  gyrus. 
The    Temporo-sphenoidal     Lobe. — The    temporo-sphenoidal 
lobe  presents  on  its  external  surface  three  well-marked  convolutions: 
viz.,-  the  superior,  the  middle,  and  the  inferior  temporal,  separated  by 
the  first  and  second  temporal  fissures.     These  three  convolutions  are 
in  a  general  way  parallel  with  each  other,  and  pursue  a  direction 
from  before  backward  and  upward.     Anteriorly,  they  are  fused  to- 
gether, but  posteriorly  their   connections    are  somewhat  different. 
The  superior  temporal  is  continuous  behind  and  above  with  the 
supra-marginal  convolution,  and  behind  and  below  with  the  angular 
convolution  or  gyre.     The  middle  temporal  blends  with  the  preceding 
and  with  the  middle  occipital.     The  inferior  temporal  is  continuous 
with  the  inferior  occipital. 

The  Occipital  Lobe. — The  occipital  lobe  is  triangular  in  shape 
and  forms  the  posterior  apex  of  the  hemisphere.  Its  base  on  the 
external  surface  is  formed  by  an  imaginary  line  drawn  from  the 
parieto-occipital  fissure  to  the  pre-occipital  notch  on  the  inferior  and 
lateral  border.  The  external  surface  presents  three  convolutions — 
the  superior,  middle,  and  inferior  occipital. 

The  inner  or  mesial  surface  of  the  hemisphere,  formed  in  part 
by  the  frontal,  the  parietal,  the  occipital,  and  the  temporal  lobes,  pre- 
sents several  convolutions  of  much  physiologic  interest,  viz. : 

1.  The    gyrus    fornicatus,    situated    between    the    calloso-marginal 

fissure  and  the  corpus  callosum.  From  its  origin  anteriorly 
at  the  base  of  the  brain  this  convolution  passes  backward, 
gradually  increasing  in  width  as  it  approaches  the  posterior 
extremity  of  the  corpus  callosum.  At  this  point  it  again  narrows 
and  descends  between  the  calcarine  and  hippocampal  fissures 
to  blend  with  the  hippocampal  convolution. 

2.  The  gyrus  hippocampus,  formed  by  the  union  of  the  posterior 

extremity  of  the  gyrus  fornicatus  and  the  median  occipito-tem- 
poral  convolution  (the  Hngual  lobule),  is  situated  just  below 
the  dentate  or  hippocampal  fissure.  Anteriorly  it  becomes 
enlarged,  and  just  behind  the  apex  of  the  temporal  lobe  turns 
backward  and  inward  to  form  a  hook-shaped  eminence,  the 
uncinate  gyrus  or  uncus. 

The  limbic  lobe  is  the  name  given  to  an  area  of  the  brain 
which  includes,  among  other  structures,  the  gyrus  fornicatus,  the 
gyrus  hippocampus,  and  the  uncus.  As  forming  a  part  of  this 
general  lobe  may  be  mentioned  the  dentate  fascia,  the  striae  and 
peduncle  of  the  corpus  callosum,  the  septum  lucidum,  the 
fornix,  and  the  infracallosal  gyrus. 


CEREBRUINI. 


507 


3.  The  temporo-occipiial  gyrus  is  bounded  by  the  cohateral  fissure 

above,  and  its  inferior  border  extends  from  the  occipital  lobe  to 
the  anterior  pole  of  the  temporal  lobe. 

4.  The  quadrate   lobule,   a  square-shaped    convolution,   is    situated 

between  the  posterior  termination  of  the  calloso-marginal  fis- 
sure and  the  parieto-occipital  fissure.  It  blends  with  the  gyrus 
fornicatus,  on  the  one  hand,  and  with  the  parietal  lobule  on  the 
other. 

5.  The  ameiis,  a  triangular  or  wedge-shaped  convolution  or  lobule, 

is  situated  on  the  mesial  surface  of  the  occipital  lobe  between  the 

parieto-occipital  and  calcarine  fissures. 
The  Insula  or  Island  of  Reil. — This  anatomic  structure  con- 
sists of  a  triangular  shaped  cluster  of  six  small  convolutions  situated 
at  the  bifurcation  of  the  Sylvian  fissure  and  concealed  from  view 
by  the  convolutions  bordering  it,  spoken  of  collectively  as  the  oper- 
culum. These  convolutions  are  connected  with  the  frontal,  the 
parietal,  and  the  temporal  lobes. 

Structure  of  the  Gray  Matter  or  the  Cortex. — The  gray  matter, 
the  cortex  of  the  cerebrum,  varies  from  two  to  four  millimeters  in 
thickness.  When  examined  with  a  lens  of  low  power,  it  presents  a 
laminated  appearance,  due  to  differences  in  color  and  arrangement 
of  its  constituent  elements.  With  higher  magnification  the  cortex 
is  seen  to  consist  of  neuroglia  cells,  nerve-cells  with  specialized 
dendrites  and  axons,  medullated  and  non-medullated  nerve-fibers, 
blood-vessels,  connective  tissue,  etc., — all  of  which  are  arranged  and 
interblended  in  a  most  intricate  manner.  Notwithstanding  the  com- 
plexity of  its  structure,  modern  histologic  methods  have  enabled 
Cajal  to  divide  it  into  four  fairly  distinct  layers  or  zones,  from  without 
inward,  as  follows  (Fig.  222): 

1.  The  Molecular  Layer. — The  most  superficial  portion  of  this  layer 

consists  mainly  of  neuroglia  or  glia  cells,  the  processes  of  which 
interlace  in  all  directions,  forming  a  distinct  sheath  just  beneath 
the  pia.  The  deeper  portions  of  this  layer  contain  a  specialized 
type  of  nerve-cell  (Cajal  cells),  of  which  there  are  several  varie- 
ties. These  cells  give  off  nerve-fibers  which  pursue  a  horizontal 
direction  for  a  variable  distance,  but  in  their  course  give  off 
collateral  branches  which  ascend  to  the  outer  surface  of  the 
layer.  Among  these  structures  are  to  be  found,  also,  dendritic 
processes  of  cells  situated  in  the  subjacent  layer.  The  terminal 
filaments  of  medullated  nerve-fibers  coming  from  nerve-cells  in 
lower  regions  of  the  encephalo-spinal  axis  are  also  present,  but 
for  the  most  part  they  pursue  a  tangential  direction. 

2.  The  Layer  0}  Small  Pyramidal  Cells. — This  layer  consists  mainly 

of  nerve-cells,  the  majority  of  which  are  pyramidal  in  shape 
and   of    small   size.     Other  cells,  however,  are  present,  which 


5o8 


TEXT-BOOK  OF  PHYSIOLOGY. 


present  a  variety  of  shapes,  for  which  reason  the  layer  was  at 
one  time  termed  the  ambiguous  layer.  The  apical  process  of  the 
pyramidal  cells  is  broad  at  the  base,  but  narrows  rapidly  as  it 
passes  upward.     It  frequently  divides  into  several   branches, 

each  of  which  develops  club- 
shaped  processes  or  gemmules, 
which  give  to  it  a  feathery  appear- 
ance. Dendrites  are  also  given 
off  from  the  sides  and  base  of 
the  cell-body.  From  the  base 
a  single  axon  descends  which  ulti- 
mately becomes  the  axis-cylinder 
of  a  medullated  nerve. 
The  Layer  of  Large  Pyramidal 
Cells. — The  nerve-cells  of  this 
layer,  as  the  name  implies,  are 
also  pyramidal  in  shape,  but  of 
large  size.  Each  cell  presents  the 
same  features  as  the  cells  of  the 
preceding  layer,  with  the  exception 
that  the  apical  process  is  larger, 
better  developed,  and  branches 
more  freely.  All  the  dendrites 
are  extensively  provided  with 
gemmules.  The  axon  is  well 
developed,  sharply  defined,  and 
smooth.  After  giving  off  collateral 
branches,  the  axon  descends  into 
the  cerebrum  and  becomes  a 
medullated  nerve-fiber. 
The  Layer  of  Polymorphous  Cells. — 
In  this  layer  the  nerve-cells  pre- 
sent a  variety  of  forms:  e.  g., 
spindle,  polygonal,  pyramidal,  etc. 
The  spindle  form  is  the  most 
common.  From  either  end  of  the 
spindle  a  large  dendrite  emerges 
which  soon  branches  and  becomes 
gemmulated.  The  axon  is  well 
defined  and  it  soon  descends  into 
the  white  matter. 
The  Number  of  Cortical  Cells. — x-Yttempts  have  been  made  by 
various  histologists  to  estimate  the  total  number  of  functional  nerve- 
cells  in  the  cerebral  cortex  of  man.  Though  the  estimates  are  widely 
different,  the  lowest  presents  numbers  which  are  beyond  compre- 


FiG.  222. — Section  of  the  Cere- 
bral Cortex  (Motor  Area) 
OF  Child,  Stained  by 
GoLGi's     Silver     Method. 

A.  Layer  of  neuroglia   cells. 

B.  Layer  of  small  pyramidal 
ganglion  cells.  C.  Layer  of 
large  pyramidal  cells.  D. 
Layer  of  irregular  smaller 
cells.— {Pier  sol.) 


CEREBRUM  509 

hension.     Thus,  Meynert's  estimate  is   612    millions;    Donaldson's 
1200  millions;  while  Thompson's  is  9200  millions. 

Structure  of  the  White  Matter.— The  white  matter  of  the 
cerebrum  consists  of  meduUated  nerve-fibers  which,  though  in- 
tricately arranged,  may  be  divided  into  three  systems:  viz.,  the 
commissural,  the  association,  and  the  projection. 

1.  The  commissural  system.     The  fibers  which  compose  this  system 

unite  corresponding  areas  of  the  cortex  of  each  hemisphere. 
The  fibers  from  the  frontal,  parietal,  and  occipital  lobes  cross 
in  the  median  line  and  form  a  band  of  transversely  arranged 
fibers,  the  corpus  callosum.  The  fibers  which  unite  the  corre- 
sponding areas  of  the  temporo-sphenoid  lobes  cross  in  the 
anterior  commissure.  All  the  commissural  fibers  are  the  axons 
of  nerve-cells  in  the  cortex,  the  terminals  of  which  are  to  be  found 
in  the  cortex  of  the  opposite  side. 

2.  The  association  system.     The  fibers  w^hich  compose  this  system 

unite  neighboring  as  well  as  distant  parts  of  the  same  hemi- 
sphere, and  may  therefore  be  divided  into  long  and  short  fibers. 
They  associate  the  inexcitable  or  association  areas  with  the 
excitable  or  projection  areas. 

3.  The  projection  system.     The  fibers  composing  this  system  unite 

certain  areas  of  the  cortex  of    the  cerebrum  with  the  basal 
ganglia,  the  pons,  medulla  oblongata,  and  spinal  cord.     They 
may  be  divided  into:  (i)  afferent  fibers  which  have  their  origin 
in  the  lower  nerve-centers  at  different  levels  and  thence  pass  to 
the  cortex;  and  (2)  efferent  fibers  which  have  their  origin  in  the 
cortex  and  thence  pass  to  the  lower  nerve-centers,  terminating 
at  different  levels.     The  former  are  also  termed  the  cortico- 
afjerent  or  cortico- petal;  the  latter,  cortico-efjerent  or  cortico-jugal. 
The  afjereiit  fibers,    the  so-called   sensor   tract,  which  transmit 
nerve  impulses  coming  from  the  general  periphery  and  the_  sense- 
organs,  pass  through  the  tegmentum  as  the  mesial  and  lateral  fillets, 
and  thence  to  the  cortex  directly  by  way  of  the  internal  capsule,  or 
indirectly  through  the  intermediation  of  the  thalamic  and  subthalamic 
nuclei.     The  distribution  of  these  fibers  to  the  various  areas  of  the 
cortex  will  be  found  in  following  paragraphs. 

The  efferent  fibers  of  the  so-called  motor  tract  which  transmit 
motor  or  volitional  nerve  impulses  from  the  cortex  to  the  pons, 
medulla,  and  spinal  cord,  emerge  from  the  layer  of  pyramidal  cells 
of  the  gray  matter  of  the  anterior  or  the  pre-central  convolution, 
the  paracentral  lobule  and  immediately  adjacent  areas.  From  this 
origin  the  axons  descend  through  the  white  matter  of  the  corona 
radiata,  converging  toward  the  internal  capsule,  into  and  through 
which  they  pass,  occupying  the  anterior  two-thirds  of  the  posterior 
limb  or  segment.     Beyond  the  capsule  they  continue  to  descend, 


5IO  TEXT-BOOK  OF  PHYSIOLOGY. 

occupying  the  middle  three-fifths  of  the  pes  or  crusta  of  the  crus 
cerebri,  the  ventral  portion  of  the  pons,  and  eventually  the  anterior 
pyramid  of  the  medulla  oblongata.  At  this  point  the  tract  divides 
into  two  portions,  viz. : 

1.  A  large  portion,  containing  from  ninety-one  to  ninety-seven  per 

cent,  of  the  libers,  which  decussates  at  the  lower  border  of  the 
medulla  and  passes  down  the  lateral  column  of  the  cord,  con- 
stituting the  crossed  pyramidal  tract. 

2.  A  small  portion,  containing  from  three  to  nine  per  cent,  of  the 

fibers,  which  does  not  decussate  at  the  medulla,  but  passes 
down  the  inner  side  of  the  anterior  column  of  the  same  side, 
constituting  the  direct  pyramidal  tract  or  column  of  Tiirck. 
After  passing  through  the  internal  capsule,  and  as  it  descends 
through  the  crus,  pons,  and  medulla,  the  cortico-efferent  tract  gives 
off  a  number  of  fibers  which  cross  the  median  line  and  arborize  around 
the  nerve-cells  in  the  gray  matter  beneath  the  aqueduct  of  Sylvius 
(the  nuclei  of  origin  of  the  third  and  fourth  cranial  nerves),  and 
around  the  nerve-cells  in  the  gray  matter  beneath  the  floor  of  the 
fourth  ventricle  (the  nuclei  of  origin  of  the  remainder  of  the  motor 
cranial  nerves).  The  remaining  fibers  go  to  form  the  crossed  and 
direct  pyramidal  tracts  and  arborize  around  the  cells  in  the  anterior 
horn  of  the  gray  matter  of  the  opposite  side  of  the  cord  at  successive 
levels.  By  this  means  the  cortex  is  brought  into  anatomic  and  phys- 
iologic relation  with  the  general  musculature  of  the  body  through  the 
various  cranial  and  spinal  motor  nerves,  (See  Fig.  210,  page  481.) 
The  jronto-cerehellar  and  the  occipito-temporo-cerebellar  tracts 
are  also  efferent  tracts  and  parts  of  the  projection  system.  The 
fronto-cerebellar,  originating  in  the  nerve-cells  of  the  cortex  of  the 
frontal  lobe,  passes  clown  to  and  through  the  internal  capsule,  occupy- 
ing the  anterior  one-third  of  the  anterior  segment.  It  then  descends 
along  the  inner  side  of  the  crus  cerebri  to  the  pons,  where  its  fibers 
arborize  around  the  cells  of  the  nucleus  pontis.  Through  the  inter- 
mediation of  these  cells  this  tract  is  brought  into  relation  with  the 
cerebellum  of  the  same  but  chiefly  of  the  opposite  side.  The  occipito- 
temporal tract,  originating  in  the  cells  of  the  cortex  of  both  the 
occipital  and  temporal  lobes,  passes  downward  and  inward  toward 
the  lenticular  nucleus,  beneath  which  it  passes  to  enter  the  outer 
one-fifth  of  the  crusta.  It  then  enters  the  pons,  and  through  the 
nucleus  pontis  also  comes  into  relation  with  the  cerebellum  of  both 
sides.     (See  Fig.  217,  page  494.) 

THE  FUNCTIONS  OF  THE  CEREBRUM. 

The  functions  of  the  cerebrum  comprehend,  in  man  at  least, 
all  that  pertains  to  sensation,  cognition,  feehng,  and  voHtion.  All 
subjective  experiences,  which  in  their  totality  constitute  mind,  are 


CEREBRUM.  511 

dependent  on  and  associated  with  the  anatomic  integrity  and  the 
physiologic  activity  of  the  cerebrum  and  its  related  sense-organs,  the 
eye,  ear,  nose,  tongue,  etc. 

From  an  examination  of  the  anatomic  development  of  the  brain 
in  different  classes  of  animals,  in  different  men  and  races  of  men, 
and  from  a  study  of  the  pathologic  lesions  and  the  results  of  ex- 
perimental lesions  of  the  brain,  evidence  has  been  obtained  which 
reveals  in  a  striking  manner  the  intimate  connection  of  the  cerebrum 
and  all  phases  of  mental  activity. 

1.  Comparative  anatomic  investigations  show  that  there  is  a  general 

connection  between  the  size  of  the  brain,  its  texture,  the  depth 
and  number  of  convolutions,  and  the  exhibition  of  mental  power. 
Throughout  the  entire  animal  series  an  increase  in  intelhgence 
goes  hand  in  hand  with  an  increase  in  the  development  of  the 
brain.  In  man  there  is  an  enormous  increase  in  size  over  that 
of  the  highest  animals,  the  anthropoid  apes.  The  most  culti- 
vated races  of  men  have  the  greatest  cranial  capacity,  that  of  the 
educated  European  or  American  being  approximately  92.1  cubic 
inches  (1835  c.c);  while  that  of  the  Austrahan  is  but  81.7  cubic 
inches  (1628  c.c).  Men  distinguished  for  great  mental  power 
usually  have  large  and  well  developed  brains;  e.g.,  that  of 
Cuvier  weighed  64.4  ounces  (1830  grams);  that  of  Abercrombie, 
63  ounces  (1786  grams).  A  large  intelligence,  however,  is  not 
incompatible  with  a  much  smaller  brain  weight;  thus,  the 
brain  of  Helmholtz  weighed  but  50.8  ounces  (1440  grams); 
that  of  Leidy,  49.9  ounces  (1415  grams);  that  of  Liebig,  47.7 
ounces  (1352  grams).  The  average  arithmetic  brain  weight  of 
96  distinguished  men  was  found  to  be  51.9  ounces  (1473 
grams)  (Spitzka). 

2.  Pathologic  lesions  and  mechanic  injuries  which  disorganize  the 

cerebrum  are  at  once  followed  by  a  disturbance  or  an  entire 
suspension  of  mental  activity.  Concussion  of  the  brain  or 
sudden  compression  from  a  hemorrhage  destroys  consciousness. 
Physical  and  chemic  alterations  of  the  gray  matter  of  the  cere- 
brum have  been  shown  to  coexist  with  insanity,  loss  of  memory, 
of  articulate  speech,  etc.  Congenital  defects  of  organization  are 
accompanied  by  a  deficiency  in  mental  capacity  and  the  higher 
instincts.  Under  such  circumstances  no  great  advance  in  brain 
development  is  possible  and  the  intelligence  remains  at  a  low 
level.  In  congenital  idiocy  the  brain  is  small,  imperfectly 
developed,  and  wanting  in  proper  chemic  composition. 

3.  Experimental  lesions  of  the  brain  in  lowxr  animals  are  attended 

by  results  similar  to  those  observed  in  disease  or  after  injury 
in  man.  Removal  of  the  cerebrum  in  the  pigeon  completely 
abolishes  intelligence  and  destroys  the  capability  of  performing 


512  TEXT-BOOK  OF  PHYSIOLOGY. 

volitional  movements.  The  pigeon  remains  in  a  state  of 
profound  stupor,  though  retaining  the  capability  of  executing 
reflex  or  instinctive  movements.  It  can  temporarily  be  aroused 
by  loud  noises,  light  placed  before  the  eyes,  pinching  of  the 
toes,  etc.,  but  it  soon  relapses  into  a  condition  of  quietude. 
Coincident  with  the  destruction  of  the  cerebrum  there  occurs 
a  loss  of  memory,  reason,  and  judgment,  and  the  animal  fails 
to  associate  the  impressions  with  any  previous  train  of  ideas. 
The  higher  the  animal  in  the  scale  of  development,  the  more 
striking  is  the  loss  of  mentality  after  removal  of  the  cerebrum. 
4.  Experimental  interference  with  the  blood-supply  to  the  cerebrum 
is  followed  by  a  diminished  or  complete  cessation  of  its  activities. 
There  is  perhaps  no  organ  of  the  body  that  is  so  directly  depend- 
ent upon  its  blood-supply  for  the  continuance  of  its  activities 
as  the  cerebrum.  The  supply  of  blood  is  furnished  by  four  large 
blood-vessels:  viz.,  the  two  carotid  and  the  two  vertebral  arteries. 
These  vessels,  after  entering  the  cavity  of  the  skull,  give  off 
branches  which  unite  to  form  the  "circle  of  Willis."  From  this 
circle,  large  branches  are  given  off  which  enter  the  cerebrum 
and  distribute  blood  to  all  its  parts.  After  passing  through  the 
capillaries  the  blood,  greatly  altered  in  chemic  composition,  is 
returned  by  large  veins.  The  large  volume  of  blood  that  is 
present  in  the  brain  and  the  marked  changes  in  composition 
that  it  undergoes  while  passing  through  the  brain  indicate  a 
very  active  and  complex  metabolism  in  this  organ.  By  means 
of  the  anatomic  arrangement  of  the  blood-vessels  at  the  base 
of  the  brain,  the  blood-supply  is  equalized.  It  also  explains 
why,  when  one,  or  even  two,  of  the  four  large  vessels  are  oc- 
cluded by  pathologic  deposits  or  surgical  procedures,  brain 
activity  continues,  though  perhaps  diminished  in  degree.  Occlu- 
sion of  all  four  vessels,  however,  is  at  onCe  followed  by  a  complete 
abolition  of  all  forms  of  cerebral  activity.  An  experiment  per- 
formed by  Brown-Sequard  illustrates  the  dependence  of  cerebral 
activity  on  the  blood-supply.  A  dog  was  beheaded  at  the 
junction  of  the  neck  and  chest.  After  a  period  of  ten  minutes 
all  evidences  of  Hfe  had  entirely  ceased.  Four  tubes  connected 
with  a  reservoir  of  warm  defibrinated  blood  were  then  connected 
with  the  four  arteries  of  the  head.  By  means  of  a  pumping 
apparatus  imitating  the  action  of  the  heart  the  blood  was  driven 
into  and  through  the  brain.  After  a  few  minutes  cerebral 
activity  returned,  as  shown  by  contraction  of  the  muscles  of 
the  face  and  eyes.  The  character  of  the  contractions  were  such 
as  to  convey  the  idea  that  they  were  directed  by  the  will.  These 
vital  manifestations  continued  for  a  period  of  fifteen  minutes, 
when  on  the  cessation  of  the  artificial  circulation  they  disap- 


THE  CEREBRUM.  513 

peared,  and  the  head  exhibited  once  again  the  usual  phenomena 
observed  in  dying:  viz.,  contraction  and   then  dilatation  of  the 
pupils  and  convulsive  movements  of  the  muscles  of  the  face. 
Localization  of  Functions  in  the  Cerebrum. — By  the  term 
localization  of  functions  is  meant  the  assignment  of  definite  phys- 
iologic functions  to  definite  anatomic  areas  of  the  cerebral  cortex. 
From  experiments  made  on  the  brains  of  animals,  by  the  observa- 
tion and  association  of  chnical  symptoms  with  pathologic  lesions  of 
the  central  nerve  system,  and  from  observation  of  the  developmental 
stages  of  the  embryonic  brain,  it  has  been  established  in  recent  years : 

1.  That  the  general  and  special  sense-organs  of  the  body  are  as- 

sociated through  afferent  nerve-tracts  with  definite  though  per- 
haps not  sharply  delimited  areas  of  the  cerebral  cortex;  and — 

2.  That  certain  areas  of  the  cortex  are  associated  through  efl'erent 

nerve-tracts  with  special  groups  of  skeletal  or  voluntary  muscles. 

Experimental  excitation  of  a  cortical  area  associated  with  a  sense- 
organ  is  undoubtedly  attended  by  the  production  of  a  sensation  at 
least  similar  to  that  produced  by  peripheral  excitation  of  the  sense- 
organ  itself;  destruction  of  the  area  is  followed  by  an  abolition  of  all 
the  sensations  associated  with  the  sense-organ.  For  these  reasons 
such  areas  are  termed  sensor. 

Excitation  of  a  cortical  area  associated  with  a  group  of  skeletal 
muscles  is  attended  by  their  contraction;  destruction  of  the  area  is 
followed  by  their  relaxation  or  paralysis.  For  these  reasons  such  areas 
are  termed  motor. 

Since  the  sense-organs  are  remote  from  the  brain  and  the  impres- 
sions made  upon  them  by  the  objective  world  can  be  utilized  by  the 
mind,  only  after  they  have  been  reproduced  in  the  cortical  areas,  it 
may  be  said  that  each  sense-organ  has  its  special  area  in  the  cortex 
by  which  it  is  represented,  or,  in  other  words,  each  sense-organ  has 
a  cortical  area  of  representation. 

Since  the  muscles  are  remote  from  the  brain  and  since  they 
contract  in  response  to  the  discharge  of  nerve  impulses  from  the 
cells  of  the  cortical  motor  areas,  it  may  be  said  that  the  activities 
of  the  motor  areas  are  represented  by  the  contractions  of  the  muscles; 
in  other  words,  that  the  cortical  motor  areas  have  areas  of  representa- 
tion in  the  general  skeletal  musculature.  It  is  usually  stated,  how- 
ever, in  the  reverse  way:  viz.,  that  the  muscle  movements  have  areas 
of  representation  in  the  cortex. 

The  cortex  of  the  cerebrum  may  therefore  be  compared  to  a 
mosaic  made  up,  partially  at  least,  of  sensor  and  motor  areas  which 
respectively  represent  sense  organs  and  motor  organs,  and  which 
bear  a  deiinite  anatomic  and  physiologic  relation  one  to  the  other. 
Their  cooperation  is  essential  to  the  normal  performance  of  all  forms 
of  cerebral  activity. 


514  TEXT-BOOK  OF  PHYSIOLOGY. 

A  knowledge  of  the  situation  of  these  areas,  the  order  of  their 
development,  the  effects  that  arise  from  their  stimulation  or  follow 
their  destruction,  are  matters  of  the  highest  importance  in  the  study 
of  cerebral  activity  and  indispensable  to  the  physician  in  the  localiza- 
tion of  lesions  which  manifest  themselves  in  perversions  or  aboHtion 
of  sensations  and  in  convulsive  seizures  or  paralyses. 

The  Sensor  Areas. — The  sensor  areas  which  should  theoret- 
ically be  present  in  the  cortex  are  primarily  those  which  receive 
and  translate  into  conscious  sensations  nerve  impulses,  developed  by 
changes  going  on  in  the  body  itself;  and  secondarily  those  which 
receive  and  translate  into  conscious  sensations  the  nerve  impulses 
developed  in  the  special  sense-organs  by  the  impact  of  the  external 
or  objective  world.  In  the  former  areas,  are  received  the  nerve  im- 
pulses that  come  from  the  skin,  mucous  membranes,  muscles,  viscera, 
etc.,  and  give  rise  to  cutaneous,  muscle,  and  visceral  sensations.  In 
the  latter  are  received  the  nerve  impulses  that  come  from  the  sense- 
organs  and  give  rise  to  tactile,  gustatory,  olfactory,  auditory,  and 
visual  sensations.  A  number  of  such  sense  areas  may  be  predicated : 
e.  g.,  areas  of  cutaneous  and  muscle  sensibility,  of  gustatory,  olfactory, 
auditory,  and  visual  sensibility. 

The  Motor  Areas. — The  motor  areas  which  should  theoretically 
be  present  in  the  cortex  are  those  which  in  consequence  of  the  dis- 
charge of  nerve  impulses  excite  contraction  of  special  groups  of 
muscles  and  which,  from  their  coordinate  and  purposive  character, 
are  conventionally  termed  vohtional.  Five  such  general  motor  areas 
may  be  predicated:  e.  g.,  one  for  the  muscles  of  the  head  and  eyes, 
one  for  the  muscles  of  the  face  and  associated  organs,  and  areas  for 
the  muscles  of  the  arm,  leg,  and  trunk.  They  are  usually  designated 
as  head  and  eye,  face,  arm,  leg,  and  trunk  motor  areas. 

The  existence  and  anatomic  location  of  these  areas  in  the  cortex 
of  animals  have  been  determined  by  the  employment  of  two  methods 
of  experimentation:  viz.,  stimulation  and  destruction  or  extirpation; 
the  first  by  means  of  the  rapidly  repeated  induced  electric  currents, 
the  second  by  the  electric  cautery  and  the  knife.  If  the  stimulation 
or  excitation  of  any  given  area  is  followed  by  contraction  and  its 
destruction  by  paralysis  of  muscles,  it  is  assumed  that  the  area  is 
motor  in  function — is  a  center  of  motion.  If  the  stimulation  of  a 
given  area  is  attended  by  phenomena  which  indicate  that  the 
animal  is  experiencing  sensation,  and  its  destruction  by  a  loss  of 
this  capability  or  the  loss  of  a  special  sense,  it  is  assumed  that  the 
area  is  sensor  in  function — is  an  area  of  special  sense.  The  animals 
generally  employed  for  experiments  of  this  character  are  dogs  and 
monkeys,  though  other  animals  have  frequently  been  employed  by 
different  investigators.  Of  all  animals,  the  monkey  is  the  most  fre- 
quently selected,  as  the  configuration  of  the  brain  in  its  general  out- 


THE  CEREBRUM. 


515 


lines  more  closely  resembles  that  of  man  than  does  the  brain  of  any 
other  animal.  The  results  therefore  which  are  obtained,  there  is 
every  reason  to  believe,  are  the  results,  in  their  general  outlines,  that 
would  follow  stimulation  of  the  human  brain  if  this  were  possible 
under  the  same  conditions.  Indeed,  the  clinical  symptoms  which  arise 
during  the  development  of  pathologic  processes,  and  the  phenomena 
which  occur  during  surgical  procedures  for  the  removal  of  growths 
and  pathologic  cortical  areas,  justify  the  conclusion  that  the  chart  of 
the  motor  and  sensor  areas  of  the  monkey  brain  may  be  transferred 
to  the  human  brain  without  introducing  any  serious  errors. 

The  Sensor  Areas  of  the  Monkey  Brain. — From  experiments 
made  on  the  brains  of  monkeys,  Fcrrier,  Schafer,  Horsley,  and  many 


Fig.  223. — Diagram  of  the  Motor  and  Sensor  Areas  on  the  Lateral  Sur- 
face OF  the  Monkey  Brain. — (After  Horsley  and  Schafer.) 


Others  have  mapped  out,  though  not  with  a  high  degree  of  definite- 
ness  and  certainty,  the  sensor  areas,  stimulation  of  which  gives  rise  to 
sensation,  destruction  to  loss  of  sensation.  A  diagrammatic  repre- 
sentation of  these  areas  is  shown  in  Fig.  223  and  Fig.  224. 

The  tactile  area  or  area  of  tactile  perception  has  not  been  accu; 
rately  or  definitely  located.  Ferrier  assigned  it  to  the  hippocampal 
region.  Schafer  and  Horsley  assigned  it  to  the  limbic  lobe,  and 
especially  to  that  portion  known  as  the  gyrus  fornicatus,  as  destruction 
of  this  convolution  was  followed  by  hemianesthesia  of  the  opposite 
side  of  the  body  which  was  more  or  less  marked  and  persistent. 
These  observers  conclude  that  the  limbic  lobe  "is  largely  if  not 
exclusively  concerned  in  the  appreciation  of  sensation,  painful  and 


5i6 


TEXT-BOOK  OF  PHYSIOLOGY. 


tactile."  Other  experimenters  question  this  conclusion  and  locate 
the  area  near  to,  if  not  within,  the  Rolandic  area.  The  difference  of 
opinion  regarding  the  location  and  probable  limitation  of  the  area 
of  tactile  sensibility  renders  necessary  additional  and  more  conclusive 
experiments. 

The  olfactory  and  gustatory  areas  or  areas  oj  oljactory  and  gusta- 
tory perception  have  been  located  in  the  uncinate  gyrus  and  the 
adjacent  region,  though  their  exact  limits  have  not  been  determined 
by  the  experiments  thus  far  performed. 

The  auditory  area  or  area  of  auditory  perception  was  located  by 
Ferrier  in  the  upper  two-thirds  of  the  superior  tcmporo-sphenoidal 


Fig.  224. — Diagram  of  the  Motor  and  Sensor  Areas  on  the  Mesial  Sur- 
face OF  THE  Monkey  Brain. — {Ajter  Horsley  and  Sckdjer.) 


convolution.  Bilateral  cauterization  of  this  region  gave  rise  to  com- 
plete deafness,  which  endured  to  the  time  of  the  animal's  death, 
more  than  a  year  later.  Unilateral  destruction  of  this  region  gave 
rise  to  deafness  in  the  opposite  ear  only.  The  results  of  experiments 
made  subsequently  by  other  observers  would  indicate  that  the  audi- 
tory area  is  somewhat  more  extended  than  that  designated  by  Ferrier, 
as  apparently  animals  recovered  their  hearing,  to  some  extent  at 
least,  after  complete  recovery  from  the  operation.  The  limit  or 
extension  of  the  area  is,  however,  unceitain. 

The  visual  area  or  area  of  visual  perception  has  been  located  in 
the  occipital  lobe,  though  in  this,  as  in  the  previous  instances,  its 
exact  limits  have  not  been  positively  determined  Experimenters 
also  are  not  in  accord  as  to  the  relative  functions  of  its  different  parts. 


THE  CEREBRUM.  517 

Ferrier  located  this  area  in  the  occipital  lobe  and  that  adjacent  por- 
tion of  the  parietal  lobe  on  the  outer  surface  known  as  the  angular 
gyrus.  He  found  that  extirpation  of  the  angular  gyrus  alone  was 
followed  by  a  temporary  blindness  of  the  opposite  eye,  which  was, 
however,  not  hemiopic  in  character.*  He  also  found  that  destruction 
of  the  occipital  lobe  together  with  the  angular  gyrus  gave  rise  to  a 
more  or  less  enduring  hemianopsia,  in  addition  to  the  transient 
blindness  of  the  opposite  eye.  From  these  and  similar  facts  he  con- 
cluded that  the  angular  gyrus  is  the  area  of  representation  for  the 
macular  or  central  region  of  the  retina,  and  the  occipital  lobe  for 
the  corresponding  halves  of  the  peripheral  portions  of  the  retina. 

It  was,  however,  found  by  Munk,  Schafer,  and  others  that  the 
angular  gyrus  was  not  concerned  in  any  way  with  vision;  that  extir- 
pation of  the  occipital  lobe  alone,  especially  if  the  line  of  division 
be  carried  a  little  further  forward  on  the  mesial  and  inferior  sur- 
faces, was  followed  by  homonymous  hemiopia  (loss  of  retinal  func- 
tion on  the  same  side),  and  therefore  homonymous  hemianopsia. 
Additional  experiments  lead  to  the  conclusion  that  the  area  for 
macular  vision  is  near  the  anterior  extremity  of  the  calcarine  fissure, 
while  the  area  for  peripheral  vision  is  in  the  posterior  portion  of  the 
mesial  surface  and  for  a  variable  distance  on  the  outer  surface. 
Moreover,  there  is  reason  to  believe  that  the  area  for  macular  vision 
is  in  relation  with  homonymous  halves  of  the  two  macules  lutese. 
The  supposed  error,  the  assignment  of  macular  vision  to  the  angular 
gyrus,  has  been  attributed  to  destruction  of  the  fibers  of  the  optic 
radiation,  which  in  their  course  to  the  occipital  lobe  pass  close  to 
this  gyrus. 

The  Motor  Areas  of  the  Monkey  Brain. — From  experiments 
made  on   the   brains  of  monkeys  Ferrier  mapped  out  a  number  of 

*  In  a  consideration  of  this  subject  certain  facts  connected  with  visual  perception, 
both  in  physiologic  and  pathologic  conditions,  must  be  kept  in  mind.  Thus, 
visual  sensation  may  arise  from  stimulation  of  either  the  central  portion,  the  macula, 
or  the  peripheral  portion  of  the  retina  or  both.  In  the  first  instance  the  vision  is 
termed  central  or  macular;  in  the  second  instance,  peripheral  or  retinal.  Macular 
vision  is  clear,  sharp,  and  distinct;  retinal  vision  progressively  indistinct  from  the 
center  toward  the  periphery.  Division  of  one  optic  tract  is  followed,  in  consequence 
of  the  partial  decussation  of  the  optic  nerve-fibers  at  the  chiasma,  by  a  loss  of  function 
in  the  outer  two-thirds  of  the  retina  of  the  same  side,  both  in  the  central  (macular) 
as  well  as  in  its  peripheral  portions,  and  the  inner  one-third  of  the  retina  of  the  oppo- 
site side.  To  this  condition  the  term  hemiopia  has  been  apphed.  .A.s  a  result  of  this 
want  of  functional  activity  of  these  retinal  portions  on  the  side  of  the  lesion,  rays 
of  light  emanating  from  olDJects  situated  in  the  opposite  side  of  the  field  of  vision  will 
not  be  perceived  when  both  eyes  are  directed  to  the  fLxation  point.  To  this  "blind- 
ness" in  the  opposite  half  of  the  field  of  vision  the  name  hemianopsia  is  given.  In 
the  lesion  under  consideration  (di\asion  of  one  optic  tract)  the  hemianopsia  is  bilateral, 
and  as  it  affects  the  corresponding  portions  associated  in  normal  vision  it  is  of  the 
homonymous  variety.  Division  of  the  right  optic  tract  is  followed  by  left  lateral  homon- 
ymous hemia>iopsia,  indicative  of  the  fact  that  objects  in  the  field  of  vision  to  the 
left  of  the  binocular  fixation  point  are  invisible. 


5i8 


TEXT-BOOK  OF  PHYSIOLOGY. 


areas  stimulation  of  which  gives  rise  to  muscle  contractions  on  the 
opposite  side  of  the  body  which  are  so  purposive  and  coordinate  in 
character  that  they  may  be  regarded  as  identical  with  those  produced 
volitionallv.    Destruction  of  these  areas  is  followed  by  paralysis.   The 
results  of  Ferrier's  earlier  work  are  represented  in  Fig.  225,  the  descrip- 
tive text  to  which  renders 
them   intelhgible.     In   a 
general  way   it   may  be 
said  that  the  upper  third 
of  the  anterior  and  pos- 
terior    central    convolu- 
tions  presides    over  the 
movements  of  the  leg  of 
the  opposite  side  of  the 
body;   the   middle  third 
over  the   movements  of 
t  h  e    arm ;    the    inferior 
third  over    the    move- 
ments of  the    face    and 
tongue.       Collectively 
these  areas  are  known  as 
the  motor  area  or  motor 
zone;  and  as  it  is  ranged 
along   the   Rolandic  fis- 
sure,    it    is     sometimes 
termed  the  Rolandic.  area. 
The    experiments    of 
Horsley     and     Schafer 
added    additional    facts 
and  enabled  them  to  con- 
struct a  new  diagramma- 
tic representation  of  the 
motor    area    and    more 
accurately     define     the 
special   areas    upon   the 
lateral    and    mesial    as- 
pects of  the  brain  of  the 
monkey.      The    bound- 
aries of  the  general  and 
special  areas,   as   deter- 
mined by  these  observers,  will  be  readily  apparent  from  an  examina- 
tion of  Fig.   223.      Their  experiments  have  enabled  them  also  to 
subdivide  the  general  into  special  areas  as  follows: 
I.  The  head  area  or  area  for  visual  direction  into  areas  excitation  of 
which  causes  "opening  of  the  eyes,  dilatation  of  the  pupils  and 


Fig.  225. — Left  Hemisphere  or  Monkey,  Show- 
ing Details  of  Motor  Areas  Indicated  by 
THE  Movements  Following  Stimulation 
of:  I.  Superior  parietal  lobule;  exciting  ad- 
vance of  the  hind  limb.  2.  Top  of  ascending 
frontal  and  parietal  convolutions;  flexion  and 
outward  rotation  of  thigh;  flexion  of  toes.  _  3. 
On  ascending  frontal  convolution  near  semilu- 
nar sulcus;  movements  of  hind  limb,  tail  and 
extremity  of  trunk.  4.  On  adjacent  margins  of 
ascending  frontal  and  parietal  convolution; 
adduction  and  extension  of  arm,  pronation  of 
hand.  5.  Top  of  ascending  frontal  near  supe- 
rior frontal  convolution;  forward  extension  of 
arm.  a,  b,  c,  d.  On  ascending  parietal;  move- 
ments of  various  muscles  of  the  forearm.  6. 
Ascending  frontal  convolution;  flexion  of 
forearm  and  supination  of  hand  which  is 
brought  toward  mouth.  7.  Retraction  and 
elevation  of  corner  of  mouth.  8.  Elevation  of 
nose  and  lip.  q  and  10.  Opening  mouth  and 
motions  of  tongue.  11.  Retraction  of  angle 
of  mouth.  12.  Middle  and  superior  frontal 
convolutions;  movements  of  head  and  eyelids. 
13  and  13'.  Anterior  and  posterior  limbs  of 
angular  gyrus;  movements  of  eyeballs.  14. 
Superior  temporo-sphenoidal  convolution,  ear 
pricked  and  head  moved.  15.  Movement  of 
lip  and  nostril. — {Ferrier.) 


THE  CEREBRUM.  519 

turning  the  head  to  the  opposite  side  with  conjugate  deviation 
of  the  eyes  to  that  side." 

2.  The  leg  area  may  be  subdivided  into  (a)  an  area  both  on  the 

lateral  and  mesial  surfaces  which  presides  over  the  movements  of 
the  hip  and  thigh;  {h)  an  area  in  the  posterior  part  which  presides 
over  the  movements  of  the  legs  and  toes;  (c)  an  area  in  the 
paracentral  lobule  for  the  movements  of  the  hallux  or  great  toe. 

3.  The  trunk  area,  situated  largely  on  the  mesial  surface,  may  be 

subdivided  into  an  anterior  and  a  posterior  area,  which  respec- 
tively preside  over  the  movements  of  the  spinal  column  as  arch- 
ing and  rotation,  and  the  movements  of  the  pelvis  and  tail. 

4.  The  arm  area  may  be  subdivided  as  follows:  {a)  an  area  supe- 

riorly which  controls  the  movements  of  the  shoulder;  {h)  an  area 
posteriorly  and  below  this,  which  controls  the  movements  of  the 
elbow;  (c)  an  area  anteriorly  and  below  the  preceding,  govern- 
ing the  movements  of  the  wrist  and  lingers;  {d)  an  area  pos- 
teriorly and  below  governing  the  movements  of  the  thumb. 

5.  The  jace  area  may  be  divided  into  an  upper  part,  comprising 

about  one-third,  and  a  lower  part,  comprising  the  remaining 
two-thirds.     In  the  upper  part  are  areas  governing  the  move- 
ments of  the  opposite  angle  of  the  mouth  and  of  the  lower  face. 
In  the  lower  part  anteriorly  there  is  an  area  governing  the  move- 
ments of  the  vocal  membranes  or  bands  (the  laryngeal  area); 
posteriorly   areas   governing   the  opening  and  closing   of   the 
mouth,  the  protrusion  and  retraction  of  the  tongue. 
Electric  stimulation  of  the  sensor  areas  is  attended  by  certain 
motor  reactions  which  vary  in  accordance  with  the  area  stimulated. 
Thus,  when  the  electrodes  are  applied  to  different  portions  of  the 
occipital  lobe  the  eyeballs  are  conjugately  turned  upward,  dow^nward, 
or  laterally  and  to  the  opposite  side;  when  placed  on  the  upper  por- 
tion of  the  superior  temporal  convolution,  the  ear  is  pricked  up  or 
retracted,  the  head  is  turned  to  the  opposite  side  and  the  pupils 
are  dilated ;  when  placed  on  the  hippocampal  convolution,  there  is 
movement  of  torsion  of  the  nostril  and  lips  of  the  same  side. 

Ferrier  assumed  that  these  movements  were  the  result  of  the 
origination  of  subjective  'sensations  and  not  an  evidence  that  the 
area  in  question  is  a  motor  area,  in  the  sense  that  this  term  is  applied 
to  the  areas  of  the  Rolandic  region,  especially  as  their  destruction 
is  not  followed  by  paralysis  of  any  of  the  corresponding  muscles. 
This  interpretation  is  supported  by  the  experiments  of  Schafer, 
which  showed  that  the  contraction  of  the  eye-muscles  which  followed 
stimulation  of  the  occipital  lobe  took  place  between  0.2  and  0.3 
second  later  than  when  the  frontal  lobe  was  stimulated;  and  that  as 
the  motor  reaction  takes  place  after  extirpation  of  the  frontal  region. 


520  TEXT-BOOK  OF  PHYSIOLOGY. 

the  route  of  the  efferent  impulse  cannot  be  to  and  through  the 
frontal  lobe,  but  probably  through  some  lower  center.  The  same 
facts  hold  true  for  the  reactions  of  the  ear-muscles  following  stimu- 
lation of  the  temporal  lobe. 

The  view  that  the  cortex  of  the  cerebrum  can  be  divided  into 
separate  and  independent  though  physiologically  related  motor  and 
sensor  areas  has  been  questioned  in  recent  years,  and  a  somewhat 
different  interpretation  given  to  the  facts.  It  is  believed  by  many 
physiologists  and  neurologists  that  the  so-called  motor  and  sensor 
areas  are  so  closely  related  that  it  is  almost  impossible  to  distinguish 
one  from  the  other  either  anatomically  or  physiologically.  Thus  the 
Rolandic  region  is  believed  to  be  both  motor  and  sensor  in  function, 
the  former,  however,  being  more  predominant  in  the  pre-central, 
the  latter  in  the  post-central,  convolution.  As  these  two  functions 
are  so  intimately  blended  and  their  anatomic  substrata  so  difficult 
of  separation,  it  is  thought  the  term  sensori-motor  should  be  em- 
ployed as  more  descriptive  and  more  in  accordance  with  the  facts 
to  the  entire  Rolandic  region. 

This  view  has  been  strengthened  by  the  results  of  the  embryo- 
logic  investigation  of  Flechig,  which  show  that  different  nerve-tracts 
become  medullated  or  receive  their  myelin  investment  at  successively 
later  periods  and  that  the  tracts  which  first  become  myelinated  and 
are  hence  first  functionally  active,  belong  to  the  afferent  system. 
Among  the  first  to  undergo  myelinization  are  three  tracts  numbered 
by  Flechsig  i,  2,  and  3,  which  arise  largely  from  the  median  nucleus 
of  the  thalamus  and  the  medial  lemniscus  and  pass  to  the  anterior 
and  posterior  convolutions,  to  the  para-central  lobule  and  foot  of 
the  superior  frontal  convolution,  and  to  the  foot  of  the  third  frontal 
convolution  respectively.  It  is  these  fibers  which  convey  nerve  im- 
pulses to  the  cortex  and  furnish  information  regarding  changes  taking 
place  in  the  body  itself  and  thus  lead  to  the  performance  of  muscle 
movements.  This  area  is  therefore  primarily  a  sensor  area,  an  area 
for  body-feelings,  cutaneous,  tactile,  muscle,  and  visceral,  and  second- 
arily a  motor  area.  The  afferent  fibers  to  this  region  become  mye- 
linated during  the  ninth  month  of  intra-uterine  life,  the  efferent  fibers 
from  it  become  mvelinated  during  the  third  month  of  extra-uterine 
hfe. 

By  the  same  method  of  reasoning  the  gustatory,  olfactory,  audi- 
tory, and  visual  sense  areas  are  to  be  regarded  as  sensori-motor  in 
character,  for  embryologic  investigations  show  that  subsequently 
to  the  myelinization  of  the  afferent  tracts  connecting  the  sense-organs 
with  the  cortex,  efferent  nerve-tracts  arise  from  or  near  to  the 
same  centers  and  undergo  myehnization.  In  other  words,  these 
areas  are  primarily  sensor  and  secondarily  motor,  and  therefore 
should   be   termed  sensori-motor.     In   Flechsig's  own  terminology 


THE  CEREBRUM. 


521 


each  cortico-petal  or  afferent  tract  is  accompanied  by  a  cortico-fugal 
or  efferent  tract. 

In  this  view  sensations,  or  the  mental  processes  the  outcome  of 
sensations,  are  the  immediate  cause  of  the  movements  of  the  mus- 
cles connected  with  both  the  sense-organs  and  skeletal  structures. 
Though  this  interpretation — viz.,  the  coincidence  of  sensor  and 
motor  areas — appears  more  in  accordance  with  the  facts  than  the 
earlier  view,  it  must  be  admitted  that  there  are  many  facts  both  of 
a  physiologic  and  pathologic  character  which  it  is  difficult  to  har- 
monize with  it. 

The  Motor  Area  of  the  Chimpanzee  Brain. — In  a  series  of 
experiments  made  by  Sherrington  and  Griinbaum  on  the  brain  of  the 
chimpanzee  it  was  discovered  that  the  so-called  motor  area  was  not 
so  widely  distributed  as  in  the  monkeys  generally,  but  was  confined 
almost  exclusively  to  the  convolution  just  in  front  of  the  fissure  of 
Rolando,  as  it  was  impossible  to  obtain  any  movement  on  direct 
stimulation  of  the  convolution  just  behind  it.  All  points  on  the 
surface  of  the  pre-central  convolution,  including  the  portion  forming 
the  wall  of  the  Rolandic  fissure  itself,  were  found  to  be  excitable 
and  productive  of  movement  when  stimulated.  The  sequence  of 
representation  from  below  upward  is  similar  to  that  observed  in  the 
monkey.  One  pecuharity,  however,  was  the  location  of  the  area 
for  conjugate  deviation  of  the  eyeballs  to  the  opposite  side.  This 
is  situated  far  forward  in  the  middle  and  inferior  frontal  convolutions, 
and  separated  from  the  areas  in  the  pre-central  convolution  by  a 
region  apparently  inexcitable.  These  facts  are  of  great  interest  and 
value  in  the  assignment  of  the  motor  areas  in  the  cortex  of  the  human 
brain,  as  in  its  development  and  configuration  the  chimpanzee  brain 
more  closely  resembles  the  human  brain  than  does  the  monkey's. 

The  Localization  of  Sensor  and  Motor  Areas  in  the  Human 
Brain. — The  observation  of  chnical  symptoms  and  their  interpreta- 
tion by  postmortem  findings,  the  phenomena  observed  during  surgical 
procedures,  and  the  results  of  embryologic  investigations,  point  to  the 
conclusion  that  corresponding  areas  both  for  sensations  and  move- 
ments exist  in  the  cerebral  cortex  of  the  human  brain,  though  it  is 
probable  that  their  locations  do  not  in  all  respects  coincide  with 
those  characteristic  of  the  monkey  or  even  the  ape  brain.  In  the  fol- 
lowing diagrams  (Figs.  226  and  227),  the  sensor  and  motor  areas 
are  at  least  provisionally  located,  in  accordance  with  recent  obser- 
vations. They  are  represented  as  limited  or  bounded  by  a  serrated 
line  to  indicate,  as  suggested  by  Mills,  that  they  are  not  sharply 
defined,  but  that  they  interfuse  or  interdigitate  with  surrounding 
regions. 

The  Sensor  Areas. — The  sensor  areas  occupy  regions  corre- 
sponding in  a  general  way  with  those  of  the  monkey  brain. 


522 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  cutaneous  and  muscle  sense  areas  have  been  assigned  to  the 
post-central,  a  portion  of  the  superior  and  inferior  parietal  convolu- 
tions on  the  lateral  aspect,  and  to  portions  of  the  frontal  convolution 
and  of  the  gyrus  fornicatus  on  the  mesial  aspect.  It  is  also  probable 
that  the  tactile  (cutaneous)  area  may  be  assigned,  though  in  less 
degree,  to  the  pre-central  convolution,  the  general  motor  area.  This 
is  in  accordance  with  the  embryologic  investigations  of  Flechsig,  who 
concludes  that  the  entire  Rolandic  region  is  to  be  regarded  as  sensor 
as  well  as  motor  in  function,  and  names  it  the  area  of  body  feelings, 
or  the  somesthetic  area. 


CONCRtTE   CONCEPT 


Fig.  226. — The  Areas  and  Centers  of  the  Lateral  Aspect  of  the 
Human  Hemi-cerebrum. — (C.  A'.  Mills.) 


The  clinical  and  postmortem  evidence  as  to  the  extent  of  the 
area  of  tactile  sensibility  and  its  coincidence  with  the  motor  area  is 
somewhat  contradictory,  and  in  some  respects  apparently  in  opposi- 
tion to  the  view  of  Flechsig.  Thus,  Dr.  C.  K.  Mills,  whose  skill  in 
interpreting  the  phenomena  of  disease  is  well  known,  states  in  this  con- 
nection in  his  work  on  nervous  diseases  that  "  innumerable  cases  have 
been  reported  of  lesions  of  the  motor  cortex  without  the  shghtest 
impairment  of  sensibility."  In  several  cases  of  excision  of  the  human 
cortex  in  the  Rolandic  region  by  surgical  operations  careful  studies 
of  the  patients  failed  to  show  any  impairment  of  sensation.  Other 
competent  observers,  however,  have  reported  a  number  of  cases  in 


THE  CEREBRUM. 


523 


which  anesthesia  more  or  less  pronounced  and  persistent  has  accom- 
pan  ed  lesions  of  the  motor  area.  The  explanation  of  these  contra- 
dictory observations  is  not  apparent. 

The  olfactory  area  has  been  assigned  to  the  uncinate  convolution, 
the  anterior  part  of  the  gyrus  fornicatus,  and  the  posterior  part  of  the 
base  of  the  frontal  lobe.  Lesions  in  this  region  are  frequently 
accompanied  by  subjective  olfactory  sensations. 

The  gustatory  area  has  been  assigned  to  the  fourth  temporal 
convolution. 

The  auditory  area  has  been  assigned  to  the  posterior  portion  of 
the  superior  temporal  convolution  and  to  the  retro-insular  convolu- 
tions, the  island  of  Reil.  Unilateral  destruction  of  this  region  is 
followed  by  only  a  partial  loss  of  hearing  in  the  opposite  ear  (owing 
to  the  partial  decussation  of  the  cochlear  nerve),  which,  however, 
may  be  recovered  from  after  a  time,  owing  probably  to  a  compensatory 
activity  of  the  insular  convolutions.  Bilateral  disease  of  this  region 
is  followed  by  complete  deafness.  Within  this  area  there  is  a  smaller 
region,  disease  of  which  is  accompanied  by  word-deajness  only,  the 
patient  being  unable  to  distinguish  the  tone  intervals  between  words 
and  syllables  and  therefore  hearing  only  confused  noises.  Object 
hearing  has  also  a  separate  area  of  representation. 

The  visual  area  has  been  assigned  to  a  triangular  shaped  area  on 
the  mesial  surface  of  the  occipital  lobe,  which  includes  the  gray  matter 
above  and  below  the  calcarine  fissure  (the  cuneus  and  upper  part  of 
the  lingual  lobe),  and  to  the  gray  matter  of  the  first  occipital  convo- 
lution on  the  lateral  aspect  of  the  occipital  lobe.  Focal  lesions  of  this 
area  on  one  side  are  followed  by  lateral  homonymous  hemianopsia, 
which,  however,  does  not  involve,  as  a  rule,  the  fovea  or  macula. 
It  is,  therefore,  the  area  of  homonymous  half-retinal  representation. 
The  location  of  the  area  for  macular  or  central  vision  is  uncertain. 
Henschen  locates  it  in  the  anterior  part  of  the  area  near  the  ex- 
tremity of  the  calcarine  fissure,  and  asserts  that  in  each  area  both 
maculae  are  represented.  From  experiments  made  on  monkeys 
Schafer  locates  it  in  the  same  region.  Beyond  the  limits  of  this 
visual  area  and  on  the  lateral  aspect  of  the  parietal  lobe  there  is  a 
region  (the  supra-marginal  convolution  and  angular  gyrus)  in  which 
impressions  of  words  and  letters  seen  have  their  representation. 
Destruction  of  this  area  by  diseases  is  follow^ed  by  word-  and  per- 
haps letter-blindness,  the  patient  being  unable  to  recognize  words 
and  letters  seen  because  of  failure  to  revive  the  memory  images  of 
words  and  letters.  The  areas  for  visual  sensations  and  optic  memory 
pictures  are  therefore  separate,  a  fact  w-hich  has  led  to  a  division  of 
the  visual  area  into  a  lower  and  a  higher  area. 

It  was  stated  in  a  previous  paragraph  that  electric  stimulation  of 
the  sensor  areas  of  the  monkey  brain  is  attended  by  certain  motor 


524 


TEXT-BOOK  OF  PHYSIOLOGY. 


reactions  which  vary  with  the  area  stimulated.  Corresponding  areas 
are  beheved  to  be  present  in  the  human  brain  and  that  their  stimula- 
tion would  be  followed  by  similar  motor  reactions.  Their  location  is 
shown  in  Figs.  226  and  227,  and  named  visual,  auditory,  olfactory, 
and  gustatory  motor. 

The  stereognostic  area  or  area  of  stereognostic  perception,  by  which 
objects  are  recognized  through  their  form  independent  of  vision  and 
by  the  sense  of  touch  alone,  has  been  located  in  the  superior  parietal 
convolution  and  the  precuneus  (Mills).  The  existence  of  such  an 
area  is  rendered  probable  by  the  fact  that  cases  have  been  recorded 
in  which  there  was  a  loss  of  this  power  (astereognosis)  unaccompanied 


Fig.  227. — The  Areas  and  Centers  of  the  Mesial  Aspect  of  the 
Human  Hemi-cerebrum. — (C.  K.  Mills.) 

by  either  sensor  or  motor  disturbances.  Postmortem  investigations 
showed  that  in  these  cases  there  was  a  destruction  of  the  superior 
parietal  convolution. 

Equilibratory,  intonation,  and  orientation  areas  have  been  pro- 
visionally located  in  the  sphenotemporal  lobe. 

The  Motor  Area. — The  general  motor  area  (Fig.  226)  is  repre- 
sented as  occupying  the  pre-central  convolution,  the  base  of  the  first 
convolution,  both  on  its  lateral  and  mesial  aspects,  and  the  paracentral 
lobule.  The  exclusion  of  the  post-central  convolution  from  the  motor 
area  is  in  accordance  with  the  embryologic  researches  of  Flechsig, 
which  indicate  that  the  efferent  fibers  which  compose  the  pyramidal 
tract  come  from  the  region  anterior  to  the  central  fissure,  and  with  the 


THE  CEREBRUM.  525 

experiments  of  Sherrington  and  Grlinbaum  on  the  brain  of  the  chim- 
panzee, which  demonstrate  that  the  post-central  convolution  is 
absolutely  inexcitable  to  electric  stimulation.  It  is  quite  probable 
that  with  the  growth  of  the  brain  in  size  and  complexity,  the  motor 
area  has  come  to  occupy  a  position  somewhat  farther  forward  in  the 
human  brain  than  in  the  monkey  brain. 

This  general  area  is  also  capable  of  subdivision  into  areas  of 
variable  size,  in  which  the  movements  of  the  face  and  associated 
structures,  the  head  and  eyes,  the  arm,  trunk,  and  leg,  are  represented. 
The  sequence  of  their  representation  from  below  upward  is  similar 
to  that  observed  in  the  monkey  and  chimpanzee.  A  localized  irritative 
lesion  of  any  one  of  these  areas  gives  rise  to  convulsive  movements  of 
the  muscles  of  the  opposite  side  of  the  body,  similar  in  character  to 
those  resulting  from  electric  stimulation  of  the  corresponding  areas  of 
the  monkey  and  ape  brains.  Destruction  of  these  areas  from  the 
growth  of  tumors,  softening,  etc.,  is  followed  by  paralysis  of  the 
muscles.  Electric  stimulation  of  the  human  brain  for  the  purpose  of 
localizing  obscure  irritative  lesions  prior  to  surgical  procedures  on 
the  brain  gives  rise  to  the  same  convulsive  movements. 

Language. — The  succession  of  motor  acts  by  which  ideas  are 
expressed,  is  known  as  language,  which  may  be  divided  into  (i) 
articulate  or  spoken,  and  (2)  written. 

The  expression  of  ideas  both  by  words  and  signs  depends  primarily 
on  the  power  of  reviving  the  images  or  memories  of  words  and  letters 
heard  and  seen;  and  secondarily  on  the  power  of  reviving  the  images 
or  memories  of  the  muscle  movements  which  were  previously  em- 
ployed in  an  eflPort  to  imitate  or  reproduce  the  words  (speech)  or 
the  verbal  signs  (writing). 

Clinico-pathologic  investigations  have  shown  that  words  and 
letters  heard  and  seen  have  areas  of  representation  in  the  cortex,  the 
former  in  the  general  auditory  area,  the  latter  in  the  supra-marginal 
convolution  and  angular  gyrus  (Fig.  226).  Destruction  of  these 
areas  is  followed  by  word-deafness  and  word-blindness  respectively. 
The  same  methods  of  investigation  have  shown  that  the  muscle 
movements  employed  to  reproduce  the  words  and  the  verbal  signs 
also  have  areas  of  representation  in  the  cortex;  the  former  in  the 
third  frontal  convolution  (Fig.  226),  and  probably  in  the  adjacent 
region,  the  island  of  Reil,  on  the  left  side  in  the  great  majority  of 
people ;  the  latter  in  front  of  the  arm  region  of  the  general  motor  area. 
Destruction  of  these  areas  is  followed  in  the  first  instance  by  a  loss 
of  the  power  of  executing  the  movements  of  the  muscles  employed 
in  speech,  and  in  the  second  instance,  of  those  employed  in  writing. 

These  different  areas  are  connected  with  one  another  by  associa- 
tion fibers,  and,  taken  collectively,  constitute  the  language  zone. 
Their  situation  and  relations  are  shown  in  Fisj.  228.     In  this  figure 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  dotted  lines  coming  from  the  ear  (a)  and  the  eye  (v)  represent 
the  auditory  and  visual  tracts  through  which  nerve  impulses  pass 
to  the  auditory  (A)  and  the  visual  centers  (V)  respectively.  Similar 
lines  coming  from  the  muscles  involved  in  speech  and  writing  might 

also  be  represented  to  indicate 
the  paths  of  the  nerve  impulses 
to  the  motor  speech  (M)  and 
the  motor  writing  center  (E). 
The  continuous  lines  on  the 
surface  of  the  cortex  represent 
nerve-fibers  which  associate 
the  auditory  and  visual  centers 
with  the  speech  and  writing 
centers  and  with  higher 
psychic  centers  (O  O)  as  well. 
The  dotted  Unes  coming  from 
the  speech  and  writing  centers 
represent  the  tracts  through 
which  nerve  impulses  pass  to 
the  muscle  of  the  larynx, 
tongue,  mouth,  and  hps,  and 
to  the  muscles  of  the  hand. 
The  anatomic  and  physiologic 
association  of  the  various  areas 
is  essential  to  the  registration 
of  the  impressions  made  on 
the  ear  and  eye  and  for  the 
expression  of  the  ideas  evolved 
from  them  by  words  (speech) 
and  signs  (writing).  Their 
collective  action  is  essential 
to  the  acquisition  of  language. 
Destruction  of  any  part  of 
this  cerebral  mechanism  is 
attended  by  an  impairment 
or  a  total  loss  either  in  the 
power  of  obtaining  auditory 
images  of  words  heard  and 
visual  images  of  words  seen, 
or  in  the  power  of  expressing 
ideas  by  speech  and  writing. 
To  this  pathologic  condition  the  term  aphasia  has  been  given. 

Aphasia. — It  was  discovered  by  Bouillaud  that  a  destructive 
lesion  of  the  third  frontal  convolution  on  the  left  side  was  accom- 
panied by  a  partial  or  complete  loss  of  the  faculty  of  articulate  speech, 


t'^iG.  228. —Diagram  Showing  the 
Relation  of  the  Centers  of 
Language  and  their  Principal 
Associations.  A.  Auditory  center. 
V.  Visual  center.  M.  Motor  speech 
center.  E.  Motor  writing  center. 
O  O.  Intellectual  center. — (Afier 
Grasset.) 


THE   CEREBRUM.  527 

the  power  to  express  ideas  with  words.  To  this  condition  the  term 
aphasia  was  given.  Though  of  hmited  apphcation  etymologically, 
the  word  is  now  employed  in  a  wider  sense  to  signify  "  partial  or  com- 
plete loss  of  the  power  of  expression  or  comprehension  of  the  con- 
ventional signs  of  language, "  words  either  spoken  or  written,  due  to 
lesions  of  different  portions  of  the  cortex,  and  especially  on  the  left 
side. 

Aphasias  are  of  many  degrees  and  kinds,  though  they  may  be  in- 
cluded in  the  two  general  divisions,  motor  and  sensor. 

i  Motor  aphasia  may  be  either  ataxic  or  agraphic.  In  ataxic 
aphasia  the  patient  is  unable  to  express  or  communicate  his  thoughts 
by  spoken  words,  owing  to  an  inabihty  to  execute  those  movements 
of  the  mouth,  tongue,  etc.,  necessary  for  speech  without  there  being 
any  paralysis  of  these  muscles.  The  lesion  is  usually  in  the  third 
frontal  convolution  and  most  frequently  associated  with  right 
hemiplegia.  In  agraphic  aphasia  the  patient  is  unable  to  com- 
municate his  ideas  by  wTiting  through  an  inability  to  execute  the 
necessary  movements,  though  retaining  his  mental  processes.  In 
this  form  of  aphasia  the  lesion  is  in  the  writing  area.  These  two  forms 
of  motor  aphasia  are  not  infrequently  associated. 

Sensor  aphasia  or  amnesia  may  be  either  visual  or  auditory.  In 
visual  aphasia  or  amnesia  the  patient  is  unable  to  recognize  a  letter 
or  word,  printed  or  written  (though  capable  of  seeing  other  objects), 
a  condition  known  as  letter-  or  word-blindness.  It  is  usually  associated 
with  lesions  in  the  neighborhood  of  the  supra-marginal  convolution. 
In  auditory  aphasia  or  amnesia  the  patient  cannot  understand 
articulate  or  vocal  speech,  though  capable  of  hearing  and  understand- 
ing other  sounds,  through  an  inability  to  distinguish  tone  intervals 
of  words  and  letters — a  condition  known  as  ivord-deafness.  It  is 
associated  with  lesions  of  the  auditory  area. 

Paraphasia  is  an  inabihty  to  recall  the  proper  words  to  associate 
with  ideas  and  necessary  to  their  expression. 

Concept  aphasia  is  the  inability  to  recall  the  names  of  objects. 
It  is  associated  with  lesions  of  the  cortex  of  the  mid-temporal  or 
third  temporal  convolution  (Mills).  This  area  is  known  as  the 
concept  or  naming  area. 

Bilateral  Representation. — Though  highly  speciahzed  move- 
ments, such  as  those  performed  by  the  arms  and  hands,  legs  and  feet, 
have  their  areas  of  representation  on  one  side  of  the  cerebrum  only, 
and  that,  opposite  to  the  side  of  the  movement,  less  highly  specialized 
movements,  such  as  the  masticatory,  phonatory,  respiratory,  and 
various  trunkal  movements,  which  require  for  their  performance 
the  cooperation  of  muscles  on  both  sides  of  the  body,  have  their 
areas  of  representation  on  both  sides  of  the  cerebrum;  the  area  of 
either  side  exciting  to  action  the  muscles  on  both  sides  of  the  body. 


528 


TEXT-BOOK  OF  PHYSIOLOGY. 


In  the  case  of  specialized  movements  the  representation  is  unilat- 
eral; in  the  case  of  the  more  general  movements  the  representation  is 
bilateral. 


Motor  and  tactile  area. 


Parietal  association  aren  ,  ,<^  ■—^'^y^ 


/ 


VV  i  ^ >  ; 


Frontal 

association 

area. 


Island  of  Reil. 


Occipito-temporal 
association  area. 


.^uditorv  area. 


Motor  and  tactile  area. 


^■om^ 


Parietal  association  uo 
(Precuneus). 


Visual  area 
(cuneus). 


Occipito-temporal 
association  area. 


Olfactory  lobe. 
Olfactory  tract. 

^  Olfactory  area. 


Gyrus  hippocampus. 


Fig.  229. — -Di.^GRAMS  to  show  the  Position  and  the  Relation  of  the 
Association  and  Projection  Areas.  The  Projection  Areas  are 
Dotted. ^(/l//fr  Flechsig.) 


Association  Centers. — The  sensor  and  motor  areas  to  which 
specific  functions  have  been  assigned  do  not  constitute  more  than 
one-third    of    the    total   cerebral    cortex.     There  yet   remain  large 


THE  CEREBRUM.  529 

regions  to  which  it  has  not  been  possible  to  assign  specific  functions 
based  on  physiologic  experiments.  Three  or  four  such  regions 
separated  by  the  sensor  and  motor  centers  are  to  be  recognized  on 
the  lateral  and  mesial  aspects  of  the  hemisphere.  In  Fig.  229 
the  location,  extent,  and  names  of  these  regions  are  represented. 
The  fibers  which  are  found  in  these  regions  belong  almost  exclusively 
to  the  association  system,  and  become  medullated  at  a  later  period 
than  do  the  fibers  of  the  projection  system;  moreover,  from  the  method 
of  their  medullization  it  would  appear  that  many  of  these  fibers  grow 
out  directly  from  the  sensor  centers  into  these  regions  and  become 
related  to  the  nerve-cells  of  their  convolutions,  while  others  grow 
out  from  adjacent  as  well  as  distant  convolutions.  From  histologic 
and  pathologic  evidence  these  regions  were  termed  by  Flechsig 
association  centers  or  areas,  implying  the  idea  that  through  the  in- 
tervention of  their  cell  mechanisms  the  sense  areas  are  indirectly 
associated  anatomically  and  physiologically,  and  together  constitute 
a  mechanism  by  which  sensations  are  associated  and  elaborated  into 
concrete  forms  of  knowledge  or  related  to  definite  forms  of  move- 
ment. 

It  has  been  assumed  by  Flechsig  that  the  frontal  association  center, 
from  its  connections  with  the  sensor  and  motor  areas  of  the  Rolandic 
region,  the  olfactory,  and  perhaps  other  regions,  is  engaged  in  as- 
sociating and  registering  body  sensations  and  vohtional  acts,  and 
that  the  knowledge  thus  gained  has  reference  largely  to  the  personal- 
ity of  the  individual;  that  the  par ieto- occipital  association  area,  from 
its  relation  to  the  visual,  auditory,  and  tactile  sense  areas,  is  engaged 
in  associating  and  registering  visual,  auditory,  and  tactile  sensations, 
and  that  the  knowledge  thus  gained  has  reference  mainly  to  the 
external  world.  These  assumptions  in  a  general  way  are  supported 
by  the  phenomena  of  disease.  In  certain  lesions  of  the  frontal  lobe 
the  symptoms  indicate  a  loss  or  change  of  ideas  regarding  personality 
rather  than  of  the  objective  w^orld,  while  the  reverse  is  true  in  disease 
of  the  parieto-occipital  lobe. 


34 


CHAPTER  XX. 
THE  CEREBELLUM. 

The  cerebellum  is  situated  in  the  inferior  fossae  of  the  occipital 
bone,  beneath  the  posterior  lobes  of  the  cerebrum,  from  which  it 
is  separated  by  the  tentorium  cerebelli,  a  semilunar  fold  of  the 
dura  mater.  It  is  partially  divided  into  hemispheres  by  a  longi- 
tudinal fissure,  more  apparent  on  the  inferior  surface,  though  united 
by  a  central  lobe,  the  vermiform  process.  Each  hemisphere  is  con- 
nected with  the  cerebrum,  the  pons,  medulla  and  spinal  cord  by  three 
bundles  of  nerve-fibers  known  respectively  as  the  superior,  middle, 
and  inferior  peduncles.  The  surface  of  the  cerebellum  presents  a 
series  of  lobes  and  fissures  of  which  the  former  have  received  more 
or  less  fanciful  names.  A  section  of  the  cerebellum  shows  that  it  is 
composed  of  gray  matter  externally  and  white  matter  internally. 
The  general  appearance  presented  on  section  is  shown  in  Fig.  230. 

Structure  of  the  Gray  Matter. — The  gray  matter  consists 
mainly  of  nerve-cells  of  varying  size  and  shape,  which  are  arranged  in 
two  layers:  viz.,  an  outer  or  molecular  and  an  inner  or  granular. 

The  molecular  layer  consists  of  stellate  and  multipolar  cells  of 
small  size,  from  which  dendrites  and  axons  pass  horizontally  and 
vertically.  The  granular  layer  consists,  as  its  name  implies,  of 
granular  shaped  cells  and  large  stellate  cells.  These  cells  are 
characterized  by  the  possession  of  dendrites  and  axons,  the  course 
and  relation  of  which  have  not  been  clearly  determined. 

The  inner  border  of  the  molecular  layer  presents  a  series  of  large 
cells  originally  described  by  Purkinje  and  known  by  his  name.  From 
the  outer  end  of  the  cell-body  one  or  more  dendrites  emerge  which 
soon  divide  and  subdivide  into  a  number  of  branches  which  pass 
toward  the  cerebellar  surface.  The  general  arrangement  of  these 
dendrites  gives  to  the  entire  cell  a  tree-like  appearance  (Fig.  231). 
From  the  inner  end  of  the  cell  an  axon  emerges  which- passes  centrally 
into  the  w^hite  matter. 

Structure  of  the  White  Matter. — The  white  matter  consists  of 
nerve-fibers  which  are  arranged  in  association  and  projection  systems. 

The  Association  System.—Th.e  fibers  which  compose  this  system 
are  of  variable  lengths  and  unite  adjacent  as  well  as  distant  regions 
of  the  cerebellar  cortex.  They  doubtless  associate  them  both  anatom- 
ically and  physiologically. 

530 


THE  CEREBELLUM. 


531 


The  Projection  System. — The  fibers  composing  this  system  con- 
nect the  cerebellar  cortex  with  certain  structures  in  the  cerebrum, 
pons,  medulla,  and  spinal  cord.  They  may  be  divided  into  efferent 
and  afferent  systems. 

The  efferent  fibers  have  their  origin  in  the  cells  of  Purkinje  and 
the  dentate  nucleus.  Some  of  these  fibers  emerge  from  the  cere- 
bellum in  the  superior 
peduncles  through 
which  they  pass  to- 
ward and  beneath  the 
corpora  quadrigemina 
to  terminate  around 
the  cells  of  the  red 
nucleus.  As  they  ap- 
proach this  nucleus 
some  of  the  fibers 
cross  the  median  line 
and  decussate  with 
those  coming  from  the 
opposite  side,  while 
others  pursue  a 
straight  direction, 
terminating  on  the 
same  side.  Through 
the  intervention  of 
fibers  which  arise  from 
the  red  nucleus  and 
ascend  to  the  cerebral 
cortex,  the  cortex  is 
thus  connected  with 
both  sides  of  the 
cerebellum,  though 
chiefly  with  the  oppo- 
site side. 

Efl'erent  fibers  also 
leave  the  cerebellum  by 
the  middle  peduncle 
and   pass   directly  to 

the  nucleus  pontis,  around  the  cells  of  which  their  terminals  arborize. 
Efferent  fibers  also  descend  the  inferior  peduncles  and  constitute  the 
tract  known  as  the  Lowenthal  and  Marchi  tract,  situated  in  the 
antero-lateral  region  of  the  spinal  cord  in  its  upper  part. 

The  afferent  fibers  come  from  a  variety  of  sources.  Those  found 
in  the  superior  peduncles  come  from  the  red  nucleus;  those  in  the 
middle  peduncles  from  the  nucleus  pontis  of  the  opposite  side,  having 


Fig.  230. — View  of  Cerebellum  in  Section, 
AND  OF  Fourth  Ventricle,  with  the 
Neighboring  Parts.  —  {From  Sappey.) 
I.  Median  groove  fourth  ventricle,  ending 
below  in  the  calamus  scriptorius,  with  the 
longitudinal  eminences  formed  by  the  fas- 
cicvili  teretes,  one  on  each  side.  2.  The  same 
groove,  at  the  place  where  the  white  streaks 
of  the  auditory  nerve  emerge  from  it  to 
cross  the  floor  of  the  ventricle.  3.  Inferior 
peduncle  of  the  cerebellum,  formed  by  the 
restiform  body.  4.  Posterior  pyramid ; 
above  this  is  the  calamvis  scriptorius.  5,  5. 
Superior  peduncle  of  cerebellum,  or  pro- 
cessus e  cerebello  ad  testes.  6,  6.  Fillet  to 
the  side  of  the  crura  cerebri.  7,  7.  Lateral 
grooves  of  the  crura  cerebri.  8.  Corpora 
quadrigemina. — -{After  Hirschfcld  and  Lc- 
veille.) 


532 


TEXT-BOOK  OF  PHYSIOLOGY. 


crossed  or  decussated  at  the  raph6  near  the  anterior  surface  of  the 
pons;  those  contained  in  the  inferior  peduncles  are  the  most  abundant 
and  important,  and  are  represented  by  (i)  the  direct  cerebellar  tract, 
which  terminates  in  the  superior  vermis  after  decussation;  (2)  the 
anterior  and  posterior  arcuate  fibers,  the  former  coming  from  the 
gracile  and  cuneate  nuclei  of  the  opposite  side,  the  latter  from  the 
same  side,  which  also  pass  to  the  superior  vermis;  (3)  the  acustico- 
cerebellar  tract,  composed  of  fibers  the  axons  of  the  sensory  end 
nuclei  (Deiters)  of  the  vestibular  portion  of  the  auditory  nerve.     It 

is  probable  that  all  these  fibers  de- 
cussate prior  to  their  final  termina- 
tion. 

The  cerebellum  through  this 
system  of  efferent  and  aft'erent  fibers 
is  brought  into  relation  with  many 
different  regions  of  the  cerebrum, 
pons,  medulla,  and  spinal  cord. 
Each  half  of  the  cerebellum  is  con- 
nected with  the  foregoing  structures 
of  the  same  side,  but  more  especially 
of  the  opposite  side. 


Fig.  231.  —  Section  of  Cere- 
bellar Cortex.  A.  Outer 
or  molecular  layer.  B. 
Inner  or  granular  laver. 
C.  White  matter,  a.  Cell 
of  Purkinje.  b.  Small  cells 
of  inner  layer,  c.  Dendrites 
of  these  cells,  d.  A  similar 
cell  lying  in  the  white 
matter. — (Stirling.) 


THE   FUNCTIONS    OF  THE    CEREBEL- 
LUM. 

From  the  observations  of  the 
results  of  experimental  lesions,  from 
analysis  of  clinico-pathologic  facts, 
and  from  its  comparative  anatomic 
development  in  different  animals,  the 
deduction  has  been  drawn  that  the 
cerebellum  coordinates  and  har- 
monizes the  action  of  those  muscles 
the  activities  of  which  are  necessary 
to  the  maintenance  of  body  equihbrium  both  during  station  and 
progression. 

By  equilibrium  of  the  body  is  understood  a  condition  which  may 
be  maintained  for  a  variable  length  of  time  without  displacement,  and 
is  possible  only  so  long  as  a  hne  passing  through  the  center  of  gravity 
falls  within  the  base  of  support.  The  support  offered  by  the  earth 
to  the  feet  neutralizes  and  counteracts  the  force  of  gravity.  In  station, 
when  the  body  is  in  the  erect  or  mihtary  position,  the  arms  by  the 
side,  the  center  of  gravity  lies  between  the  sacrum  and  the  last  lumbar 
vertebra,  and  the  line  of  gravity  falls  between  the  feet  and  within 
the  base  of  support.  The  entire  skeleton  for  the  time  being  is  ren- 
dered fixed  and  rigid  at  all  its  joints  by  the  combined  action  of  the 


THE   CEREBELLUM.  533 

muscles  connected  with  it.  That  this  position  may  be  maintained  all 
the  different  groups  of  antagonistic  but  cooperative  muscles  must  be 
accurately  coordinated  in  their  actions.  Any  failure  in  this  respect  is  at 
once  attended  by  a  disturbance  of  the  equilibrium  and  displacement. 

In  progression,  walking,  running,  dancing,  etc.,  the  body  is  trans- 
lated from  point  to  point  by  the  alternate  action  of  the  legs.  Whether 
the  direction  of  the  translation  be  linear  or  curvilinear,  as  the  legs 
change  their  position  from  moment  to  moment,  the  center  of  gravity 
also  changes,  and  at  once  the  equihbrium  is  menaced.  If  it  is  to  be 
maintained  and  displacement  prevented  there  must  be  a  prompt 
readjustment  in  the  relation  of  all  parts  of  the  body  so  that  the  line 
of  gravity  falls  again  within  the  base  of  support.  The  more  com- 
plicated the  movements  of  progression,  or  the  narrower  the  base  of 
support,  the  greater  is  the  danger  to  the  equilibrium,  and  hence  the 
necessity  for  rapid  and  compensatory  changes  in  coordinated  muscle 
activity.  All  movements  of  this  character,  in  man  at  least,  are  pri- 
marily volitional  and  require  for  their  performance  the  constant  exercise 
of  the  attention.  With  frequent  repetition  they  gradually  come  to 
be  performed  independently  of  consciousness  and  fall  into  the  cate- 
gory of  secondary  or  acquired  reflexes. 

Though  coordinating  power  is  exhibited  by  the  spinal  cord, 
medulla,  and  basal  ganglia,  it  is  only  in  the  cerebellum  that  this 
power  attains  its  highest  development  and  differentiation.  To  it 
is  assigned  the  power  of  selecting  and  grouping  muscles,  not  in  any 
restricted  part,  but  in  all  parts  of  the  body,  and  coordinating  their 
actions  in  such  a  manner  as  to  preserve  the  equilibrium. 

The  Results  of  Experimental  Lesions. — If  the  cerebellum  in 
its  totality,  coordinates  and  harmonizes  the  action  of  the  muscles 
on  the  opposite  sides  of  the  body,  any  derangement  of  its  structure  or 
its  connections  with  the  cord,  medulla,  pons,  or  basal  ganglia  should 
at  once  be  followed  by  incoordination  of  muscles  and  a  want  of  har- 
mony in  their  action.  Experimental  lesions  of  the  cerebellum  are 
attended  by  such  results.  The  phenomena  observed  are  many  and 
complex.  They  differ  in  extent  and  character  in  different  animals 
and  in  accordance  with  the  extent  and  location  of  the  lesion,  though 
the  note  of  incoordination  runs  through  them  all. 

Removal  of  one  lateral  half  of  the  cerebellum  in  the  dog  is  followed 
by  an  inability  to  maintain  the  equihbrium  necessary  to  the  erect 
position.  On  attempting  to  stand,  the  animal  at  once  falls  toward 
the  side  of  the  lesion,  the  muscles  of  which  at  the  same  time  contract 
and  give  to  the  body  a  distinctly  curved  condition  (Fig.  232).  The 
anterior  hmbs  are  extended  to  the  opposite  side.  On  making  efforts 
to  regain  the  standing  position,  the  animal  may  roll  over  around  the 
long  axis  of  its  body.  Conjugate  deviation  of  the  eyes  is  frequently 
observed  as  well  as  nvstagmus. 


534 


TEXT-BOOK  OF  PHYSIOLOGY. 


After  a  few  days  the  symptoms  partially  subside  and  the  animal 
acquires  the  power  of  sitting  on  the  abdomen  when  the  anterior 
limbs  are  widely  extended  (Fig.  233).  As  the  days  go  by  the  improve- 
ment continues,  and  the  animal  recovers  the  power  of  walking,  though 
each  step  is  attended  with  tremor  and  oscillations  of  the  body.     Any 


Fig.  232. — Attitude  Assumed  After   Destruction  of  the  Left  Half 
OF  THE  Cerebellum. — {Morat  and  Doyoii,  after  Thomas.) 


change  in  the  center  of  gravity  such  as  results  when  one  leg  is  Hfted 
may  result  in  a  fall  toward  the  side  of  the  lesion,  owing  to  an 
inabihty  to  promptly  bring  about  the  necessary  compensatory 
muscle  actions.  With  time  the  animal  continues  to  improve  in  its 
power  of  adjustment,  though  it  never  completely  recovers  it.  Move- 
ments of  progression  are  apt 

\ 


to  be  characterized  by  stiffness 
and  accompanied  by  tremor 
suggestive  of  volitional  efforts. 
Total  removal  of  the  cere- 
bellum is  followed  by  a  differ- 
ent train  of  symptoms.  The 
extensor  muscles  apparently 
preponderate  in  their  action, 
for  the  limbs  are  extended 
and  abducted,  the  head  and 
neck  are  retracted,  and  opis- 
thotonos is  established.  In 
time  these  effects  also  partially 
subside,  though  all  attempts 
at  walking  are  permanently 
accompanied  by  tremor  and  oscillations.  The  characteristic  effect 
which  follows  section  of  the  peduncles  is  again  incoordination, 
manifesting  itself  in  deviation  of  the  head,  eyes,  inability  to  walk, 
tremor  on  exertion,  etc.  The  effects  vary,  however,  according  to  the 
peduncle  divided.  Section  of  the  middle  peduncle  gives  rise  to  the 
most  pronounced  eft'ects.  The  head  and  the  anterior  part  of  the 
body  are  at  once  drawn  toward  the  pelvis  on  the  side  of  the  section. 


Fig.  233. 
the 


—Attitude  in  Repose  after 
Complete  Removal  of  the 
Cerebellum  but  during  the 
Period  of  Restoration  of  Func- 
tion.—  {Morat  and  Doyon,  after 
Thomas.) 


THE  CEREBELLUM. 


535 


A  voluntary  effort  on  the  part  of  the  animal  causes  it  to  lose  all 
control  of  its  muscles  and  the  body  is  rotated  in  the  direction  of  its 
longitudinal  axis  from  40  to  60  times  a  minute  before  it  comes  to  rest. 
According  as  the  lesion  is  made  from  behind  or  before,  the  rotation 
is  from  or  to  the  side  of  the  section.  In  time  these  symptoms 
subside,  though  the  animal  never  completely  recovers. 

The  partial  recovery  of  the  power  of  coordination,  observed  after 
removal  of  a  portion  or  the  whole  of  the  cerebellum,  indicates  that 
the  centers  in  the  cord,  medulla,  pons,  and  cerebrum  endowed  with 
corresponding  though  less  developed  power,  develop  compensatory 
activity  and  acquire  to  some  extent  the  capabilities  of  the  cerebellum 
itself  (Fig.  234). 

Clinico- pathologic  facts  partly  corroborate  the  results  of  phys- 
iologic investigations.  In  various  forms  of  uncomplicated  cere- 
bellar disease,  vertigo,  tremor  on  making  voluntary  efforts,  difficulty 
in  maintaining  the  erect 
position,  unsteadiness  in 
walking,  opisthotonos, 
pleurothotonos,  are 
among  the  symptoms 
generally  observed. 

Comparative  anatomic 
investigations  reveal  a 
remarkable  correspond- 
ence between  the  de- 
velopment of  the  cerebel- 
lum and  the  complexity 
of  the  movements  ex- 
hibited by  animals.     In 

those  animals  whose  movements  are  complex  and  require  for  their 
performance  the  cooperation  of  many  groups  of  muscles  the  cere- 
bellum attains  a  much  greater  development  in  reference  to  the  rest 
of  the  brain  than  in  animals  whose  movements  are  relatively  simple 
in  character.  This  relative  increase  in  the  development  of  the 
cerebellum  is  found  in  many  animals,  such  as  the  kangaroo,  the 
shark,  the  swallow,  and  the  predaceous  birds  generally. 

The  Coordinating  Mechanism. — Though  it  is  not  known  how 
the  cerebellum  selects  and  coordinates  groups  of  muscles  for  the  per- 
formance of  any  complex  movement,  it  is  known  that  its  activity  is 
largely  reflex  in  origin  and  excited  by  impulses  reflected  to  it  from 
peripheral  organs.  In  this  as  in  other  forms  of  reflex  activity  the 
mechanism  involves  (i)  afferent  nerves,  e.  g.,  cutaneous,  muscle,  optic, 
and  vestibular,  and  their  related  end-organs,  tactile  corpuscles, 
muscle  spindles,  retina,  and  semicircular  canals,  all  indirectly  con- 
nected with  (2)  the  cerebellar  centers;  (3)  efferent  nerves  indirectly 


Fig.  234. — Progression  after  Destructiox 
OF  THE  Vermis.  —  {Moral  and  Doyon, 
after  Thomas.) 


536  TEXT-BOOK  OF  PHYSIOLOGY. 

connected  with  (4)  the  general  musculature  of  the  body.  Both  station 
and  progression  are  directly  dependent  on  the  development  and  trans- 
mission of  afferent  impulses  from  the  previously  mentioned  periph- 
eral sense-organs  to  the  cerebellum.  Tactile,  muscle,  visual,  and 
labyrinthine  impressions  and  sensations  not  only  cooperate  in  the 
development  and  organization  of  the  motor  adjustments  necessary 
to  the  maintenance  of  the  equilibrium  and  locomotive  coordination, 
but  even  after  their  organization  they  are  necessary  to  the  excitation 
of  cerebellar  activity.  The  manner  in  which  they  lead  to  the  develop- 
ment of  this  capabihty  on  the  part  of  the  cerebellum  is  conjectural. 
Their  ever-present  influence  is  shown  by  the  effects  which  follow  their 
removal,  as  the  following  facts  indicate. 

The  prevention  of  the  development  of  tactile  impulses  by  freezing 
or  anesthetizing  the  soles  of  the  feet,  and  the  blocking  of  normally  de- 
veloped impulses  through  destruction  of  afferent  pathways  in  diseases 
of  the  spinal  cord  lead  at  once  to  marked  impairment  in  the  coordinat- 
ing power.  The  removal  of  the  skin  from  the  hind  legs  of  the  frog, 
previously  deprived  of  its  cerebrum,  destroys  its  coordinating  power, 
which  it  would  otherwise  possess  in  a  high  degree. 

The  blocking  in  consequence  of  destructive  lesions  of  the 
spinal  cord,  of  the  impulses,  which  come  from  the  muscles,  tendons, 
etc.,  and  which  inform  us  of  the  activity  and  the  degree  of  activity 
of  our  muscles,  the  location  of  the  hmbs,  the  amount  of  effort  necessary 
to  produce  a  given  movement,  etc.,  also  gives  rise  to  much  incoor- 
dination. A  blocking  of  both  tactile  and  muscle  impulses  frequently 
exists  in  degeneration  or  sclerosis  of  the  posterior  columns  of  the 
spinal  cord.  The  coordinating  power  is  so  much  impaired  in  this 
disease  that  the  patient  is  unable  to  maintain,  without  strained  effort, 
the  erect  position  and  especially  if  the  directive  power  of  the  eyes  be 
removed  by  closure  of  the  hds.  Walking  becomes  extremely  difficult ; 
the  gait  is  irregular  and  jerky,  and  equilibrium  is  maintained  only 
by  keeping  the  eyes  fixed  on  the  ground  in  front  and  by  artificially 
increasing  the  basis  of  support  by  the  use  of  canes. 

An  interference  with  the  development  of  the  customary  visual 
impressions  which  in  a  measure  maintain  the  sense  of  relation  of  the 
individual  to  surrounding  objects  also  gives  rise  to  equihbratory  dis- 
turbances. A  rapid  change  in  the  relation  of  the  individual  to  sur- 
rounding objects  or  the  reverse;  a  change  in  the  direction  of  one 
optic  axis  from  the  use  of  a  prism  or  from  paralysis  of  an  eye  muscle ; 
the  destruction  of  an  eye; — these  and  similar  conditions  frequently 
give  rise  to  such  marked  disturbances  of  the  equihbratory  power  that 
displacement  is  difficult  to  prevent. 

An  interference  with  the  development  of  the  so-called  labyrin- 
thine impressions  by  destruction  of  the  semicircular  canals  gives  rise 
to  the  most  remarkable  disturbances  in  this  respect.     Section  of  one 


THE  CEREBELLUM.  537 

horizontal  canal*  in  the  pigeon  is  followed  by  oscillations  of  the 
head  in  a  horizontal  plane  around  a  vertical  axis.  Bilateral  section 
so  increases  these  oscillations  that  the  pigeon  is  unable  to  maintain 
equilibrium  and  forced  to  fall  and  turn  continuously  around  the 
vertical  axis.  Bilateral  section  of  the  posterior  vertical  canals  gives  rise 
to  oscillations  around  a  horizontal  axis  which  frequently  become  so 
exaggerated  as  to  eventuate  in  the  turning  of  backward  somersaults, 
head  over  heels.  Similar  phenomena  follow  division  of  the  superior 
vertical  canals. 

Bilateral  destruction  of  both  sets  of  canals  is  attended  by  extra- 
ordinary disturbances  in  the  equihbrium.  From  the  moment  of  the 
operation  the  animal,  the  pigeon,  loses  all  control  of  its  motor  mechan- 
isms. It  can  neither  maintain  a  fixed  attitude  nor  execute  orderly 
movements  of  progression ;  its  activity,  continuous  and  uncontrollable, 
is  characterized  by  spinning  around  a  vertical  axis,  turning  somer- 
saults, dashing  itself  against  surrounding  objects  until  hfe  is  endan- 
gered. If  the  animal  be  protected  from  injury,  these  disturbances 
gradually  subside,  and  in  the  course  of  a  few  months  the  equihbratory 
power  is  so  far  regained  that  standing  and  walking  at  least  become 
possible.  In  this  condition,  however,  the  coordinating  power  is 
directly  dependent  on  visual  impulses,  for  with  the  closure  of  the 
eyes  all  the  previous  motor  disturbances  at  once  recur.  These  and 
similar  facts  indicate  that  the  semicircular  canals  are  the  peripheral 
sense-organs  from  which  come  the  nerve  impulses  most  essential  to 
the  excitation  of  the  cerebellar  coordinative  centers  in  their  control  of 
equilibrium  and  of  progression. 

The  cerebellum  may  therefore  be  regarded  as  the  essential,  most 
highly  differentiated  portion  of  the  coordinating  mechanism  con- 
cerned in  the  maintenance  of  equihbrium,  during  both  station  and 
progression.  The  manner  in  which  the  cerebellum  accomplishes 
this  result  is  unknown,  though  it  is  certain,  from  the  foregoing  facts, 
that  its  special  mode  of  activity  is  dependent  on  the  excitatory  action 
of  nerve  impulses  reflected  from  a  variety  of  peripheral  sense-organs. 

*  The  physiologic  anatomy  of  the  semicircular  canals  is  described  in  the  chapter 
devoted  to  the  ear,  to  which  the  reader  is  referred. 


CHAPTER  XXI. 
THE  CRANIAL  NERVES. 

The  nerve-trunks  which  serve  as  channels  of  communication 
between  the  encephalon  and  the  structures  of  the  head,  the  face, 
and  in  part  the  organs  of  the  thorax  and  abdomen,  pass  through 
foramina  in  the  walls  of  the  cranium,  and  for  this  reason  are  termed 
cranial  nerves. 

According  to  the  classification  now  generally  adopted,  there  are 
twelve  cranial  nerves  on  either  side  of  the  median  line,  which,  enu- 
merated from  before  backward,  are  as  follows  (Fig.  235): 

First  or  Olfactory.  Seventh  or  Facial. 

Second  or  Optic.  Eighth  or  Auditory. 

Third  or  Oculo-motor.  Ninth  or  Glosso-pharyngeal. 

Fourth  or  Patheticus.  Tenth  or  Pneumogastric  or  Vagus. 

Fifth  or  Trigeminal.  Eleventh  or  Spinal  Accessory. 

Sixth  or  Abducens.  Twelfth  or  Hypoglossal. 

The  cranial  nerves  may  be  classified  physiologically  in  accordance 
with  their  functional  manifestations  into  three  groups,  viz. : 

1.  Nerves  of  Special  Sense:  e.g.,  Olfactory,    Optic,    Auditory,    Gustatorj'    (Glosso- 

pharyngeal). 

2.  Nerves  of  General  SensibiKty:  e.g.,  Large     root     of     the     Trigeminal,     Glosso- 

pharyngeal, and  Pneumogastric. 

3.  Nerves  of  Motion:  e.g.,  Oculo-motor,  Patheticus,  the  small  root  of  the  Trigeminal, 

Facial,  Spinal  Accessory,  and  Hypoglossal. 

Though  this  classification  in  the  main  holds  true,  it  must  be  borne 
in  mind  that  modern  investigations  have  demonstrated  that  the  glosso- 
pharyngeal and  pneumogastric  nerves  contain  even  at  their  junction 
with  the  medulla  oblongata  a  number  of  efferent  or  motor  fibers, 
and  to  this  extent  are  mixed  nerves. 

The  Origins  of  the  Cranial  Nerves. — In  accordance  with 
modern  views  as  to  the  origins  of  nerves  in  general,  it  may  be  stated 
that— 

The  nerves  0}  special  sense  have  their  origin  respectively  in  the 
neuro- epithelial  cells  in  the  mucous  membrane  of  the  olfactory  region 
of  the  nose,  in  the  ganghon  cells  of  the  retina,  in  the  cells  of  the  spiral 
ganghon  of  the  cochlea  and  the  ganglion  of  Scarpa,  and  in  the  cells 
of  the  petrous  and  jugular  gangha.  From  the  cells  of  these  gangha 
dendrites  pass  peripherally  to  become  associated  with  speciahzed 
end- organs,  while  axons  pass  centrally  in  well-defined  bundles  to 

538 


THE  CRANIAL  NERVES. 


539 


become   related  by  means  of  their  end-tufts  with  primary  basal 
gangha. 

The  nerves  of  general  sensibility  have  their  origin  in  the  ganglia 
on  their  trunks,  and  in  this  respect  resemble  the  spinal  nerves.  From 
the  ganglion  cell  there  emerges  a  short  axon  process  which  soon 
divides  into  a  central  and  a  peripheral  branch.  The  former  passes 
toward  and  into  the  gray  matter  located  beneath  the  floor  of  the 
fourth  ventricle,  where  its  end- 
tufts  arborize  about  nerve-cells. 
The  latter  (the  peripheral  branch) 
passes  toward  the  general  periph- 
ery to  be  distributed  to  skin  and 
mucous  membranes  (Fig.  236). 

The  nerves  0}  motion  have 
their  origin  in  the  nerve-cells  in 
the  gray  matter  beneath  the 
aqueduct  of  Sylvius  and  beneath 
the  floor  of  the  fourth  ventricle 
(Fig.  237).  The  axons  emerging 
from  these  cells  course  per- 
ipherally to  be  distributed  to 
skeletal  muscles.  In  some  of  the 
motor  nerves,  and  in  some 
sensory  nerves  as  well,  there  are 
to  be  found  efferent  fibers  of 
smaller  size  which  have  a  similar 
origin  and  which  become  related 
through  the  intervention  of 
sympathetic  ganglia  (peripheral 
neurons)  with  visceral  muscles 
and  glands.  These  nerves  have 
been  termed  autonomic  nerves. 

The  Cortical  Connections 
of  the  Cranial  Nerves. — Each 
of  these  three  groups  of  cranial 
nerves  has  special  connections 
with  the  cerebral  cortex. 

The  nerves  of  special  sense  for  the  most  part  terminate  in  primary 
basal  gangha,  around  the  cells  of  which  their  central  end-tufts  ar- 
borize. From  these  cells  axons  arise  which  pass  upward  and  directly 
or  indirectly  come  into  physiologic  relation  with  sensor  nerve-cells 
in  the  cerebral  cortex. 

The  nerves  of  general  sensibility  terminate  in  the  gray  matter 
beneath  the  floor  of  the  fourth  ventricle,  around  the  nerve-cells  of 
which   their  end-tufts   arborize.     These  groups   of  nerve-cells   are 


Fig.  2- 


-Superficial  Origin  of  the 
Cranial  Nerves  from  the  Base 
OF  THE  Encephalon.  I.  Olfactory. 
2.  Optic.  3.  Motor  oculi.  4. 
Patheticus.  5.  Trigeminal.  6.  Abdu- 
cens.  7.  Facial.  7'.  Nerve  of 
Wrisberg.  8.  Auditory.  9.  Glosso- 
pharyngeal. 10.  P'neumogastric. 
II.  Spinal  accessory.  12.  Hypo- 
glossal.— {Morat  and  Doyon.) 


S40 


TEXT-BOOK  OF  PHYSIOLOGY. 


known  as  sensor  end-nuclei.  Though  once  regarded  as  the  centers 
of  origin  of  the  sensor  nerves,  they  are  now  regarded  as  the  centers 
of  origin  of  axons  which  pass  upward  to  the  cortex  of  the  cerebrum, 
where  they  also  come  into  physiologic  relation  with  sensor  nerve- 
cells. 


Fig.  236. — Ganglia  of  Origin  of  the 
Sensor  Cranial  Nerves,  i.  Tri- 
geminal (ganglion  of  Gasser).  2. 
Nerve  of  Wrisberg.  3.  Auditory 
(ganglion  of  Scarpa).  4.  Glosso- 
pharyngeal (ganglion  of  Andersch). 
5.  Pneumogastric  (ganglion  plexi- 
formis). — {After  Moral  and  Doyon.) 


Fig.  237. — Nuclei  of  Origin  of  the 
Motor  Cranial  Nerves.  1. 
Motor  oculi.  2.  Patheticus.  3. 
Motor  root  of  the  trigeminal.  4. 
Abducens.  5.  Facial.  6.  Mixed 
nucleus  for  efferent  fibers  of  the 
glosso-pharyngeal  vagus  and  spinal 
accessory.  7.  Hypoglossus.  8. 
Spinal         accessory.  9.  Spinal 

nerves. — (After  Moral  and  Doyon.) 


The  axons  in  both  of  these  classes  of  nerves  thus  originate  in  the 
cells  of  the  central  nerve  system  and  continue  upward  to  the  cere- 
brum, the  primary  afferent  path. 

The  motor  nerves  which  have  their  origin  in  the  cells  of  the  gray 
matter  beneath  the  aqueduct  of  Sylvius  and  beneath  the  floor  of  the 
fourth  ventricle  are  in  physiologic  relation  with  nerve-cells  in  the 


THE  CRANIAL  NERVES. 


541 


motor  region  of  the  cortex  through  descending  axons  contained  in  the 
pyramidal  tract.  The  end-tufts  of  these  axons  arborize  around  the 
nerve-cells.  The  efferent  path  beginning  in  the  cerebral  cortex  is 
thus  continued  by  the  motor  nerves  to  the  general  periphery. 

The  three  groups  of  nerves,  those  of  special  sense,  of  general 
sensibility,  and  the  motor  nerves,  are  neurons  of  the  first  order;  the 
nerve-cells  and  fibers  which  constitute  the  cerebral  connections  are 
neurons  of  the  second  order.  It  is  possible  that  the  sensor  cells 
in  the  cerebral  cortex  are  neurons  of  a  third  order. 


FIRST  PAIR.     THE  OLFACTORY. 

The  first  cranial  nerve,  the  olfactory,  is  situated  in  the  upper 
third  of  the  nasal  fossa,  in  the  regio  oljactoria.  It  consists  of  from 
20  to  30  branches,  the  fibers  of  which  are  non-meduUated. 

Origin. — The  olfactory  nerve  is  composed  of  centrally  coursing 
axons  which  have  their 
origin  in  the  central  ends 
of  bipolar,  rod-shaped,  or 
spindle-shaped  nerve-cells 
interspersed  among  the  epi- 
thelial cells  covering  the 
mucous  membrane  in  the 
regio  ■  olfactoria ;  the  per- 
ipheral ends  of  these  cells 
give  oft'  a  number  of  den- 
drites which  are  spread  out 
to  form  a  delicate  feltwork 
over  the  surface  of  the 
mucous  membrane.  From 
their  origin  the  axons 
gradually  converge  to  form 
bundles  which  ascend  to 
the  cribriform  plate  of  the 
ethmoid  bone,  through  the 
foramina  of  which  they  pass 
to  become  related  by  their 
end-tufts  mth  structures  in 
the  gray  matter  of  the 
olfactory  bulb  (Fig.  238). 

Cortical  Connections. 
— The   olfactory  bulb  and 

olfactory  tract,  formerly  called  the  olfactory  nerve,  are  portions  of 
the  cerebrum  (the  olfactory  lobe)  which  arise  embryologically  by  a 
protrusion  of  the  walls  of  the  cerebral  ca\dty.    The  bulb  is  oval-shaped 


Fig.  238. — The  Relation  of  the  Olfactory 

Nerves  to  the  Olfactory  Tract,  i 
Olfactory  nerve-cell.  2.  Axon  process.  3. 
Epithelial  cells.  4.  Glomerulus.  5.  Mitral 
cells.  6.  Centrally  coursing  axons  of  the 
olfactory  tract. — {Moral  and  Doyon.) 


542 


TEXT-BOOK  OF  PHYSIOLOGY. 


il'illfS, 


and  consists  of  both  gray  and  white  matter.  It  rests  on  the  cribriform 
plate  of  the  ethmoid  bone  and  is  embraced  by  the  olfactory  nerves. 
As  seen  on  sagittal  section,  there  is  just  beneath  the  surface  a  layer 
of  large  pyramidal  and  spindle-shaped  cells  (termed  also  mitral  cells), 
each  provided  with  an  apical  and  two  lateral  dendrites.  The  apical 
dendrite  passes  toward  the  surface  and  ends  in  a  brush-  or  basket-Hke 
expansion  which  interlaces  with  the  end-tufts  of  the  olfactory  nerves, 

forming  what  are 
known  as  the  olfac- 
tory glomerules.  The 
lateral  dendrites  end 
free. 

The  axons  of  the 
pyramidal  cells  pass 
toward  the  center  of 
the  bulb  and  bend  at 
right  angles,  after 
which  they  pursue  a 
horizontal  direction 
toward  and  into  the 
olfactory  tract.  This 
tract  is  about  five 
centimeters  in  length, 
prismatic  in  shape  on 
cross-section  and  di- 
visible into  a  ventral 
and  a  dorsal  portion. 
It  emerges  from  the 
posterior  extremity  of 
the  bulb,  passes  back- 
ward to  the  posterior 
part  of  the  anterior 
lobe,  where  it  divides 
into  three  roots:  viz., 
a  lateral  or  external, 
a  mesial  or  internal,  a 
middle  or  dorsal.  The 
fibers  of  the  lateral  and  mesial  roots  are  derived  almost  exclusively 
from  the  ventral  portion  of  the  tract,  the  fibers  of  which  come  from  the 
mitral  cells  in  the  bulb.  The  lateral  root-fibers  pass  outward  into  the 
fossa  of  Sylvius  and  come  into  relation  with  nerve-cells  in  the  inferior 
extremity  of  the  gyrus  hippocampus  and  the  gyrus  uncinatus.  The 
mesial  fibers  pass  inward  and  come  into  relation  with  nerve-cells 
in  the  pre-callosal  part  at  least  of  the  gyrus  fornicatus.  The  fibers 
thus  far  considered  are  undoubtedly  true  olfactory  fibers,  pursuing 


Fig. 


239. — Olfactory  Lobe  of  the  Human  Brain. — 
Bii.  Olfactory  bulb.  T.  Tract.  Tr.o.  Trigone.  R. 
Rostrum  of  corpus  callosum.  p.  Peduncle  of  cor- 
pus callosum,  passing  into  G.s.,  gyrus  subcallosus 
(diagonal  tract,  Broca).  Br.  Broca's  area.  P.p. 
Fissura  prima.  F.s.  Fissura  serotina.  C.a.  Posi- 
tion of  anterior  commissure.  L.t.  Lamina  ter- 
minalis.  Ch.  Optic  chiasma.  T.o.  Optic  tract. 
p.olf.  Posterior  olfactory  lobule  (or  anterior  per- 
forated space).  7n.r.  Mesial  root.  l.r.  Lateral 
root  of  tract. — (His.) — (After  Quain.) 


THE  CRANIAL  NERVES.  543 

a  centripetal  direction,  carrying  nerve  impulses  from  the  olfactory  cells 
to  the  cerebrum  (Fig.  239). 

Histologic  and  embryologic  methods  of  research  have  shown  that 
some  of  the  fibers  in  the  olfactory  tract  are  centrifugal  in  direction. 
They  originate  in  the  olfactory  cortical  areas,  pass  toward  the 
peripher}'  as  far  as  the  anterior  commissure,  where  they  cross  to 
become  the  dorsal  root,  enter  the  olfactory  tract,  and  finally  terminate 
in  the  bulb.  This  tract  serves  to  connect  the  cortex  with  the  bulb  of 
the  opposite  side,  and  carries  impulses  from  the  cortex  to  the  bulb. 
The  two  opposite  cerebral  olfactory  areas  are  also  united  by  com- 
missural fibers  which  decussate  at  the  anterior  commissure. 

Functions. — The  olfactor}'  nerves,  including  the  olfactory  tract, 
are  channels  of  communication  between  the  olfactor}'  region  in  the 
nose  and  the  cerebral  cortex.  The  stimulus  to  its  excitation  is  the 
impact  and  chemic  action  of  gaseous  or  volatile  organic  matter 
on  the  dendrites  of  the  olfactory  cells.  The  energy  set  free  develops 
nerve  impulses  which,  travehng  through  the  entire  olfactory  tract 
to  the  cortex,  evoke  the  sensation  of  odor.  The  sensitiveness  of  the 
olfactory  end-organ  to  the  action  of  many  substances  is  remarkable, 
responding,  for  example,  to  the  tto i. oinr  of  a  gram  of  oil  of  roses  and 
to  the  o-.Te^TT.Toir  of  a  gram  of  mercaptan. 

Division  or  destruction  of  the  olfactory  path  at  any  point  is  fol- 
lowed by  an  abolition  of  the  sense  of  smell  on  the  corresponding  side. 
Destructive  lesions  of  the  hippocampal  and  uncinate  gyri  are  fol- 
lowed bv  similar  results. 


SECOND  PAIR.     THE  OPTIC. 

The  second  cranial  nerve,  the  optic,  consists  of  centrally  coursing 
axons  of  neurons,  which  connect  the  essential  part  of  the  organ  of 
vision,  the  retina,  with  sensory  end-nuclei  or  ganglia  situated  at  the 
base  of  the  cerebrum. 

Origin. — The  axons  which  constitute  the  optic  nerve  have 
their  origin  in  the  ganglionic  cells  in  the  anterior  part  of  the  retina. 
Through  their  dendrites  these  cells  are  brought  into  relation  pos- 
teriorly with  successive  layers  of  cells  which  collectively  constitute  the 
retina.  Though  the  retina  is  said  to  consist  of  ten  or  eleven  layers, 
it  may  be  reduced  practically  to  three,  viz.  (Fig.  240) : 

1.  The  layer  of  visual  cells. 

2.  The  layer  of  bipolar  cells. 

3.  The  layer  of  ganglionic  cells. 

The  visual  cells  present  peripherally  modified  dendrites,  known 
as  the  rods  and  cones;  centrally  they  give  off  an  axon  which  after 
a  short  course  terminates  in  an  end-tuft.  The  bipolar  cells  also 
possess  dendrites  and  an  axon;  the  former  interlace  with  the  end- 


544 


TEXT-BOOK  OF  PHYSIOLOGY. 


tufts  of  the  visual  cell  axon,  the  latter  with  the  dendrites  of  the  gang- 
lion cell.  The  retina  may  be  regarded  therefore  as  the  peripheral 
end-organ  in  which  the  optic  nerve  originates.  From  their  origin  the 
axons  turn  backward,  at  the  same  time  converging  to  form  a  distinct 
bundle  which  passes  through  the  chorioid  coat  and  sclera.  After 
emerging  from  the  eyeball  the  nerve-bundle  (the  optic  nerve)  passes 
backward  as  far  as  the  sella  turcica,  traversing  in  its  course  the  or- 
bit cavity  and  the  optic  foramen.  At  the  sella  turcica  there  is  a 
union  and  partial  decussation  in  man  and  other  mammals  of  the 
two  nerves,  forming  the  optic  chiasm. 

Decussation  of  the  Optic  Nerves. — The 
results  of  various  methods  of  research  would  seem 
to  indicate  that  the  fibers  from  the  nasal  third  of 
the  retina  of  the  left  eye  cross  in  the  chiasm  to 
unite  with  the  fibers  from  the  temporal  two-thirds 
of  the  retina  of  the  right  eye.  In  a  'similar 
manner  the  fibers  from  the  nasal  third  of  the 
retina  of  the  right  eye  cross  in  the  chiasm  and 
unite  with  the  fibers  from  the  temporal  two-thirds 
of  the  retina  of  the  left  eye.  Posterior  to  the 
chiasm  the  crossed  and  uncrossed  fibers  form 
the  so-called  optic  tracts,  which  after  winding 
around  the  crura  cerebri  enter  the  optic  'basal 
ganglia.  Transection  of  the  optic  nerve  shows 
that  it  is  composed  of  an  enormous  number  of 
non-medullated  nerve-fibers,  estimated  by;Salzer 
at  from  450,000  to  800,000,  enclosed  in  a  sheath 
of  the  dura  mater.  In  the  central  portion  of  the 
nerve  there  is  seen  a  distinct  bundle  of  fibers, 
triangular  in  shape,  that  come  from  the  region  of 
the  macula  lutea.  At  the  chiasm  this  bundle  of 
fibers  undergoes  a  partial  decussation  similar  to 
that  of  the  fibers  coming  from  the  more  peripheral 
portions  of  the  retina.  In  the  left  optic  tract, 
therefore,  fibers  from  four  different  regions  are  to> 
be  found:  viz.,  the  two-thirds  of  the  temporal  side  of  the  left  retina, 
the  temporal  half  of  the  left  macula,  the  nasal  third  of  the  right 
retina,  and  the  nasal  half  of  the  right  macula.  Corresponding  fibers 
are  to  be  found  in  the  right  optic  tract.  As  the  optic  tract  passes 
around  the  crus  cerebri  it  divides  into  a  lateral  or  outer,  and  a  mesial 
or  inner  bundle,  which  then  terminate  in  the  optic  basal  ganglia. 
The  fibers  of  the  lateral  bundle  are  traceable  into  the  external 
geniculate  body  (the  pre-geniculum),  the  pulvinar  of  the  optic 
thalamus,  and  the  anterior  quadrigeminal  body  (the  pregeminum). 
These  are  in  all  probabihty  the  true  visual  fibers.     The  fibers  of 


Fig.  240. — Retinal 
Cells,  s',  z'. 
Visual  cells  with 
their  peripheral 
terminations.  5. 
Rods.  z.  Cones. 
b.  Bipolar  cells. 
g.  Ganglion  cells 
from  which  arise 
the  axons  of  the 
optic  nerve. 


THE  CRANIAL  NERVES. 


545 


VISUAL  NEW 


VISUAL  nELD 


the  mesial  bundle  are  traceable  into  the  internal  geniculate  body 
(the  post-geniculum)  and  the  posterior  quadrigeminal  body  (the  post- 
geminum). 

Cortical   Connections. — ^After  entering   the  basal   gangha  the 
visual  fibers  terminate  in  end-tufts  which  arborize   around  nerve- 
cells.     From  these  cells  new  axons  arise  which  ascend  through  the 
posterior  part  of  the  internal 
capsule,  at  the  same  time  curv- 
ing backward  to  form  the  op- 
tic radiation  of  Gratiolet,  and 
terminate   finally  around 
nerve-cells  in  the  gray  matter 
of  the  cuneus  and  in  the  gray 
matter  bordering  the  calcarine 
fissure,  both  situated  on  the 
mesial  aspect  of  the  occipital 
lobe. 

Cenlrijugal  Fibers  of  the 
Optic  Nerve. — All  the  fibers 
previously  alluded  to  have 
been  aft'erent  or  centripetal  in 
direction;  but  the  optic  nerve 
also  contains  efferent  or  centri- 
fugal fibers  which  come  from 
nerve-cells  in  the  basal  ganglia 
and  ramify  around  special 
cells,  the  amacrine  cells,  in 
the  retina.  Their  function  is 
unknown.  It  has  been  sug- 
gested that  they  regulate  the 
vascular  supply  to  the  retina. 
Centrifugally  coursing  fibers 
also  connect  the  visual  areas 
of  the  cortex  ^vith  the  basal 
ganglia. 

Functions.  —  The  optic 
nerve  apparatus  in  its  entirety  connects  the  visual  cells  of  the  retina 
with  the  cells  of  the  cerebral  cortex.  The  excitation  of  this  appa- 
ratus evokes  the  sensation  of  light  and  its  dift"erent  quahties — colors. 

The  specific  physiologic  stimulus  of  the  visual  retinal  cells  is  the 
impact  of  the  undulations  of  the  ether.  The  energy  set  free  excites 
in  the  optic  nerve,  nerve  impulses  which  are  transmitted  first  to  the 
optic  ganglia  and  then  to  the  cerebral  cortex,  where  they  evoke  the 
sensations  of  light  and  color. 

Iris  Reflex. — The  optic  nerve  also  assists  in  the  automatic  reg- 
35 


Fig.  241. — Diagram  Illustrating  Left 
Homonymous  Lateral  Hemianopsia 
FROM  A  Lesion  of  the  Right  Optic 
Tract  or  the  Right  Cuneus.  The 
Shaded  Lines  in  the  Visual  Fields 
Indicate  the  D.\rkened  Area. 


546 


TEXT-BOOK  OF  PHYSIOLOGY. 


ulation  of  the  size  of  the  pupil.  Some  of  its  fibers  form  the  afferent 
part  of  the  reflex  nerve  mechanism  by  which  the  circular  fibers  of  the 
iris  (the  sphincter  pupillae)  are  excited  to  contraction.  These  fibers 
arborize  around  nerve- cells  in  the  anterior  quadrigeminal  body,  from 
which  axons  descend  in  the  posterior  longitudinal  bundle.  In  their 
course  they  give  off  collateral  branches  to  the  nuclei  of  origin  of  the 
oculo-motor  nerve,  from  which  nerve-fibers  pass  to  the  iris. 

Light  falling  on  the  retina  generates  nerve  impulses  which,  when 
conducted  through  the  aflPerent  and  efferent  pathways  just  mentioned, 
stimulate  the  circular  fibers  to  contraction.  The  extent  of  the  con- 
traction will  depend  on  the  in- 
tensity of  the  fight.  In  the 
absence  of  all  fight  the  muscle 
completely  relaxes. 

Hemiopia  and  Hemian- 
opsia.— Division  of  the  optic 
nerve  between  the  eyeball 
and  the  optic  chiasm  is  fol- 
lowed by  complete  blindness 
in  the  eye  of  the  corresponding 
side.  Owing  to  the  partial 
decussation  of  the  fibers  in  the 
chiasm,  division  of  an  optic 
tract  is  followed  by  a  loss  of 
sight  in  the  outer  two-thirds  of 
the  eye  of  the  same  side  and  in 
the  inner  third  of  the  eye  of  the 
opposite  side.  To  this  loss  of 
visual  power  in  the  retina  the 
term  hemiopia  is  given.  In 
consequence  of  this  loss  of  vis- 
ual power  in  the  retina  there  is 
a  corresponding  obscuration 
or  total  obliteration  of  nearly  one-half  of  the  visual  field,  to  which 
the  term  hemianopsia  is  given.  If,  for  example,  the  right  optic 
tract  is  divided  there  will  be  hemiopia  in  the  outer  two-thirds  of  the 
right  eye  and  the  inner  third  of  the  left  eye,  with  lejt  lateral  hemian- 
opsia, and  as  the  portions  of  the  retina  which  are  affected  are 
associated  in  vision  the  loss  of  the  visual  fields  is  spoken  of  as 
homonymous  hemianopsia  (Fig.  241).  A  destructive  lesion  of  the 
cerebral  visual  area,  the  cuneus  and  the  adjacent  gray  matter  on 
the  right  side,  is  also  followed  by  left  lateral  hemianopsia.* 


Fig.  242. — Diagram  to  Show  the  Exist- 
ence OF  Hemianopsia.  The  lesion  is 
supposed  to  be  in  the  right  optic  tract. 


*  It  should  be  borne  in  mind  that  in  both  instances  the  retina  itself  is  unaffected. 
The  impact  of  light  generates,  as  usual,  nerve  impulses  which  proceed  as  far  back- 
ward as  the  point  of  division  or  destruction.  In  consequence  those  portions  of  the 
cerebral  cortex  stimulation  of  which  evokes  the  sensation  of  light  remain  unaffected 


THE  CRANIAL  NERVES.  547 

The  existence  of  an  homonymous  hemianopsia  becomes  evident 
when  the  individual  is  directed  to  focus  the  vision  on  an  object  placed 
directly  in  front  and  with  its  center  in  the  median  plane  of  the  body, 
when  if  the  lesion  be  on  the  right  side,  the  left  half  of  the  object  will 
be  invisible.  The  reason  for  this  will  be  apparent  on  reference  to 
Fig.  242.  All  the  hght  rays  emanating  from  the  left  half  of  the 
object  fall  on  the  retina  on  the  side  of  the  injury,  and  hence  there 
will  be  no  sensation.  If,  however,  the  object  be  moved  to  the  right 
without  change  in  the  position  of  the  head,  the  entire  object  will  be 
visible,  as  all  the  rays  fall  on  the  normal  side.  If,  on  the  contrary,  the 
object  be  moved  to  the  left,  it  will  be  invisible  for  the  opposite  reason. 

Hemianopsia  may  be  the  result  of  either  destruction  of  the  optic 
tract  or  of  the  cortical  visual  area.  The  seat  of  lesion  in  any  given 
case  is  indicated  by  a  peculiarity  of  the  iris  reflex  pointed  out  by 
Wernicke,  which  will  be  referred  to  in  connection  with  the  considera- 
tion of  the  oculo-motor  nerve. 

THIRD  PAIR.     THE  OCULO-MOTOR. 

The  third  cranial  nerve,  the  oculo-motor,  consists  of  some 
15,000  peripherally  coursing  nerve-fibers  which  serve  to  bring  the 
nerve-cells  from  which  they  arise  into  relation  with  a  large  portion  of 
the  general  musculature  of  the  eye. 

Origin. — The  axons  composing  the  third  nerve  arise  from  a  series 
of  seven  or  eight  groups  of  nerve-cells,  located  in  the  gray  matter 
beneath  the  floor  of  the  aqueduct  of  Sylvius.  From  each  of  these 
groups  or  nuclei,  bundles  of  axons  emerge,  which  after  a  short  course 
unite  to  form  the  common  trunk.  The  large  majority  of  the  fibers  in 
the  nerve  come  directly  from  the  nuclei  of  the  same  side;  the  remain- 
der come  from  a  group  of  cells  on  the  opposite  side  of  the  median 
line.     There  is  thus  a  partial  decussation  of  its  fibers  (Fig.  243). 

The  different  groups  of  cells,  the  nuclei  of  origin,  are  arranged 
in  a  serial  manner.  The  anatomic  arrangement  of  these  nuclei 
would  indicate  that  each  nucleus  is  related  to  an  individual 
member  of  the  eye-group  of  muscles.  Clinical  observation  and  the 
investigation  of  the  results  of  pathologic  processes  have  not  only 
shown  that  this  is  the  case,  but  have  succeeded  in  locating  the 
position  of  the  nucleus  for  any  given  muscle.  Though  there  is  some 
dift'erence  of  opinion  in  regard  to  the  exact  location  of  one  or  two  of 
the  nuclei,  the  tabulation  subjoined  is  approximately  correct. 

Enumerating  them  from  before  backward,  the  nuclei  occur  in 
the  following  order : 

1.  The  sphincter  pupillae. 

2.  The  tensor  chorioideae  (the  accommodation  nucleus). 

and  the  individual  does  not  become  aware  through  sensation,  of  the  presence  of  a 
luminous  body  in  the  left  side  of  the  visual  field. 


548 


TEXT-BOOK  OF  PHYSIOLOGY. 


3.  The  convergence  nucleus,  a  common  nucleus  for  the  conjoint 
action  of  the  two  internal  recti  muscles. 

The  superior  rectus. 


The  inferior  rectus. 
The  levator  palpebrae. 
The  inferior  oblique. 
Cortical  Connections. — The 
oculo-motor  nuclei  are  in  histologic 
and  physiologic  relation  with  the 
motor  area  of  the  cerebrum.  Nerve- 
cells  in  the  cortex  give  off  axons 
which,  entering  the  pyramidal  tract, 
descend  through  the  internal  capsule, 
and  the  crus  cerebri,  from  which  they 
cross  to  the  opposite  side.  The  end- 
tufts  arborize  around  the  nuclei  of 
the  oculo-motor  nerve  with  the  ex- 
ception of  the  nucleus  for  the  iris 
sphincter. 

Distribution. — After  their  origin 
the  axons  converge  to  form  a  com- 
mon trunk,  which  emerges  from  the 
base  of  the  encephalon,  on  the  inner 
side  of  the  crus  cerebri,  in  front  of 
the  pons  Varohi.  The  nerve  then 
passes  forward  through  the  sphenoid 
fissure  into  the  orbit  cavity,  where 
it  divides  into  a  superior  and  an 
inferior  branch.  The  former  is  dis- 
tributed to  the  superior  rectus  and 
the  levator  palpehrcE  muscles;  the 
latter  is  distributed  to  the  internal 
and  inferior  recti  and  inferior  oblique 
muscles  (Fig.  244). 

From  the  inferior  branch  a  short 
bundle  of  fibers  passes  to  the  ciliary 
or  ophthalmic  ganglion,  where  they 
terminate,  arborizing  around  the 
ganglion  cells.  These  fibers  are 
smaller  in  size  than  those  constitut- 
ing the  bulk  of  the  nerve  and  belong  to  the  system  known  as 
the  autonomic.  These  cells  give  origin  to  new  axons,  the  ciliary 
nerves,  which  enter  the  eyeball,  pass  forward  between  the  sclera  and 
chorioid  coat,  and  terminate  in  the  cihary  muscle  and  iris.  The 
ciliary  nerves  are  not  portions  of  the  third  nerve  proper,  but  periph- 


FiG.  243. — Diagrammatic  View  of 
THE  Situation  and  Relation 
OF  THE  Nuclei  of  Origin  of 
THE  Oculo-motor  and  Path- 
eticus  (Trochlearis)  Nerves. 
The  oculo-motor  nuclei  consist 
of  an  anterior  nucleus,  the 
Edinger-Westphal  nucleus  (a 
and  h),  and  a  posterior  nucleus; 
the  posterior  nucleus  has  a  dor- 
sal, a  ventral,  and  a  mesial 
portion;  the  decussation  of 
fibers  from  the  dorsal  portion 
of  the  posterior  nucleus  is  also 
shown.  The  decussation  of 
the  fibers  of  the  fourth  nerve  is 
also  represented.— (£(fiw^er.) 


THE  CRANIAL  NERVES. 


549 


eral  sympathetic  neurons.  As  the  ciHary  ganghon  receives  filaments 
from  the  cavernous  plexus  of  the  sympathetic  and  filaments  which 
become  a  part  of  the  trigeminal  nerve,  it  is  probable  that  the  ciliary 
nerves  contain  not  only  motor,  but  vaso-motor  and  sensor  fibers. 

Properties. — Stimulation  of  the  nerve  near  its  exit  from  the  en- 
cephalon  is  followed  by  contraction  of  the  muscles  to  which  it  is  dis- 
tributed with  the  following  results,  viz.  : 

1.  Diminution  in  the  size  of  the  pupil. 

2.  Accommodation  of  the  eye  for  near  vision. 

3.  Elevation  of  the  upper  eyelid. 

4.  Internal  deviation  and  rotation  upward  and  inward  of  the  anterior 

pole  of  the  eye,  combined  with  a  small  amount  of  torsion  toward 
the  mesial  line,  due  to 
preponderating  action 
of  the  internal  rectus 
and  inferior  oblique 
muscles. 
Division  of    the   nerve 

experimentally  or  compres- 
sion     from     a     pathologic 

lesion    is    followed     by    a 

relaxation   of  the  muscles, 

with  the    following  effects, 

viz. : 

I.  Dilatation  of  the  pupil, 
the  iris  responding 
neither  to  light  nor  to 
efforts  of  accommoda- 
tion. 
Loss  of  the  accommoda- 
tive power. 
Fahing  of  the  upper  eye- 
hd  (ptosis). 

4.  External  deviation  and  rotation  downward  and  outward  of  the 

anterior  pole  of  the  eyeball  combined  with  a  small  amount  of 
torsion  toward  the  mesial  line  due  to  the  unopposed  action  of 
external  rectus  and  the  superior  obhque  muscles. 

5.  Double  vision  or  diplopia.     The  image  of  the  eye  of  the  paralyzed 

side  is  projected  to  the  opposite  side  of  the  true  image  and  to 
the  upper  part  of  the  visual  field.  Owing  to  the  slight  mesial 
torsion  the  false  image  is  inchned  away  from  the  true  image. 

6.  Immobihty  and  slight  protrusion  of  the  eyeball. 

Function. — The  function  of  the  third  nerve  is  to  transmit  nerve 
impulses  from  the  nuclei  of  origin  to  all  the  muscles  of  the  eye  except 
the  external  rectus  and  superior  oblique  and  excite  them  to  activity. 


2. 


3 


Fig.  244. — Intra-orbital  Portion  of  the 
Third  Nerve,  i.  Optic  nerve.  2.  Third 
nerve.  3.  Superior  branch.  4.  Inferior 
branch.  5.  Abducens.  6.  Trifacial.  7. 
Ophthalmic  branch  divided.  8.  Nasal 
branch.  9.  Ciliary  ganglion.  10.  Motor 
branch  to  this  ganglion  from  the  inferior 
branch  of  the  third  nerve.  11.  Sensory 
fibers.  12.  Sympathetic  fibers.  13.  Ciliary 
nerves. — (Sappey.) 


55° 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  majority  of  the  ocular  movements,  the  power  of  accommoda- 
tion, the  variations  in  the  size  of  the  pupil  in  accordance  with  varia- 
tions in  the  intensity  of  the  hght,  the  power  of  convergence  of  the 
visual  axes,  are  all  excited  by  the  transmission  of  nerve  impulses  by 
the  constituent  fibers  of  the  nerve  from  their  related  nuclei.  This 
is  made  evident  by  the  effects  which  follow  stimulation  and  division 
of  the  nerve  or  lesions  of  the  nuclei  themselves. 

The  central  nuclei  can  be  excited  to  activity  (i)  by  nerve  impulses 
descending  the  motor  tract,  from  the  cerebral  cortex,  (2)  by  nerve 
impulses  coming  through  various  afferent  nerves.  This  holds  true 
more  especially  for  the  sphincter  pupillas  nucleus. 

The  Iris  Reflex  or  the  Pupillary  Reflex. — These  are  terms 
applied  to  the  variations  in  the  size  of  the  pupil  that  follow  vari- 

Gasserian  GangUon 


Cil.ary  ^<  N.ulc.scJ  3rJ  Nerac 

J\/erves  Ciliary  Ganglion 


Sympathetic. 
Postganglionic 
fibers 


Superior 
Cervical 
Ganglion    Vv 


y~  ThoracicJveroe 


Pirganglionic 

Jibers 

fromSpin  al  Cord 


Fig.  245. — Diagram  Showing  the  Structures  Involved  in  the  Iris  Reflex. 


ations  in  the  intensity  of  the  light.  In  the  absence  of  light  the 
pupil  'widely  dilates,  due  largely  to  the  relaxation  of  the  sphincter 
pupillcB  muscle  and  partly  to  a  contraction  of  the  radiating  fibers  of 
the  iris  which  collectively  constitute  the  dilatator  pupiUa  muscle. 
With  the  entrance  of  light  into  the  eye,  the  pupil  narrows  in  con- 
sequence of  the  contraction  of  the  sphincter  pupilloe  caused  by  a 
stimulation  of  the  peripheral  end  of  the  optic  nerve,  the  degree 
of  contraction  depending  within  limits  on  the  intensity  of  the  light. 

The  action  is  a  reflex  one  and  the  mechanism  involved  includes 
the  retina,  the  optic  nerve,  the  anterior  quadrigeminal  body,  the  third 
nerve,  the  ciliary  nerves  (the  peripheral  sympathetic  neurons),  and 
the  sphincter  pupillae  muscle.  (  Fig.  245.)  In  this  mechanism  the 
optic  nerve  is  the  afferent  path,  the  motor  oculi  the  efferent  path, 
and  the  anterior  quadrigeminal  body  the  intermediate  center.     These 


THE  CRANIAL  NERVES.  551 

facts  are  demonstrated  by  the  entire  loss  of  the  reflex  which  fol- 
lows division  or  destruction  of  any  part  of  this  arc.  The  anterior 
quadrigeminal  body  appears  to  be  in  relation  through  its  axons  and 
their  collateral  branches  with  the  sphincter  pupUlce  nucleus  of  both 
sides,  inasmuch  as  stimulation  of  one  retina  is  followed  by  narrow- 
ing of  the  pupils  of  both  eyes.  To  this  simultaneous  contraction  of 
the  pupils  the  term  "consensual"  has  been  given.  Contraction  of 
both  pupils  also  occurs  as  an  associated  movement  in  the  conver- 
gence of  the  eyes  during  accommodation.  The  dilatation  of  the 
pupil  is,  however,  not  due  exclusively  to  the  relaxation  of  the 
sphincter  pupillae  muscle,  but  partly  to  the  contraction  of  the  dila- 
tator pupillcB  muscle,  which  is  kept  normally  in  a  state  of  tonic 
contraction  by  impulses  emanating  from  a  nerve-center  in  the 
medulla  oblongata. 

The  axons  which  arise  in  this  center  pass  down  the  cord,  emerge 
through  the  first  thoracic  nerve,  and  then  ascend  to  the  superior 
cervical  ganglion  (see  Fig.  245),  in  which  their  terminal  branches 
arborize  around  its  nerve-cells.  From  these  cells  new  axons  of  the 
sympathetic  system  arise  which  pass  successively  to  the  ophthalmic 
division  of  the  fifth  nerve,  the  nasal  nerve,  the  long  ciliar}-  nerve  and 
the  iris. 

Experimental  research  renders  it  highly  probable  that  the  dilatator 
center  is  in  a  state  of  continuous  activity  and  the  dilatator  muscle  in 
a  state  of  tonic  contraction.  Whatever  the  normal  stimulus  may  be, 
the  center  is  increased  in  activity  by  dyspneic  blood,  by  severe  muscle 
exercise,  by  emotional  excitement,  and  by  stimulation  of  various 
sensoiy  nerves.  That  the  afferent  pathway  just  alluded  to  transmits 
the  impulses  to  the  iris  is  shown  by  the  fact  that  division  in  any  part 
of  the  course  is  followed  by  narrowing,  stimulation  by  active  dilata- 
tion of  the  pupil. 

The  variations  in  the  size  of  the  pupil,  though  largely  a  reflex 
act  under  the  control  of  the  oculo-motor  nerve,  are  nevertheless 
partly  due  to  the  active  cooperation  of  the  dilatator  nerves  and  their 
related  muscle.  The  size  of  the  pupil  necessary  from  moment  to 
moment  for  the  admission  of  just  that  amount  of  light  essential  to 
the  formation  and  perception  of  a  distinct  image  is  the  result  of  two 
nicely  adjusted  and  delicately  balanced  forces. 

Wernicke's  Pupillary  Reaction. — It  was  stated  on  page  531 
that  a  modification  of  the  pupillary  reaction  is  observed  in  some 
cases  of  hemianopsia  which  indicates  approximately  the  seat  of  lesion. 
This  reaction  is  present  only  when  the  lesion  is  along  the  course  of 
the  optic  tract.  In  these  cases,  if  a  fine  ray  of  light  is  projected  into 
the  eye  in  such  a  manner  that  it  falls  entirely  on  the  non-responsive 
side  of  the  retina,  there  will  be  an  absence  of  a  pupillary  response, 
owing  to  the  break  in  the  reflex  arc.     If,  however,   the  light    be 


552 


TEXT-BOOK  OF  PHYSIOLOGY. 


thrown  on  the  sensitive  side  of  the  retina  the  usual  response,  a  con- 
traction of  the  sphincter  and  a  narrowing  of  the  pupil,  is  at  once 
observed. 

FOURTH  PAIR.     THE  PATHETICUS. 

The  fourth  cranial  nerve,  the  patheticus,  consists  of  peripherally 
coursing  axons  which  serve  to  bring  the  cells  from  which  they  arise 
into  relation  with  the  superior  obHque  muscle. 

Origin. — The  axons  of  this  nerve  arise  from  a  group  of  cells 
located  beneath  the  aqueduct  of  Sylvius  just  posterior  to  the  last 

nucleus  of  the  third  nerve. 
After  emerging  from  the  nu- 
cleus the  nerve-fibers  pass  down- 
ward for  a  short  distance,  then 
curve  dorsally  around  the  aque- 
duct of  Sylvius,  and  enter  the 
valve  of  Vieussens,  where  they 
completely  decussate  with  the 
nerve-fibers  of  the  opposite  side. 
Cortical  Connections. — 
The  nucleus  of  the  pathetic  nerve 
is  in  histologic  and  physiologic 
connection  with  the  motor  area 
of  the  cerebral  cortex.  Nerve 
cells  in  this  region  give  off  axons 
which  enter  the  pyramidal  tract 
and  descend  through  the  inter- 
nal capsule  and  the  crus  cerebri, 
after  which  they  cross  to  the 
opposite  side.  Their  end-tufts 
arborize  around  the  cells  of  the 
nuclei  already  described. 

Distribution. — After  its 
decussation  the  nerve-trunk 
emerges  just  below  the  posterior 
quadrigeminal  body,  crosses  the 
superior  cerebellar  peduncle,  and  winds  around  the  crus  cerebri  to  the 
anterior  border  of  the  pons  Varohi.  It  then  enters  the  orbit  cavity 
through  the  sphenoid  fissure  and  finally  terminates  in  the  superior 
oblique  muscle.  In  its  course  the  nerve  receives  filaments  from  the 
cavernous  plexus  of  the  sympathetic  and  the  ophthalmic  division  of 
the  trigeminal. 

Properties. — Stimulation  of  the  nerve-trunk  is  followed  by  spas- 
modic contraction  of  the  superior  oblique  muscle,  the  anterior  pole 
of  the  eyeball  being  turned  downward  and  outward,  combined  with 
slight  torsion  away  from  the  middle  line. 


Fig.  246. — Distribution  of  the  Patheti- 
cus. I.  Olfactory  nerve.  II.  Optic 
ner\-es.  III.  Motor  oculi  communis. 
IV.  Patheticus,  by  the  side  of  the 
ophthalmic  branch  of  the  fifth,  and 
passing  to  the  superior  oblique  muscle. 
VI.  Motor  oculi  externus.  i.  Gang- 
lion of  Gasser.  2,  3,  4,  5,  6,  7,  8,  9,  10. 
Ophthalmic  division  of  the  fifth 
nerve,  with  its  branches. — {Hirsch- 
jeld.) 


THE  CRANIAL  NERVES. 


553 


Division  of  the  nerve  is  followed  by  a  relaxation  or  paralysis  of 
the  muscle.  In  consequence  of  the  now  unopposed  action  of  the 
inferior  oblique  muscle,  the  anterior  pole  of  the  eyeball  is  turned 
upward  and  inward  with  shght  torsion  toward  the  middle  line.  The 
diplopia  consequent  upon  this  paralysis  is  homonymous,  the  images 
appearing  one  above  the  other.  The  image  of  the  paralyzed  eye  is 
below  that  of  the  normal  eye  and  its  upper  end  inclined  toward  that 
of  the  normal  eye. 

Function. — The  function  of  this  nerve  is  to  transmit  nerve  im- 
pulses to  the  superior  oblique  muscle  and  to  excite  it  to  contraction. 


FIFTH  PAIR.     THE  TRIGEMINAL. 

The  fifth  cranial  nerve,  the  trigeminal,  consists  of  both  aflferent  and 
efferent  axons  which  for  the  most  part  are  separate  and  distinct. 
The  afferent  axons  constitute  by  far  the  major  portion,  the  efferent 
fibers    the    minor 
portion,     of     the 
nerve. 

Origin  of  the 
Afferent  Axons. 

The  afferent 
axons  have  their 
origin  in  the  mon- 
axonic  cells  in  the 
ganglion  of  Gas- 
ser,  which  rests 
on  the  apex  of  the 
petrous  portion  of 
the  temporal 
bone.  The  cells 
of  this  ganglion 
give  origin  to  a 
short  process 
which  soon  di- 
vides into  two 
branches,  one  of 
which  passes  cen- 
trally, the  other 
peripherally  (Fig.  247).  The  centrally  directed  branches  collec- 
tively form  the  so-called  large  or  sensor  root;  the  peripherally 
directed  branches  collectively  constitute  the  three  main  divisions  of 
the  nerve:  viz.,  the  ophthalmic,  the  superior  maxillary,  and  the 
inferior  maxillary.  Branches  of  the  carotid  plexus  of  the  sym- 
pathetic enter  the  nerve  in  the  neighborhood  of  the  ganglion  of 
Gasser  and  accompany  some  of  its  branches  to  their  terminations. 


Fig.  247. — Scheme  of  Origin  and  Constitution  of  the 
Trigeminal  Nerve,  i.  Centrally  coursing  fibers.  2,3, 
4.  Peripherally  coursing  fibers  of  the  cells  of  the  ganglion 
of  Gasser.  R,  N.  Nuclei  of  origin  of  the  efferent  fibers. 
6.  IMotor  root.     Central  terminations  of  the  large  root. 


554 


TEXT-BOOK  OF  PHYSIOLOGY. 


Distribution. — i.  The  Central  Branches. — The  axons  of  the 
large  root  pass  backward  into  the  pons  Varolii  on  its  lateral  aspect. 
After  entering  the  pons  each  axon  divides  into  two  branches,  one 
of  which  passes  upward  a  short  distance,  the  other  passes  down- 
ward, descending  as  far  as  the  second  cervical  segment.  Both 
branches  give  off  a  number  of  collaterals,  some  of  which  terminate 
in  fine  end-tufts  around  nerve-cells  in  the  substantia  gelatinosa. 

2.  The  Peripheral  Branches. — The  peripheral  axons  emerge  from 
the  peripheral  end  of  the  ganglion  of  Gasser  in  three  distinct  and 
separate  branches,  each  of  which  is  distributed  to  a  different  region 
of  the  face  and  head. 

The  ophthalmic  branch  passes 
forward     and     subdivides 
into  three  large  branches, 
the  frontal,  the  lachrymal, 
and  the   nasal.     The  ulti- 
mate   termination    of    the 
branches   of    these    nerves 
is  as  follows:  viz.,  the  con- 
junctiva  and   skin   of   the 
upper   eyelid,   the   cornea, 
the    skin    of    the  forehead 
and  the  nose,  the  lachrymal 
gland    and    caruncle,    and 
the  mucous  membrane  of 
the  nose  (Fig.  248). 
The      superior      maxillary 
branch    passes    forward 
through    the   foramen   ro- 
tundum,  crosses  the  sphe- 
no-maxillary   fossa,    enters 
the  infra-orbital  canal,  and 
emerges  at  the  infra-orbital 
foramen.     In  its  course  it 
gives  off  a  number  of  branches  which  are  distributed  as  follows : 
viz.,  to  the  integument   and  conjunctiva  of   the  lower  lid,  the 
nose,  cheek,  and  upper  lip,  the  palate,  the  teeth  of  the  upper 
jaw,  and  the  alveolar  processes  (Fig.  249). 
The  inferior  maxillary  branch  passes  through  the  foramen  ovale, 
after  which  it  subdivides  into  three  branches — the  auriculo-tem- 
poral,    the    lingual,    and    the    inferior    dental.      The  ultimate 
branches  are  distributed  as  follows:  viz.,  the  external  auditory 
meatus,  the  side  of  the  head,  the  mucous  membrane  of  the 
mouth,  the  anterior  portion  of  the   tongue,  the   arches  of  the 
palate,  the  teeth  and  alveolar  process  of  the  lower  jaw  and  the 
integument  of  the  lower  part  of  the  face  (Fig.  250). 


Fig.  248. — Ophthalmic  Branch  of  the 
Fifth,  i.  Ganglion  of  Gasser.  2. 
Ophthalmic  division  of  the  fifth.  3. 
Lachrymal  branch.  4.  Frontal  branch. 
5.  E.xternal  frontal. 
7.  Supra-trochlear. 
9.  External  nasal. 
— (Hirschfeld.) 


6.  Internal  frontal. 

8.  Nasal  branch. 

10.  Internal  nasal. 


THE  CRANIAL  NERVES. 


555 


The  afferent  axons  thus  serve  to  bring  into  relation  the  skin, 
mucous  membranes  of  the  head  and  face,  and  other  sentient  struc- 
tures, with  certain  sensor  end-nuclei  in  the  pons,  medulla  oblongata, 
and  adjoining  structures. 

Cortical  Connections. — The  nerve-cells  around  which  the  end- 
tufts  of  the  centrally  coursing  axons  ramify  collectively  constitute 
the  "sensor  end-nuclei"  of  the  trigeminal  nerve.  From  these  cells 
new  axons  arise  which  cross  the  median  line,  enter  the  fillet  or 
lemniscus,  and  ascend  directly  to  the  sensor  area  of  the  cerebral 
cortex. 

Properties. — Irritative  pathologic  lesions,  e.  g.,  pressure  by 
tumors,  aneurysms,  neuritis,  degenerative  changes  in  the  ganghon 


Fig.  249. — I.  Superior  maxillary  nerve.  2,  3,  4,  5.  Dental  nerves.  6.  Spheno- 
palatine ganglion.  7.  Vidian  nerve.  8.  Large  superficial  petrosal.  9.  Carotid 
branch  of  large  petrosal.  10.  Oculo-motor.  11.  Superior  cer\'ical  ganglion.  12. 
Carotid  branches  of  this  ganglion.  13.  Facial.  14.  Glosso-pharyngeal.  15. 
Jacobson's  nen'e,  and  16,  17,  18,  19,  branches  to  the  sympathetic,  fenestra 
rotunda,  Eustachian  tube.  20.  Deep  e.xternal  petrosal.  21.  Deep  internal 
petrosal . — {Hirschjeld . ) 


cells,  or  lesions  which  in  any  way  gradually  impair  the  physical  or 
chemic  integrity  of  the  nerve-fibers,  give  rise  to  a  variety  of  painful 
sensations  referable  to  the  seat  of  the  lesion  or  to  one  or  more  regions 
in  the  peripheral  distribution  of  the  nerve.  Many  of  the  various 
forms  of  trigeminal  neuralgia  are  caused  by  lesions  of  this  character. 
Exposure  of  the  dental  nerves  from  caries  of  the  teeth,  the  presence 
of  minute  foreign  bodies  in  the  conjunctiva,  operative  procedures  in 
the  nasal  chambers,  all  testify  to  the  extreme  sensibility  of  the  nerve. 
Division  of  the  large  root,  within  the  cranium  is  followed  at  once  by 
complete  abolition  of  all  sensibility  in  the  head  and  face  to  which 
its  branches  are  distributed.  The  skin  and  mucous  membranes,  the 
eye,  nose,  or  teeth  may  be  experimentally  injured  without  any  evi- 


5S6  TEXT-BOOK  OF  PHYSIOLOGY. 

dences  of  pain  on  the  part  of  the  animal.  Various  reflexes,  e.  g., 
those  of  mastication,  insahvation,  deglutition,  the  afferent  paths  of 
which  are  formed  in  part  by  the  fifth  nerve,  are  often  seriously  im- 
paired. At  the  same  time  the  lacrimal  secretion  diminishes  and  the 
pupil  contracts.  The  same  results  are  observed  in  human  beings 
in  w^hom  the  nerve  has  been  divided  for  relief  from  neuralgia.  Anes- 
thesia or  a  loss  of  sensibility  may  also  be  caused  by  pathologic  lesions 
of  the  nerve-trunks  or  of  the  sensor  end-nuclei. 

Division  of  the  large  root  at  or  near  the  ganglion  of  Gasser  has 
not  infrequently  been  followed  by  an  alteration  in  the  nutrition  of 
the  eye  and  nose.  In  the  course  of  twenty-four  hours  the  eye  becomes 
vascular  and  inflamed;  the  cornea  becomes  opaque;  ulceration  sets 
in  which  may  lead  to  complete  destruction  of  the  eyeball.  The 
mucous  membrane  of  the  nose  becomes  swollen,  vascular,  and  liable 
to  hemorrhage  on  the  slightest  irritation.  The  degenerative  changes 
may  lead  to  a  complete  loss  of  the  sense  of  smell.  These  results  were 
formerly  attributed  to  a  loss  of  trophic  influence  which  it  was  believed 
the  nerve  exercised  over  these  structures.  Modern  experimentation 
and  various  surgical  procedures  have  demonstrated  that  the  nutritive 
disorders  are  septic  in  origin,  made  possible  by  the  anesthetic  condi- 
tion and  by  the  changed  vascular  supply  from  division  of  the  vaso- 
motor fibers  which  join  the  nerve  at  or  near  the  ganglion. 

Origin  of  the  Efferent  Axons. — The  efferent  axons  arise  for 
the  most  part  from  nerve-cells  located  in  the  gray  matter  beneath 
the  upper  half  of  the  floor  of  the  fourth  ventricle.  A  group  of  cells 
known  as  the  superior  or  accessory  nucleus,  situated  posterior  to 
the  corpora  cjuadrigemina,  give  origin  to  axons  which  descend  and 
join  the  axons  from  the  chief  motor  nucleus  (Fig.  247). 

Distribution. — From  their  origin  the  fibers  pass  forward  through 
the  pons  and  emerge  on  its  lateral  aspect,  forming  the  so-called  small 
root  of  the  fifth  nerve.  This  then  passes  forward  beneath  the  ganglion 
of  Gasser,  leaves  the  cavity  of  the  skull  through  the  foramen  ovale, 
and  joins  the  inferior  maxillary  division  already  described.  Its 
axons  are  ultimately  distributed  to  the  muscles  of  mastication:  viz., 
the  masseter,  the  temporal,  the  external  and  internal  pterygoids,  the 
mylohyoid,  and  the  anterior  portion  of  the  digastric.  A  few  axons 
are  also  distributed  to  the  tensor  tympani  and  tensor  palati  muscles 
(Fig.  250).  The  efferent  or  peripherally  coursing  axons  thus  serve 
to  bring  the  nerve-cells  from  which  they  arise  into  relation  with  the 
muscles  of  mastication. 

Cortical  Connections. — The  nuclei  of  origin  of  the  small  root 
are  in  histologic  and  physiologic  relation  with  the  lower  third  of  the 
motor  area  of  the  cerebral  cortex.  Nerve-cells  in  this  region  give 
off  axons  which  enter  the  pyramidal  tract,  descend  through  the  in- 
ternal capsule  and  the  crus  cerebri,  after  which  they  cross  to  the 


THE  CRANIAL  NERVES. 


557 


opposite  side.     Their  end-tufts  arborize  around  the  cells  of  nuclei  in 
the  medulla  oblongata. 

Properties. — Stimulation  of  the  small  root  gives  rise  to  convulsive 
movements  of  the  muscles  of  mastication.  Division  of  the  nerve  is 
followed  by  a  paralysis  of  these  muscles.     Contraction  or  paralysis  of 


Fig.  250. — Inferior  Maxillary  Branxh  of  the  Trigeminal  Nerve,  i.  Branch 
to  the  masseter  muscle.  2.  Filament  of  this  branch  to  the  temporal  muscle.  3. 
Buccal  branch.  4.  Branches  anastomosing  with  the  facial  nerve.  5.  Filament 
from  the  buccal  branch  to  the  temporal  muscle.  6.  Branches  to  the  e.xternal 
pterygoid  muscle.  7.  Middle  deep  temporal  branch.  8.  Auriculo-temporal 
nerve.  9.  Temporal  branches.  10.  Auricular  branches.  11.  Anastomosis  with 
the  facial  nerve.  12.  Lingual  branch.  13.  Branch  of  the  small  root  to  the 
mylo-hyoid  muscle.  14.  Inferior  dental  nerve,  with  its  branches  (15,  15).  16. 
Mental  branch.  17.  Anastomosis  of  this  branch  with  the  facial  nerve. — (Hirsch- 
jeld.) 


the    tensor   tympani    and    tensor    palati    muscles    would    also    be 
observed  under  the  same  conditions. 

Functions. — The  fifth  nerve,  by  virtue  of  its  transmitting  nerve 
impulses  from  the  periphery  to  the  cerebral  cortex,  where  they  evoke 
sensation,  endows  all  the  parts  to  which  it  is  distributed  with  sensi- 
bility; it   also   endows   the   muscles   of   mastication   with   motihty 


558 


TEXT-BOOK  OF  PHYSIOLOGY, 


Throughrthe  relation  of  its  central  end-tufts  with  the  motor  nuclei  in 
the  medulla  and  pons,  it  assists  in  the  reflex  acts  of  mastication  and 
insalivation. 

SIXTH  PAIR.     THE  ABDUCENS. 

The  sixth  cranial  nerve,  the  abduccns,  consists  of  peripherally 
coursing  axons  which  serve  to  bring  the  nerve-cells  from  which  they 
arise  into  relation  with  the  external  rectus  muscle. 

Origin. — The  axons  arise  from  a  group  of  cells  located  in  the 
gray  matter  beneath  the  upper  half  of  the  floor  of  the  fourth  ventricle. 

It  is  quite  probable  that  a  few 
fibers  in  each  nerve-trunk  come 
from  the  nucleus  on  the  opposite 
side  of  the  middle  line. 

Distribution. — The  nerve- 
fibers  pass  forward  from  their 
origin  through  the  gray  and 
white  matter  and  emerge  through 
the  groove  between  the  medulla 
oblongata  and  the  pons  Varolii 
just  external  to  the  anterior 
pyramid.  The  nerve  then  passes 
through  the  sphenoid  fissure  into 
the  orbit  cavity,  where  it  is 
distributed  to  the  external  rectus 
muscle  (Fig.  251).  In  its  course 
the  nerve  receives  filaments  from 
the  carotid  plexus  of  the  sym- 
pathetic. 

Cortical  Connections. — 
The  nucleus  of  the  sixth  nerve 
is  in  histologic  and  physiologic 
connection  vAih.  the  motor  area 
of  the  cerebral  cortex.  From 
nerve-cells  in  this  region  axons  are  given  off  which  enter  the 
pyramidal  tract,  descend  through  the  internal  capsule  and  crus 
cerebri,  after  which  they  cross  to  the  opposite  side,  w^here  their 
end-tufts  arborize  around  the  cells  of  the  nucleus  already  described. 
Properties. — Stimulation  of  the  nerve  is  followed  by  spasmodic 
contraction  of  the  external  rectus  muscle  and  external  deviation  of 
the  eyeball.  Division  of  the  nerve  is  followed  by  paralysis  or  relaxa- 
tion of  the  muscle.  As  a  result  of  the  unopposed  action  of  the 
internal  rectus  the  anterior  pole  of  the  eyeball  is  turned  toward  the 
middle  hne  (internal  strabismus).  In  consequence  of  this  deviation 
there  is  homonymous  diplopia.     The  images  are  on  the  same  level 


Fig.  251. — Distribution  of  the  Motor 

OCULI  EXTERNUS  OR  AbDUCENS.   I. 

Trunk  of  the  motor  oculi  communis, 
with  its  branches  (2,  3,  4,  5,  6,  7).  8. 
Motor  oculi  externus,  passing  to 
the  external  rectus  muscle.  9.  Fila- 
ments of  the  motor  oculi  externus 
anastomosing  with  the  sympathetic. 
10.  Ciliary  nerves. — (Hirschfeld.) 


THE  CRANIAL  NERVES.  559 

and  parallel.     The  image  of  the  paralyzed  eye  lies  external  to  that 
of  the  normal  eye. 

Function. — The  function  of  this  nerve  is  to  transmit  nerve  im- 
pulses to  the  external  rectus  muscle  and  excite  it  to  contraction. 

SEVENTH  PAIR.    THE  FACIAL. 

The  seventh  cranial  nerve,  the  facial,  consists  of  peripherally 
coursing  nerve-fibers,  which  serve  to  bring  the  nerve-cells  from 
which  they  arise  into  relation  with  most  of  the  superficial  muscles  of 
the  head  and  face. 

The  muscles  supplied  by  this  nerve,  as  stated  by  the  general 
anatomists,  are  as  follows:  The  occipito-frontalis,  corrugator  super- 
cilii,  orbicularis  palpebrarum,  levator  labii  superioris,  alaeque  nasi, 
zygomatici,  the  pyramidalis  nasi,  the  compressor  nasi,  the  depressor 
alae  nasi,  levator  anguli  oris,  buccinator,  orbicularis  oris,  depressor 
anguli  oris,  depressor  labii  inferioris,  the  levator  menti,  the  posterior 
belly  of  the  digastric,  the  stylo-hyoid,  and  the  platysma  myoides. 

Origin. — The  nerve-fibers  or  axons  composing  the  seventh  nerve 
arise  for  the  most  part  from  a  nucleus  of  large  multipolar  nerve-cells 
situated  about  five  milhmeters  beneath  the  upper  half  of  the  floor 
of  the  fourth  ventricle  toward  the  middle  line. 

From  this  nucleus,  which  is  about  four  millimeters  long,  axons 
emerge  which  at  first  pass  inward  and  backward  as  far  as  the  epen- 
dyma  of  the  ventricle ;  they  then  turn  on  themselves,  forming  an  arch 
that  encloses  the  nucleus  of  the  sixth  nerve;  they  then  course  down- 
ward and  outAvard,  emerging  from  the  pons  at  its  lower  border  between 
the  olivary  and  restiform  bodies.  As  the  axons  approach  the  floor 
of  the  ventricle  collateral  branches  are  given  oft'  which,  crossing  the 
median  line,  arborize  around  the  nerve-cells  of  the  opposite  facial 
nucleus. 

Clinical  observations  and  histologic  investigations,  however,  render 
it  probable  that  the  libers  distributed  to  the  occipito-frontalis,  the  cor- 
rugator supercihi,  and  the  upper  half  of  the  orbicularis  palpebrarum, 
are  derived  from  the  oculo-motor  nucleus,  and,  descending  the 
posterior  longitudinal  bundle,  enter  the  trunk  of  the  facial  as  it 
turns  to  pass  forward  through  the  pons.  It  is  also  probable,  for 
similar  reasons,  that  the  fibers  distributed  to  the  orbicularis  oris  are 
derived  from  the  hypoglossal  nucleus. 

Cortical  Connections. — The  nucleus  of  the  facial  nerve  is  in 
histologic  and  physiologic  connection  with  the  facial  region  of  the 
general  motor  area  of  the  cerebral  cortex.  From  the  cells  of  this 
region  axons  descend  through  the  pyramidal  tract,  the  internal  cap- 
sule, and  the  crus  cerebri,  beyond  wliich  they  cross  to  the  opposite 
side  and  arborize  around  the  cells  of  the  nucleus  already  described. 

Distribution. — From  its  superficial  origin  the  trunk  of  the  nerve 
passes  into  the  internal  auditory  meatus  beside  the  auditory  nerve. 


56o 


TEXT-BOOK  OF  PHYSIOLOGY. 


After  passing  forward  and  outward  for  a  short  distance  through  the 
bone  above  and  between  the  cochlea  and  vestibule,  the  nerve  makes 
a  sharp  bend,  forming  the  genu  facialis,  turns  backward  and  enters 
the  aqueduct  of  Fallopius,  the  general  course  of  which  it  follows  as 
far  as  the  stylo-mastoid  foramen.     After  emerging  from  this  foramen 


Fic.  252. — Superficial  Branches  of  the  Facial  and  the  Fifth. — i.  Trunk  of  the 
facial.  2.  Posterior  auricular  nerve.  3.  Branch  which  it  receives  from  the 
cervical  plexus.  4.  Occipital  branch.  5,  6.  Branches  to  the  muscles  of  the  ear. 
7.  Digastric  branches.  8.  Branch  to  the  stylo-hyoid  muscle.  9.  Superior  ter- 
minal branch.  10.  Temporal  branches.  11.  Frontal  branches.  12.  Branches 
to  the  orbicularis  palpebrarum.  13.  Nasal  or  suborbital  branches.  14.  Buccal 
branches.  15.  Inferior  terminal  branch.  16.  Mental  branches.  17.  Cervical 
branches.  18.  Superficial  temporal  nerve  (branch  of  the  fifth).  19,  20.  Frontal 
nerves  (branches  of  the  fifth).  21,  22,  23,  24,  25,  26,  27.  Branches  of  the  fifth. 
28,  29,  30,  31,  32.  Branches  of  the  cervical  nerves. — {Hirschjeld.) 

the  nerve  passes  downward  and  forward  as  far  as  the  parotid  gland, 
within  which  it  terminates  by  dividing  into  two  main  branches,  the 
temporo-facial  and  the  cervico-facial,  the  ultimate  branches  of  which 
are  distributed  as  previously  stated  to  the  superficial  muscles  of  the 
head  and  face  (Fig.  252). 


THE  CRANIAL  NERVES. 


561 


The  Pars  Intermedia  or  Nerve  of  Wrisberg. — The  facial  nerve 
at  the  genu,  the  point  where  it  turns  backward  to  enter  the  aque- 
duct of  Fallopius,  presents  a  shght  enlargement,  grayish  in  color, 
and  in  which  nerve-cells  are  contained.  This  enlargement  is  known 
as  the  geniculate  ganglion.  The  cells  of  this  ganglion,  originally 
bipolar,  present  single  axons  which  soon  divide  into  centrally  and 
peripherally  coursing  branches.  The  former  collectively  constitute 
the  pars  intermedia  or  nerve  of  Wrisberg,  which,  entering  and  passing 
through  the  pons,  termi- 
nates around  the  sensor 
end-nucleus  of  the  glosso- 
pharyngeal nerve ;  the  latter, 
the  peripherally  directed 
branches,  enter  the  sheath 
of  the  facial  and  accompany 
it  as  far  as  a  point  about 
five  millimeters  above  the 
stylo-mastoid  foramen. 

From  its  mode  of  origin 
the  nerve  of  Wrisberg  can 
not  be  regarded  as  an 
integral  part  of  the  facial 
nerve  proper,  but  is  to  be 
regarded  as  an  independent 
sensor  nerve.  As  to  the 
true  function  and  relation 
of  this  nerve  there  is  much 
conflict  of  opinion. 

Branches  of  the  Fa- 
cial.— In  the  aqueduct  of 

Fallopius  the  facial  gives  off  the  following  branches :  the  greater  and 
lesser  petrosals,  the  stapedius,  and  the  chorda  tympani  (Fig.  2  5 3). 

1.  The  greater  petrosal  nerve  is  given  off  near  the  geniculate  ganglion. 

It  then  passes  forward  into  the  spheno-maxillary  fossa  and  ter- 
minates in  the  spheno-palatine  ganglion  by  an  arborization  of 
its  fibers  around  the  ganglion  cells. 

2.  The  lesser  petrosal  nerve  is  given  off  at  a  point  somewhat  external 

to  the  preceding.  It  leaves  the  skull  by  a  small  foramen  and 
terminates  in  the  otic  ganglion  by  an  arborization  of  its  fibers 
around  the  ganglion  cells. 

3.  The  stapedius  branch  leaves  the  aqueduct  of  Fallopius  somewhat 

further  down  by  a  small  foramen,  enters  the  pyramid  of  the 
middle  ear,  and  is  finally  distributed  to  the  stapedius  muscle. 

4.  The  chorda  tympani  is  given  off  from  the  facial  at  a  point  about 

five  millimeters  above  the  stylo-mastoid  foramen.     It  then  passes 
36 


Fig.  253. — Chorda  Tympani  Nerve,  i,  2,  3, 
4.  Facial  nerve  passing  through  the  aquae- 
ductus  Fallopii.  5.  Ganglioform  enlarge- 
ment. 6.  Great  petrosal  nerve.  7.  Spheno- 
palatine gangHon.  8.  Small  petrosal  nerve. 
9.  Chorda  tympani.  10,  11,  12,  13.  Various 
branches  of  the  facial.  14,  14,  15.  Glosso- 
pharyngeal nerve. — {Hirschjeld.) 


562  TEXT-BOOK  OF  PHYSIOLOGY. 

upward  and  forward  and  enters  the  tympanum  through  the  iter 
chordae  posterius,  crosses  the  tympanic  membrane  between  the 
malleus  and  incus,  leaves  the  tympanum  by  the  iter  chordae 
anterius  or  canal  of  Huguier,  and  finally  joins  the  lingual  branch 
of  the  fifth.  Some  of  its  fibers  can  be  traced  to  the  dorsum  of 
the  tongue,  others  to  the  submaxillary  and  sublingual  ganglia, 
where  they  terminate  in  tufts  around  the  ganglion  cells. 

Properties. — Electric  stimulation  of  the  trunk  of  the  nerve  after 
its  emergence  from  the  stylo-mastoid  foramen  produces  convulsive 
movements  in  all  the  muscles  to  which  its  branches  are  distributed. 
The  same  results  follow  stimulation  of  the  intracranial  portion  of  the 
nerve  in  an  animal  recently  killed. 

Irritative  pathologic  lesions — e.  g.,  tumors,  aneurysms,  etc. — 
situated  along  the  course  of  the  nerve  or  at  its  nuclear  origin,  fre- 
quently give  rise  to  spasmodic  movements  of  the  facial  muscles  which 
may  be  tonic  or  clonic  in  character. 

Division  of  the  facial  nerve  after  its  emergence  from  the  stylo- 
mastoid foramen  is  followed  by  a  complete  relaxation  or  paralysis 
of  the  superficial  facial  muscles.  The  same  result  follows  compres- 
sion of  the  nerve-trunk  in  any  part  of  its  intracranial  course. 

The  phenomena  presented  by  an  individual  suffering  from  division 
or  compression  of  the  facial  nerve,  and  which  collectively  constitute 
facial  paralysis,  are  as  follows:  a  relaxed  and  immobile  condition  of 
the  side  of  the  face  corresponding  to  the  lesion ;  separation  of  the  eye- 
lids from  paralysis  of  the  orbicularis  palpebrarum  and  the  unopposed 
contraction  of  the  levator  palpebrae  muscles;  abohtion  of  the  act  of 
winking;  drooping  of  the  angle  of  the  mouth;  an  escape  of  saliva 
from  the  mouth;  contraction  of  the  muscles  and  distortion  of  the 
opposite  side  of  the  face;  on  attempting  to  laugh  or  talk  the  dis- 
tortion of  the  face  is  increased;  during  mastication  the  food  accu- 
mulates between  the  teeth  and  cheek,  from  paralysis  of  the  buccina- 
tor; articulation  is  impaired  from  paralysis  of  the  orbicularis  oris 
muscle,  the  labial  sounds  especially  being  imperfectly  produced. 

Properties  of  the  Branches  Given  off  in  the  Aqueduct  of 
Fallopius. — The  great  petrosal  nerve  is  in  all  probabihty  composed 
of  efferent  fibers  (vaso-dilatator  and  secretor)  which  leave  the  pons 
by  way  of  the  nerve  of  Wrisberg,  or  pars  intermedia,  to  be  distributed 
around  the  cells  of  the  spheno-palatine  ganglion;  for  stimulation 
either  of  this  nerve  or  of  the  ganglion  is  followed  by  the  same  results  : 
viz.,  dilatation  of  the  blood-vessels  of,  and  secretion  from,  the  mucous 
membrane  of  the  nose,  soft  palate,  upper  part  of  the  pharynx,  roof 
of  mouth,  gums,  and  upper  lip. 

The  small  petrosal  nerve  is  also  composed  of  efferent  fibers; 
shortly  after  leaving  the  facial  it  is  joined  by  a  small  nerve,  derived 
from  Jacobson's  branch  of  the  glosso-pharyngeal,  which  is  also  effer- 


THE  CRANIAL  NERVES.  563 

ent  in  function;  for  stimulation  of  Jacobson's  nerve  as  well  as  stimula- 
tion of  the  otic  ganglion  is  followed  by  the  same  result:  viz.,  dilatation 
of  the  blood-vessels  of,  and  secretion  from,  the  mucous  membrane  of  the 
cheek,  lower  lip,  and  gums,  and  of  the  parotid  and  the  orbit  glands. 

The  stapedius  nerve,  distributed  directly  to  the  stapedius  muscle, 
is  motor  in  function. 

The  Chorda  Tympani. — Stimulation  of  the  chorda  tympani  nerve 
in  the  tympanic  cavity  produces  dilatation  of  the  blood-vessels  of, 
and  an  increased  production  and  discharge  of  saliva  from,  the  sub- 
maxillary and  subhngual  glands. 

Division  of  this  nerve  is  followed  by  a  contraction  of  the  blood- 
vessels and  a  diminution  of  the  secretion.  From  these  results  it  is 
certain  that  the  chorda  tympani  contains  both  vaso-dilatator  and  secre- 
tor  fibers.  Nicotin  applied  to  the  submaxillary  and  sublingual  gang- 
lia abolishes  the  effects  of  stimulation  of  the  chorda  tympani.  It 
does  not  prevent  the  same  effects  when  the  ganglia  themselves  are 
stimulated.  It  is  clear,  therefore,  that  the  vaso-dilatator  and  secretor 
fibers  arborize  around  the  cells  of  the  gangha  and  are  not  distributed 
directly  to  the  gland  structures.  It  is  highly  probable  that  the  efferent 
fibers  in  the  chorda  tympani  emerge  from  the  pons  by  way  of  the 
pars  intermedia,  or  nerve  of  Wrisberg. 

Division  of  the  chorda  in  the  tympanum  is  also  followed  by  a  loss 
of  taste  in  the  anterior  two-thirds  of  the  tongue.  For  this  and  other 
reasons  the  chorda  tympani  has  long  been  regarded  as  the  nerve  of 
taste  for  this  region.  The  specific  physiologic  stimulus  to  the  chorda 
tympani  nerve  is  organic  matter  in  solution  acting  on  the  peripheral 
terminations  of  the  nerve  in  the  mucous  membrane  of  the  tongue. 
The  exact  pathway  for  these  afferent  or  gustatory  fibers  beyond  the 
geniculate  ganglion  has  long  been  a  subject  of  much  discussion. 
According  to  some  observers  these  fibers  enter  the  great  petrosal 
nerve,  pass  forward  as  far  as  the  spheno-palatine  ganglion,  then  into 
the  superior  maxillary  division  of  the  trigeminal,  and  so  to  the  brain. 
According  to  others,  these  fibers  pass  into  the  pars  intermedia,  into 
the  pons,  where  they  terminate  around  the  sensor  end-nucleus  of 
the  glosso-pharyngeal.  The  evidence  for  and  against  either  of  these 
two  views  is  most  conflicting  and  insufficient  to  justify  positive  state- 
ments one  way  or  the  other.  To  the  writer  the  weight  of  evidence 
seems  to  favor  the  view  that  the  gustatory  fibers  have  their  origin  in 
the  geniculate  ganglion ;  that  they  pass  centrally  through  the  pars  in- 
termedia ;  that  they  are  similar  in  function  to  the  glosso-pharyngeal ; 
and  that  they  are  indeed  but  aberrant  branches  of  this  nerve. 

Functions. — The  facial  is  the  motor  nerve  to  the  muscles  of  the 
face.  As  these  muscles  express  ideas  and  emotions  the  nerve  has  been 
termed  the  nerve  of  expression.  Because  of  the  presence  of  efferent 
fibers  which  leave  the  main  trunk  by  way  of  the  chorda  tympani 
nerve,  it  regulates  the  caliber  of  the  blood  vessels  of  the  submaxillary 


564 


TEXT-BOOK  OF  PHYSIOLOGY. 


and  sublingual  glands  and  excites  the  glands  to  activity. 
influences  hearing  by  its  action  on  the  stapedius  muscle. 


It  also 


EIGHTH  PAIR.    THE  AUDITORY. 

The  eighth  cranial  nerve,  the  auditory,  consists  of  the  centrally 
coursing  axons  of   neurons  which   connect  the  essential  organ  of 

hearing  with    sensor  end-nuclei 
,--    10  in  the  pons  Varolii.     This  nerve 

..-■''  consists  of  two  portions:  viz.,  a 

cochlear  or  auditory  and  a  ves- 
tibular or  equilibratory. 

Origin. — The  axons  compris- 
ing the  cochlear  portion  have 
their  origin  in  the  bipolar  nerve- 
cells  of  the  spiral  gangHon  located 
in  the  spiral  canal  near  the  base 
of  the  osseous  lamina  spiralis 
(Fig.  254).  From  this  origin 
they  pass  centrally  into  the  cen- 
tral canal  of  the  modiolus,  at  the 
base  of  which  they  emerge  in 
well-defined  bundles  and  enter 
the  internal  auditory  meatus. 
Dendritic  processes  from  these 
cells  pass  peripherally  to  termi- 
nate on  the  ciliated  epithelial  cells 
of  the  organ  of  Corti. 

The  axons  comprising  the 
vestibular  portion  have  their 
origin  in  the  bipolar  nerve-cells 
of  the  ganglion  of  Scarpa  located 
in  the  internal  auditory  meatus. 
From  this  origin  they  pass  cen- 
trally in  connection  with  th6 
cochlear  portion.  Dendritic  pro- 
cesses from  these  cells  pass  per- 
ipherally into  the  internal  ear, 
where  they  terminate  on  epithe- 
lial cells  situated  on  the  inner 
surface  of  the  utricle  and  saccule 
and  in  the  ampullae  of  the  semicircular  canals. 

The  common  trunk  of  the  auditory  nerve,  consisting  of  both 
cochlear  and  vestibular  divisions  after  emerging  from  the  internal 
auditory  meatus,  passes  backward,  inward,  and  downward  as  far 


Fig.  254. — Origin  and  Termination 
OF  THE  Auditory  Nerve.  i. 
Cochlea.  2.  Spiral  ganglion  (Corti). 
3.  Cochlear  nerve.  4.  Ventral 
acoustic  nucleus.  5.  Lateral  acoustic 
nucleus.  6.  Semicircular  canals.  7. 
Ganglion  of  Scarpa.  8.  Vestibular 
nerve.  9.  Dorso-external  nucleus 
(Deiters).  10.  Dorso-internal  nu- 
cleus.— (After  Moral  and  Doyon.) 


THE  CRANIAL  NERVES.  565 

as  the  lateral  aspect  of  the  pons  where  the  two  divisions  again 
separate. 

The  cochlear  nerve,  the  external  root,  passes  to  the  outer  side 
of  the  restiform  body  and  enters  the  ventral  acoustic  nucleus  and  the 
lateral  acoustic  nucleus,  around  the  cells  of  which  its  end-tufts 
arborize.  The  vestibular  nerve,  the  internal  root,  passes  on  the 
inner  side  of  the  restiform  body  to  the  dorsal  portion  of  the  pons, 
where,  after  bifurcating,  the  end-tufts  of  the  axons  arborize  around 
the  dorso-internal  or  chief  auditory  nucleus  and  the  dorso-external 
or  Deiters'  nucleus.  Some  of  the  fibers  of  the  vestibular  branch 
descend  through  the  pons  and  medulla  as  far  as  the  cuneate  nucleus. 

Cortical  Connections. — The  cochlear  nerve  is  ultimately  con- 
nected with  the  cerebral  acoustic  area,  in  the  temporal  lobe  of  the 
opposite  side  through  the  intermediation  of  the  auditory  tract.  This 
tract  is  complex  and  involved.  In  a  general  way  it  may  be  said  to 
consist  in  part  of  fibers  which  come  direct  from  the  cochlear  branch. 
After  passing  through  the  ventral  nucleus  and  the  trapezoid  body 
they  cross  the  median  hne,  enter  the  lemniscus  or  fillet,  and  finally 
terminate  in  the  pre-  and  post-geminal  bodies.  In  their  course  they 
give  off  collateral  branches  to  these  various  nuclei  through  which 
they  pass.  Other  fibers  taking  their  origin  from  cells  in  these  various 
nuclei  proceed  to  the  cortex  where  they  terminate. 

Properties. — Stimulation  of  the  cochlear  nerve  is  unattended  by 
either  motor  or  sensor  phenomena.  Division  of  the  nerve  is  fol- 
lowed by  a  loss  of  the  sense  of  hearing.  Irritative  pathologic  lesions 
give  rise  to  sensations  of  sound  of  varying  character  and  intensity. 
Degeneration  of  the  nerve  or  destruction  by  tumors,  etc.,  will  also 
be  followed  by  a  loss  of  the  sense  of  hearing. 

Experimental  lesions  of  the  semicircular  canals  involving  a 
destruction  of  the  physiologic  relations  of  the  vestibular  nerve  are 
followed  by  a  loss  of  the  coordinating  and  equilibratory  power. 
Disordered  movements,  such  as  rotation  to  the  right  or  left,  somer- 
saults backward  and  forward,  follow  destruction  of  these  canals. 
Pathologic  lesions  in  the  peripheral  distribution  of  the  nerve  are 
attended  in  man  by  disturbances  of  equilibrium ;  e.  g.,  vertigo,  a  sense 
of  swaying,  pitching,  and  staggering. 

Functions. — The  function  of  the  cochlear  nerve  is  to  convey 
nerve  impulses  from  its  origin  to  the  pons,  from  which  they  are 
transmitted  by  the  auditor\'  tract  to  the  acoustic  area  in  the  cerebral 
cortex.  The  specific  physiologic  stimulus  to  the  development  of 
these  impulses  is  the  impact  of  atmospheric  undulations  on  the  tym- 
panic membrane,  received  and  transmitted  by  the  chain  of  bones 
to  the  structures  of  the  internal  ear, — the  organ  of  Corti, — with  which 
the  peripheral  terminations  of  the  nerve  are  connected.  The  function 
of  the  vestibular  nerve  is  the  transmission  of  nerve  impulses  to  the 


566  TEXT-BOOK  OF  PHYSIOLOGY. 

pons,  whence  they  are  transmitted  to  the  cortex  of  both  the  cerebrum 
and  cerebelhim  and  to  other  centers.  The  specific  physiologic  stim- 
ulus is  supposed  to  be  a  variation  in  pressure  in  the  ampullae  of  the 
semicircular  canals  caused  by  movements  of  the  endolymph  induced  by 
changes  in  the  position  of  the  head  and  body.  The  impulses  carried 
by  the  vestibular  nerve  give  rise  refiexly  to  certain  adaptive  and  pro- 
tective movements  by  which  the  equilibrium  of  the  body  in  both 
dynamic  and  static  conditions  is  maintained. 

NINTH  NERVE.     THE  GLOSSO-PHARYNGEAL. 

The  ninth  cranial  nerve,  the  glosso-pharyngeal,  consists,  as  shown 
by  both  histologic  and  experimental  methods  of  research,  of  both 
afferent  and  efferent  nerve-fibers,  of  which  the  former,  however,  are 
by  far  the  more  abundant.  Near  its  exit  from  the  cavity  of  the 
skull  the  nerve  presents  two  ganglionic  enlargements  known  as  the 
petrosal  and  jugular  ganglia. 

Origin  of  the  Afferent  Fibers. — The  afferent  fibers  serve  to 
bring  certain  end-nuclei  in  the  medulla  oblongata  into  anatomic  and 
physiologic  relation  with  portions  of  the  mucous  membrane  of  the 
tongue,  pharynx,  and  middle  ear.  The  afferent  fibers  are  axons  of 
the  monaxonic  cells  of  the  petrosal  and  jugular  ganglia.  The  single 
axon  from  each  of  these  cells  soon  divides  into  two  branches,  one 
of  which  passes  centrally,  the  other  peripherally.  The  centrally 
directed  branches  collectively  form  the  so-called  roots,  four  or  five 
in  number,  which  enter  the  medulla  between  the  olivary  and  resti- 
form  bodies.  The  peripherally  directed  branches  collectively  form 
the  two  main  divisions,  from  the  distribution  of  which,  to  the  tongue 
and  pharynx,  the  nerve  takes  its  name. 

Distribution. — The  axons  of  the  centrally  directed  branches 
after  entering  the  medulla  pass  toward  its  dorsal  aspect,  where  they 
bifurcate,  give  off  collateral  branches,  and  terminate  in  fine  end- 
tufts  in  the  immediate  neighborhood  of  two  groups  of  nerve-cells, 
the  sensor  end-nuclei.  The  axons  of  the  peripherally  directed 
branches,  after  emerging  from  the  base  of  the  skull  through  the 
jugular  foramen,  pass  forward  and  inward  under  cover  of  the  stylo- 
pharyngeal muscle;  winding  around  this  muscle  they  divide  into 
terminal  branches  which  are  distributed  to  the  mucous  membrane  of 
the  posterior  one-third  of  the  tongue,  pharynx,  soft  palate,  uvula, 
and  tonsils  (Fig.  257). 

Origin  of  the  Efferent  Fibers. — The  efferent  fibers  serve  to 
bring  the  nerve-cells  from  which  they  arise  into  connection  with  a 
portion  of  the  musculature  of  the  fauces  and  phar}'nx.  These  nen-e- 
cells  are  located  in  the  lateral  portion  of  the  formatio  reticularis  at 
some  distance  below  the  floor  of  the  fourth  ventricle.     Thev  consti- 


THE  CRANIAL  NERVES. 


0^7 


tute  the  upper  portion  of  a  collection  of  cells  known  as  the  nucleus 
amhiguus. 

Distribution. — From  this  origin  the  efferent  fibers  pass  dorsally 
to  near  the  sensor  end-nuclei,  then  turn  outward  and  forward  and 
finally  emerge  from  the  medulla  in  intimate  association  with  the 
afferent  fibers.  They  are  ultimately  distributed  to  the  stylo-pharyn- 
geus,  the  palato-glossus  and  to  the  middle  constrictor  muscles  of  the 
pharynx.  In  addition  to  the  foregoing  eft'erent  fibers  the  glosso- 
pharyngeal nerve  contains  at  its  emergence  from  the  medulla  both 
vaso-motor  and  secretor  fibers. 

Jacobson's  Nerve. — This  is  a  small  branch  which  leaves  the 
glosso-phar}'ngeal  at  the  petrous  ganglion.  After  passing  through  a 
small  canal  in  the  base  of  the  skull  it  enters  the  tympanic  cavity, 
within  which  it  gives  off  branches  to  the  great  and  lesser  petrosal 
nerves,  to  the  mucous  membrane  of  the  foramen  ovale,  the  foramen 
rotundum,  and  to  the  Eustachian  tube. 

Cortical  Connections. — The  motor  nucleus  is  doubtless  con- 
nected with  the  general  motor  area  of  the  cortex  through  fibers  de- 
scending in  the  pyramidal  tract.  The  exact  location  of  the  cortical 
area  for  the  phar}mx  is  not  well  determined,  but  is  most  likely  to  be 
found  in  the  lower  part  of  the  general  motor  area  near  the  termination 
of  the  Rolandic  fissure.  The  cortical  connections  of  the  afferent 
tract  are  unknown. 

Properties. — Electric  stimulation  of  the  glosso-pharyngeal  trunk 
calls  forth  evidence  of  pain  and  contraction  of  the  stylo-pharyngeus 
and  middle  constrictor  muscles.  Division  of  the  nerve  abolishes 
sensibility  in  the  mucous  membrane  to  which  it  is  distributed,  impairs 
the  sense  of  taste  in  the  posterior  third  of  the  tongue,  and  gives  rise 
to  paralysis  of  the  above-mentioned  muscles. 

Stimulation  of  Jacobson's  nerve  is  followed  by  dilatation  of  the 
blood-vessels  of,  and  secretion  from,  the  mucous  membrane  of  the 
lower  lip,  cheek,  and  gums,  and  from  the  parotid  gland.  Division 
of  the  nerve  is  followed  by  the  opposite  results.  The  course  of  the 
fibers  which  give  rise  to  these  results  is  by  way  of  the  lesser  petrosal 
to  the  otic  ganglion,  around  the  cells  of  which  the  fibers  arborize. 
From  the  cells  of  this  ganghon  non-medullated  fibers  pass  to  the 
blood-vessels  and  gland  cells. 

Functions. — The  afferent  fibers  of  the  glosso-pharyngeal  trans- 
mit nerve  impulses  from  the  parts  to  which  they  are  distributed  to 
the  cerebral  cortex,  where  they  evoke  sensations  of  pain  and  sensations 
of  taste;  they  also  assist  in  all  probabihty  in  the  performance  of 
certain  reflexes  connected  with  deglutition.  The  efferent  fibers  trans- 
mit impulses  to  muscles,  exciting  them  to  activity,  and  to  the  otic 
ganglion,  which  in  turn  dilates  blood-vessels  and  excites  secretion. 


568  TEXT-BOOK  OF  PHYSIOLOGY. 


TENTH  NERVE.    THE  PNEUMOGASTRIC   OR  VAGUS. 

The  tenth  cranial  nerve,  the  pneumogastric  or  vagus,  consists, 
as  shown  by  histologic  methods  of  research,  of  both  afferent  and 
efferent  fibers,  independent  of  those  derived  in  its  course  from  adjoin- 
ing motor  or  efferent  nerves.  Near  the  exit  of  the  nerve  from  the 
cavity  of  the  cranium  it  presents  two  ganghonic  enlargements  known 
respectively  as  the  ganglion  of  the  root  (the  jugular)  and  the  ganglion 
of  the  trunk  (the  plexiform). 

Origin  of  the  Afferent  Fibers. — The  afferent  fibers  take  their 
origin  in  the  monaxonic  cells  of  the  gangha  on  the  root  and  trunk. 
The  single  axon  from  each  of  these  cells  soon  divides  into  two 
branches,  one  of  which  passes  centrally,  the  other  peripherally.  The 
centrally  directed  branches  collectively  form  the  so-called  roots,  ten 
to  fifteen  in  number,  which  enter  the  medulla  between  the  restiform 
body  and  the  lateral  column.  The  peripherally  directed  branches 
collectively  form  a  portion  of  the  common  trunk  of  the  nerve. 

Distribution. — The  axons  of  the  centrally  directed  branches 
after  entering  the  medulla  pass  toward  its  dorsal  aspect,  where  they 
bifurcate,  give  off  collaterals,  and  terminate  in  fine  end-tufts  in  the 
immediate  neighborhood  of  two  groups  of  nerve-cells,  the  vagal 
sensor  end-nuclei. 

The  axons  of  the  peripherally  directed  branches  unite  to  form  a 
portion  of  the  common  trunk,  which,  as  it  descends  the  neck  and 
enters  the  thorax  and  abdomen,  gives  off  a  number  of  branches  which 
are  ultimately  distributed  to  the  mucous  membrane  of  the  esophagus, 
larynx,  lungs,  stomach,  and  intestine,  and  also  to  the  heart.  The 
afferent  fibers  thus  serve  to  bring  into  anatomic  and  physiologic 
relation  the  mucous  membrane  of  these  organs  with  certain  sensor ' 
end-nuclei  in  the  medulla  oblongata. 

Origin  of  the  Efferent  Fibers. — The  efferent  fibers  take  their 
origin  from  nerve-cells  located  in  the  lateral  portion  of  the  jormatio 
reticularis  at  some  distance  below  the  floor  of  the  fourth  ventricle. 
These  cells  constitute  the  lower  portion  of  the  nucleus  amhiguus. 

Distribution. — From  their  origin  the  efferent  axons  pass  dor- 
sally  to  near  the  sensor  end-nuclei,  then  turn  outward  and  forward, 
and  finally  emerge  from  the  medulla  in  close  association  with  the 
afferent  branches.  They  are  ultimately  distributed  to  the  levator 
palati,  azygos  uvulae,  and  palato-pharyngus  muscles;  to  the  superior 
and  inferior  constrictor  muscles  of  the  pharynx,  and  to  the  muscles  of 
the  esophagus;  to  the  muscle-fibers  of  the  stomach  and  perhaps  the 
intestines;  and  to  the  non-striated  muscle-fibers  of  the  bronchial 
tubes.  Among  the  efferent  fibers  are  some  which  are  distributed  to 
the  gastric  glands  and  to  the  pancreas. 

According  to  Beevor  and  Horsley,  in  the  monkey  the  motor  fibers 
for  the  levator  palati  come  from  the  spinal  accessory  nerve. 


THE  CRANIAL  NERVES.  569 

The  efferent  fibers  thus  serve  to  bring  the  nerve-cells  from  which 
they  arise  into  anatomic  and  physiologic  connection  with  a  portion  of 
the  musculature  of  the  aHmentary  canal  and  its  diverticulum,  the 
lung. 

Communicating  Branches. — At  or  near  the  ganglia  the  vagus 
receives  communicating  branches  from  the  eleventh  nerve,  the  spi- 
nal accessor}',  the  facial,  the  hypoglossal,  and  the  anterior  branches 
of  the  two  upper  cervical  nerves.  Owing  to  this  manifold  origin  of 
the  efferent  fibers  in  the  trunk  and  peripheral  branches  of  the  vagus, 
it  is,  in  some  instances,  difficult,  if  not  impossible,  to  determine  to 
which  of  these  nerves  a  given  muscle  contraction  is  to  be  referred. 

Vagal  Branches. — As  the  vagus  passes  down  the  neck  it  gives 
off  the  following  main  branches  (Fig.  255) : 

1.  The  pharyngeal  ?terves  which,  after  entering  into  the  formation  of 

the  phar}'ngeal  plexus,  are  distributed  to  the  mucous  membrane 
and  to  the  muscles  of  the  pharynx;  e.  g.,  superior  and  inferior 
constrictors,  the  levator  palati,  and  the  azygos  uvulse. 

2.  The  superior  laryngeal  nerve  which,  entering  the  larynx  through 

the  thyro-hyoid  membrane,  is  distributed  to  the  mucous  mem- 
brane fining  the  interior  of  the  larv'nx  and  to  the  crico-thyroid 
muscle.  From  the  superior  laryngeal  and  the  main  trunk  small 
branches  are  given  oft'  which  in  the  rabbit  unite  to  form  a  single 
nerve,  the  so-called  depressor  nerve.  It  is  distributed  to  the 
heart-muscle.  Though  this  anatomic  arrangement  is  not  found 
in  man,  there  are  many  reasons  for  believing  that  analogous 
fibers  are  present  in  the  vagus  trunk  of  man  and  other  animals. 

3.  The  inferior  laryngeal  nerve  which  is  distributed  ultimately  to  all 

the  muscles  of  the  larynx  except  the  crico-thyroid  and  to  the 
inferior  constrictor  of  the  pharynx. 

4.  The  cardiac  nerves  which,  after  entering  into  the  formation  of  the 

cardiac  plexus,  are  distributed  to  the  heart. 

5.  The  pulmonary  nerves  distributed  to  the  mucous  membrane  of  the 

bronchial  tubes  and  their  ultimate  terminations,  the  lobules  and 
air-cells,  as  well  as  to  their  non-striated  muscle-fibers. 

6.  The  gastric  and  intestinal  nerves,  distributed  to  the  mucous  mem- 

brane and  muscular  walls  of  the  stomach  and  intestines.  Other 
fibers  in  all  probabiHty  pass  to  the  liver,  spleen,  kidney,  and 
suprarenal  bodies. 

Properties  of  the  Pneumogastric  or  Vagus  Nerve  and  its 
Various  Branches. — Faradization  of  the  vagus  nerve  close  to  the 
medulla  oblongata  gives  rise  to  sensations  of  pain  and  to  contraction 
of  the  musculature  of  a  portion  of  the  alimentary  tract:  viz.,  the 
palate,  phar}'nx,  esophagus,  stomach,  and  possibly  of  the  intestine 
and  of  the  pulmonary  apparatus.  Division  of  the  nerve  is  followed 
by  a  loss  of  sensibihty  in  the  mucous  membrane  of  the  ahmentary 


S70 


TEXT-BOOK  OF  PHYSIOLOGY. 


tract  and  of  the  pulmonary  apparatus,  together  with  a  loss  of  motility 
of  the  structures  above  mentioned. 


Fig.  255. — Distribution  of  the  Pneumogastric. — i.  Trunk  of  the  left  pneumo- 
gastric.  2.  Ganglion  of  the  trunk.  3.  Anastomosis  with  the  spinal  accessory. 
4.  Anastomosis  with  the  subhngual.  5.  Pharyngeal  branch  (the  auricular  branch 
is  not  shown  in  the  figure) .  6.  Superior  laryngeal  branch.  7.  External  laryngeal 
nerve.  8.  Laryngeal  plexus,  q,  9.  Inferior  larj'ngeal  branch.  10.  Cervical  car- 
diac branch.    II.  Thoracic  cardiac  branch.    12,  13.  Pulmonary  branches.   14.  Lin- 

,'.  gual  branch  of  the  fifth.  15.  Lower  portion  of  the  subhngual.  16.  Glosso-pharyn- 
geal.  17.  Spinal  accessory.  18,19,20.  Spinal  nerves.  21.  Phrenic  nerve.  22,23. 
Spinal  ner\'es.     24,  25,  26,  27,  28,  29,  30.  Sympathetic  ganglia. — {Hirschjeld.) 

■  Stimulation  of  the  trunk  of  the  nerve  in  different  parts  of  its 
course  produces  a  variety  of  results  dependent  to  some  extent  on  the 
presence  of  anastomosing  branches  from  adjoining  nerves. 


THE  CRANIAL  NERVES.  571 

The  Pharyngeal  Nerves. — Faradization  of  the  pharyngeal  nerves 
consisting  of  both  afferent  and  efferent  fibers,  gives  rise  to  sensations 
of  pain,  contraction  of  the  pharyngeal  muscles,  and  perhaps  to  vomit- 
ing. Division  of  these  nerves  is  followed  by  a  loss  of  sensibiHty  in 
the  parts  to  which  they  are  distributed  and  by  paralysis  of  the  muscles 
with  a  consequent  impairment  of  deglutition. 

The  Superior  Laryngeal  Nerve. — Faradization  of  the  superior 
laryngeal  nerve  gives  rise  to  sensations  of  pain,  and  to  contraction  of 
the  crico-thyroid  muscle.  Through  reflected  impulses  it  causes  con- 
traction of  the  muscles  of  deglutition,  and  of  the  muscles  concerned 
in  the  act  of  coughing;  inhibition  of  the  inspirator}'  movement  and 
arrest  of  respiration  in  the  condition  of  expirator}'  standstill,  with 
perhaps  a  tetanic  contraction  of  the  expiratory  muscles;  and  con- 
traction of  the  laryngeal  muscles  with  closure  of  the  glottis.  Periph- 
eral stimulation  of  this  nerve — e.  g.,  the  contact  of  foreign  particles — 
gives  rise  to  a  similar  series  of  phenomena.  Division  of  these  nerves 
is  followed  by  a  loss  of  sensibility  in  the  laryngeal  mucous  membrane, 
paralysis  of  the  crico-thyroid  muscle  with  a  consequent  lowering  of 
the  pitch,  and  a  diminution  in  the  clearness  of  the  voice.  In  conse- 
quence of  the  loss  of  the  sensibiHty  there  is  an  inability  to  perceive 
the  entrance  of  foreign  bodies  into  the  lar}'nx. 

The  Depressor  Nerve. — Stimulation  of  the  peripheral  end  of  the 
depressor  nerve  is  without  effect;  stimulation  of  the  central  end  re- 
tards and  even  arrests  the  heart's  pulsations  and  lowers  the  general 
blood-pressure.  These  two  effects,  though  associated,  are  neverthe- 
less independent  of  each  other.  If  the  vagus  nerves  be  divided  on 
both  sides  between  the  origin  of  the  depressor  and  the  origin  of  the 
cardiac  nerves,  and  the  former  stimulated,  there  will  be  a  fall  of 
pressure  without  retardation  of  the  heart.  The  effect  on  the  heart 
is  attributed  to  a  stimulation  of  the  cardio-inhibitory  mechanism  in 
the  medulla  oblongata. 

The  fall  of  general  blood-pressure  was  formerly  attributed  to  a 
sudden  dilatation  of  the  splanchnic  blood-vessels  alone,  in  conse- 
quence of  a  depression  of  that  portion  of  the  general  vaso-motor  center 
which  maintains  through  the  splanchnic  nerves  a  tonic  contraction  of 
their  walls.  It  has  been  satisfactorily  demonstrated  that  this  is  not 
the  sole  cause ;  for  after  division  of  the  splanchnic  nerves,  stimulation 
of  the  depressor  causes  a  still  further  fall  of  from  30  to  40  per  cent.' 
in  the  general  pressure  (Porter  and  Beyer).  Evidently,  not  anyone, 
but  all  portions  of  the  vaso-motor  center  are  subject  to  the  effects  of 
depressor  stimulation. 

The  Inferior  Laryngeal  Nerves. — Faradization  of  the  inferior 
laryngeal  nerves  produces  effects  which  vary  in  accordance  with  the 
strength  of  the  stimulus,  with  different  animals,  and  with  the  same 
animal  at  different  periods  of  life.     In  the  adult  dog  and  in  man,  the 


572  TEXT-BOOK  OF  PHYSIOLOGY. 

glottis  is  kept  widely  open  for  respiratory  purposes  by  the  tonic  con- 
traction of  the  abductor  muscles  (the  crico-arytenoids) ;  for  phonatory 
purposes  the  glottis  is  closed  and  the  vocal  membranes  approximated 
by  the  contraction  of  the  adductor  muscles.  It  has  been  shown  that 
these  opposed  groups  of  muscles  have  independent  nerve-supplies; 
that  two  sets  of  fibers  in  the  common  trunk  can  be  separated  and 
stimulated  independently  of  each  other.  Feeble  stimulation  of  the 
common  trunk  produces  a  still  further  abduction  of  the  vocal  cords. 
With  an  increase  in  the  strength  of  the  stimulus,  however,  the  reverse 
obtains:  namely,  adduction  which  increases  until  the  glottis  is  com- 
pletely closed.  Division  of  the  nerves  is  followed  by  paralysis  of  both 
the  phonatory  and  respiratory  muscles,  the  abductors  and  adductors, 
with  the  result  of  seriously  impairing  both  phonation  and  respiration 
and  not  infrequently  causing  death.  The  fibers  of  the  inferior  laryn- 
geal nerve  are  derived  from  the  eleventh  nerve,  the  spinal  accessory. 

The  Cardiac  Nerves. — Faradization  of  the  trunk  of  the  vagus  or 
of  the  peripheral  end  of  the  divided  nerve  gives  rise  to  a  diminution 
in  the  frequency  and  force  of  the  heart's  contractions;  and  if  the  stim- 
ulation be  sufficiently  powerful,  completely  arrests  it  in  the  phase  of 
diastole.  To  these  results  the  term  inhibition  is  applied.  Division 
of  the  vagi  or  of  the  cardiac  branches  is  follo\Yed  by  an  increase  in 
the  number  of  the  contractions  from  loss  of  inhibitor  influences. 
The  inhibitor  fibers  of  the  vagus  are  generally  believed  to  be  derived 
from  the  spinal  accessory,  though  this  has  been  questioned.  Accord- 
ing to  the  recent  investigations  of  Schaternikoiif  and  Friedenthal, 
they  come  direct  in  the  vagus,  from  a  nucleus  near  the  vagal  motor 
nucleus  in  the  medulla,  the  spinal  accessory  sending  no  branches  to 
the  heart.  In  the  frog  and  other  batrachia  the  vagus  contains  also 
accelerator  or  augmentor  fibers  derived  from  the  sympathetic;  hence 
stimulation,  especially  if  feeble,  may  increase  the  heart's  action  or 
may  only  retard,  but  not  arrest,  the  heart. 

The  Pulmonary  Nerves. — The  pulmonary  nerves,  given  off  from 
the  trunk  after  its  entrance  into  the  thorax,  do  not  lend  themselves 
readily  to  experimentation.  Division  of  both  vagi  in  the  neck  above 
the  point  of  exit  of  the  pulmonary  branches  is  followed  by  a  decrease 
in  the  frequency  of  the  respiratory  acts,  with  an  increase  in  their  depth. 
At  the  same  time  there  is  a  loss  of  sensibility  of  the  mucous  membrane 
of  the  trachea  and  lungs  and  a  paralysis  of  non-striated  muscle-fibers. 
Stimulation  of  the  central  end  of  the  vagus  increases  the  frequency, 
but  decreases  the  amplitude,  of  the  respiratory  movements.  If  the 
stimulation  be  increased  in  intensity  the  respiratory  movements  in- 
crease in  frequency  until  the  inspiratory  muscles  pass  into  the  con- 
dition of  tetanus. 

Feeble  stimulation  of  the  vagus  not  infrequently  inhibits  the 
inspiratory  movement  and  increases  the  expiratory  until  there  is  a 


THE  CRANIAL  NERVES.  573 

complete  cessation  of  movement  in  the  condition  of  expiratory  stand- 
still. The  effect  thus  produced  is  similar  to,  if  not  identical  with, 
that  produced  by  stimulation  of  the  superior  laryngeal  nerv^e.  This 
would  seem  to  indicate  the  presence  in  the  vagus  trunk  of  two  sets 
of  afferent  fibers  coming  from  the  lungs  through  the  pulmonary 
branches,  one  of  which  inhibits  inspiration,  the  other  expiration. 

Faradization  of  the  trunks  of  the  pulmonary  branches  or  stimula- 
tion of  their  peripheral  terminations  in  the  mucous  membrane  of 
the  bronchial  tubes  or  alveoli  by  the  inhalation  of  chemic  vapors 
causes  arrest  of  respiratory  movements,  a  fall  of  blood-pressure,  and 
a  reflex  inhibition  of  the  heart  (Brodie). 

Gastric  Nerves. — Stimulation  of  the  peripheral  end  of  a  divided 
vagus  nerve  causes  a  distinct  contraction  of  the  right  half  of  the 
stomach  and  secretion  from  the  gastric  glands.  Division  of  the  nerve 
abolishes  the  sensibility  of  the  mucous  membrane  of  the  stomach, 
impairs  motihty,  and  interferes  with  the  secretion  of  the  gastric  juice. 

Similar  experimentation  on  the  trunk  of  the  vagus  has  shown  that 
the  nerve  excites  contraction  of  the  upper  part  of  the  small  intestine 
and  of  the  gall-bladder,  the  secretion  of  the  pancreas,  the  renal  cir- 
culation, the  secretion  of  urine,  etc. 

Functions. — The  afferent  fibers  transmit  nerve  impulses  from  the 
area  of  their  distribution  to  the  medulla  and  thence  through  cortical 
connections  to  the  sensor  cerebral  areas,  where  they  evoke  sensations. 

The  efferent  fibers  transmit  impulses  outward  which  excite  con- 
traction of  the  muscles  of  the  esophagus,  the  stomach,  the  small  intes- 
tine, and  the  gall-bladder,  and  the  muscles  of  the  bronchial  tubes 
excite  secretion  from  the  glands  of  the  stomach,  pancreas,  and 
kidney  and  exert  an  inhibitor  influence  on  the  activity  of  the 
heart.  The  efferent  fibers  belong  to  the  autonomic  system  of  nerves 
and  are  not  connected  with  the  ganglia  of  the  vagus,  but  with  local 
peripheral  ganglia. 

ELEVENTH  PAIR.     THE  SPINAL  ACCESSORY. 

The  eleventh  cranial  nerve,  the  spinal  accessory,  consists  of 
peripherally  coursing  fibers  which  bring  the  nerve-cells  from  which 
they  arise  into  relation  with  separate  but  functionally  related  muscles. 
It  consists  of  two  portions,  the  medullary  or  bulbar  and  the  spinal. 

Origin. — The  axons  comprising  the  medullary  portion  arise  from 
a  group  of  nerve-cells  in  the  lower  part  of  the  nucleus  ambiguus. 
From  this  origin  the  axons  pass  forward  and  outward  to  emerge  from 
the  medulla  just  below  and  in  series  with  the  roots  of  the  vagus  nerve. 

The  axons  comprising  the  spinal  portion  have  their  origin  in 
nerve-cells  in  the  lateral  margin  of  the  anterior  horn  of  the  gray 
matter  in  the  cervical  portion  of  the  cord  as  far  down  as  the  fifth 


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TEXT-BOOK  OF  PHYSIOLOGY. 


cervical  vertebra.     From  this  origin  the  fibers  pass  to  the  surface  of 

the  cord  to  emerge  between  the 
ventral  and  dorsal  roots  in  from 
six  to  eight  filaments,  after  which 
they  unite  from  below  upward 
to  form  a  distinct  nerve.  This 
enters  the  cranial  cavity  through 
the  foramen  magnum,  where  it 
joins  with  the  medullary  portion 
to  form  the  common  trunk, 
which  then  passes  forward  to 
emerge  from  the  cranium 
through  the  jugular  foramen. 

Distribution. — After  emerg- 
ing from  the  cranial  cavity  the 
nerve  soon  separates  into  two 
branches : 

1.  An  internal  or  anastomotic 
branch,  consisting  chiefly 
of  filaments  coming  from 
the  medulla  oblongata.  It 
soon  enters  the  trunk  of 
the  vagus,  from  which 
fibers  pass  to  the  muscles  of 
the  pharynx,  to  the  muscles 
of  the  larynx  through  the 
inferior  laryngeal  nerve, 
and  to  the  heart  accord- 
ing to  most  authorities. 

2.  An  external  branch,  consist- 
ing chiefly  of  the  accessory 
fibers  from  the  spinal  cord. 
It  is  distributed  to  the 
sterno-cleido-mastoid  and 
trapezius  muscles. 

Cortical  Connections. — The 
nucleus  of  origin  of  the  medul- 
lary branch  at  least  is  in  relation 
with  nerve-cells  in  the  lower 
third  of  the  general  cerebral 
motor  area,  the  axons  of  which 
descend  in  the  pyramidal  tract. 
Properties. — Faradization  of 
the  nerve  near  its  origin  gives 
rise  to  muscle  contraction.     Destruction  of  the   medullary  root  is 


Fig 


256. — Spinal  Accessory  Nerve. 
I.  Trunk  of  the  facial  nerve.  2,  2. 
Glosso  -  pharyngeal  nerv-e.  3,  3. 
Pneumogastric.  4,  4,  4.  Trunk  of 
the  spinal  accessory.  5.  Sublingual 
nerve.  6.  Superior  cervical  gang- 
lion. 7,  7.  Anastomosis  of  the  first 
two  cervical  nerves.  8.  Carotid 
branch  of  the  sympathetic.  9,  10,  11, 
12,  13.  Branches  of  the  glosso- 
pharyngeal. 14,15.  Branches  of  the 
facial.  16.  Otic  ganglion.  17. 
Auricular  branch  of  the  pneumogas- 
tric. 18.  Anastomosing  branch  from 
the  spinal  accessory  to  the  pneumo- 
gastric. 19.  Anastomosis  of  the  first 
pair  of  cervical  nerves  with  the  sub- 
lingual. 20.  Anastomosis  of  the  spi- 
nal accessory  with  the  second  pair  of 
cervical  nerves.  21.  Pharyngeal 
plexus.  22.  Superior  laryngeal 
nerve.  23.  External  laryngeal 
nerve.  24.  Middle  cervical  gang- 
lion.— {Hirschjeld.) 


THE  CRANIAL  NERVES.  575 

followed  by  impairment  of  deglutition  and  a  loss  of  the  power  of 
producing  vocal  sounds  on  account  of  paralysis  of  the  constrictor 
muscles  of  the  larynx.  According  to  some  authorities,  there  is  also  an 
acceleration  of  the  heart's  action  from  a  loss  of  inhibitor  influences. 

Stimulation  of  the  external  branch  gives  rise  to  contraction  of 
the  sterno-cleido-mastoid  and  trapezius  muscles,  though  division  of 
the  branch  does  not  give  rise  to  complete  paralysis,  as  they  are  sup- 
plied with  motor  fibers  also  from  the  cervical  nerves.  In  consequence 
of  division  of  the  external  branch  animals  experience  extreme  short- 
ness of  breath  during  exercise,  from  a  want  of  coordination  of  the 
muscles  of  the  fore-limbs  and  the  muscles  of  respiration. 

Functions. — The  spinal  accessory  nerve  transmits  nerve  impulses 
outward  which  influence  the  movements  of  deglutition,  and  the 
vocal  movements  of  the  larynx,  which  inhibit  the  action  of  the  heart 
and  which  control  respiratory  movements  associated  with  sustained 
or  prolonged  muscle  efforts. 

TWELFTH  PAIR.    THE  HYPOGLOSSAL. 

The  twelfth  cranial  nerve,  the  hypoglossal,  consists  of  peripher- 
ally coursing  nerve-fibers  which  serve  to  connect  the  nerve-cells 
from  which  they  arise  with  the  musculature  of  the  tongue. 

Origin. — The  axons  composing  the  hypoglossal  nerve  arise  from 
a  collection  of  nerve-cells  situated  beneath  the  floor  of  the  fourth 
ventricle.  This  nucleus  is  elongated  and  extends  from  the  medullary 
striae  downward  as  far  as  the  lower  border  of  the  olivary  body.  It  is 
located  ventro-laterally  to  the  spinal  canal.  After  leaving  the  cells 
of  the  nucleus  the  axons  pass  forward  and  outward  toward  the  surface 
of  the  medulla,  from  which  they  emerge  in  ten  or  twelve  small  bundles 
or  filaments  in  the  groove  between  the  olivary  body  and  the  anterior 
pyramid.     Beyond  this  point  they  unite  to  form  a  common  trunk. 

Distribution.- — The  common  trunk  thus  formed  passes  out  of 
the  cranial  cavity  through  the  anterior  condyloid  foramen.  In  its 
course  it  receives  filaments  from  the  first  and  second  cervical  nerves, 
the  sympathetic  and  vagus.  It  is  finally  distributed  to  the  intrinsic 
muscles  of  the  tongue  and  to  the  genio-hyo-glossus,  hyo-glossus,  and 
stylo-hyoid  muscles.  Branches  derived  from  the  cervical  plexus 
pass  to  muscles  which  elevate  and  depress  the  hyoid  bone. 

Cortical  Connections. — The  hypoglossal  nerve  nuclei  are  con- 
nected with  nerve-cells  in  the  lower  third  of  the  general  motor  area 
around  the  inferior  termination  of  the  fissure  of  Rolando  by  axons 
which  descend  in  the  pyramidal  tract. 

Properties. — Faradization  of  the  nerve  gives  rise  to  convulsive 
movements  of  the  muscles  to  which  it  is  distributed.  Division  of  the 
nerve  is  followed  by  a  loss  of  motion  and  an  interference  with  deglu- 


576 


TEXT-BOOK  OF  PHYSIOLOGY. 


tition,  mastication,  and  articulation,  especially  in  the  pronunciation 
of  the  consonantal  sounds.  In  hemiplegia,  complicated  with  paraly- 
sis of  the  tongue  from  injury  to  the  hypoglossal  tract,  the  opposite 
side  of  the  tongue  is  involved  in  the  paralysis.  On  protrusion  of 
the  tongue  the  tip  is  deviated  to  the  paralyzed  side,  due  to  the 
unopposed  action  of  the  muscle  of  the  opposite  side. 


Fig.  257. — Distribution  or  the  Hypoglossal  Nerve. — i.  Root  of  the  fifth  nerve. 
2.  GangHon  of  Gasser.  3,  4,  5,  6,  7,  9,  10,  12.  Branches  and  anastomoses  of  the 
fifth  nerve.  11.  Submaxillary  ganglion.  13.  Anterior  belly  of  the  digastric 
muscle.  14.  Section  of  the  mylo-hyoid  muscle.  15.  Glosso-pharyngeal  Nerve. 
16.  Ganglion  of  Andersch.  17,  18.  Branches  of  the  glosso-pharyngeal  nerve. 
19,  19.  Pneumogastric.  20,  21.  Gangha  of  the  pneumogastric.  22,  22.  Superior 
laryngeal  branch  of  the  pneumogastric.  23.  Spinal  accessory  nerve.  24.  Sublin- 
gual nerve-  25.  Descendens  noni.  26.  Thyro-hyoid  branch.  27.  Terminal 
branches.  28.  Two  branches,  one  to  the  gendo-hyo-glossus  and  the  other  to  the 
genio-hyoid  muscle. — (Sappey.) 


Function. — The  hypoglossal  nerve  transmits  nerve  impulses 
from  its  center  of  origin  to  the  intrinsic  and  extrinsic  muscles  of  the 
tongue,  endowing  them  with  motility.  The  coordinate  activity  of 
these  muscles  favorably  assists  mastication,  articulation,  and  deg- 
lutition. 


CHAPTER  XXII. 
THE  SYMPATHETIC  NERVE  SYSTEM. 

The  sympathetic  nerve  system  consists  of  a  number  of  gangHa 
united  one  to  another  by  intervening  cords  of  nerve-fibers.  These 
gangHa  may  for  convenience  of  description  be  divided  into  three 
groups:  viz.,  the  vertebral  or  lateral,  the  pre-vertebral  or  collateral, 
and  the  peripheral  or  terminal. 

The  vertebral  ganglia  are  arranged  in  the  form  of  chains,  one  on 
each  side  of  the  vertebral  column.  The  number  of  ganglia  in  the 
chain  varies  in  animals  of  different  and  in  animals  of  the  same 
species.  In  man  the  number  varies  from  20  to  22.  Each  chain  may 
be  divided  into  a  cervical,  a  thoracic,  a  lumbar,  a  sacral,  and  a 
coccygeal  portion.  The  cervical  portion  is  usually  described  as  con- 
sisting of  three  gangha — a  superior,  a  middle,  and  an  inferior.  This 
statement  is  open  to  question,  hov^ever,  as  the  middle  one  is  fre- 
quently absent  and  the  inferior  one  is  regarded  by  some  anatomists 
as  belonging  to  the  pre-vertebral  series.  The  thoracic  portion  con- 
sists of  ten  or  eleven  gangha,  the  lumbar  and  sacral  portions  of  four 
each  and  the  coccygeal  portion  of  one,  the  so-called  ganghon  impar. 

The  pre-vertebral  ganglia  are  also  united  in  the  form  of  a  chain 
situated  in  the  abdominal  cavity.  The  ganglia  constituting  this 
chain  are  known  as  the  semilunar,  the  renal,  the  superior  and  inferior 
mesenteric,  and  hypogastric. 

The  peripheral  gangha  are  in  more  or  less  close  relation  with  the 
tissues  and  organs  in  different  parts  of  the  body.  As  members  of 
this  group  may  be  mentioned  the  cihary  or  ophthalmic,  the  spheno- 
palatine, the  otic,  the  submaxillary  and  the  sublingual  gangha;  the 
gangUa  in  walls  of  the  heart,  the  respiratory  organs,  the  intestines, 
bladder,  etc. 

The  general  arrangement  of  the  sympathetic  gangha,  their  inter- 
connecting cords  and  branches,  is  shown  in  Figs.  258  and  259. 

Structure  of  the  Ganglia. — Each  ganglion  consists  of  a  capsule 
or  stroma  of  connective  tissue  in  which  are  contained  large  numbers 
of  nerve-cells,  nerve-fibers  medullated  and  non-medullated,  and 
blood-vessels.  The  nerve-cehs  give  origin  to  two  or  more  dendrites, 
which,  perforating  a  nucleated  capsule  by  which  each  cell  is  sur- 
rounded, branch  and  rebranch  and  interlace  to  form  a  pericapsular 
plexus.  Each  cell  gives  origin  also  to  an  axon,  which  as  it  leaves 
37  577 


578 


TEXT-BOOK  OF  PHYSIOLOGY. 


Fig.  258. — Cervical  and  Thoracic  Portion  of  the  Sympathetic. — i,  i,  i.  Right 
pneumogastric.  2.  Glosso-pharyngeal.  3.  Spinal  accessory.  4.  Divided  trunk 
of  the  subungual.  5,  5,  5.  Chain  of  gangha  of  the  sympathetic.  6.  Superior 
cervical  ganghon.  7.  Branches  from  this  ganglion  to  the  carotid.  8.  Nerve  of 
Jacobson.  9.  Two  filaments  from  the  facial,  one  to  the  spheno-palatine  and  the 
other!  to  the  otic  ganglion.     10.  Motor  oculi  externus.     11.  Ophthalmic  ganglion, 


THE  SYMPATHETIC  NERVE  SYSTEM. 


579 


the  cell  becomes  invested  with  a  sheath  continuous  with  the  capsule 
surrounding  the  cell-body.  It  is,  however,  wanting  in  a  medullary 
sheath,  and  hence  the  nerve  presents  a  gray  color.  Such  a  structure, 
in  its  entirety,  is  known  as  a  sympathetic  neuron. 

Structure  of  the  Interconnecting  Cords. — The  interconnecting 
cords  are  composed  of  non-medullated  and  medullated  nerve-fibers. 
The  former  are  the  axons  of  cells  found  in  the  ganglia  more  centrally 
located ;  the  latter,  as  will  be  stated  later,  are  derived  from  the  spinal 
nerves,  from  the  fibers  of  which,  however,  they  differ  in  character, 
being  much  smaller  and  finer.  The  fibers  of  the  interconnecting 
cords,  as  a  rule,  transmit  nerve  impulses  from  the  more  centrally  to 
the  more  peripherally  located  ganglia,  and  are  therefore  termed 
ratni  efjerentes.  In  the  vertebral  chain  some  of  the  cords  transmit 
nerve  impulses  upward,  others  downward,  others  again  forward,  to 
the  pre-vertebral  and  peripheral  gangha. 

Among  the  rami  eft'erentes,  interconnecting  cords,  there  are  some 
which  possess  special  interest  for  the  physiologist,  viz. : 

1.  The  cervical,  which  connects  the  thoracic  ganglia  with  the  superior 

cervical  ganglion.     It  is  composed  mainly  of  medullated  nerve- 
fibers  which  are  derived  originally  from  the  spinal  nerves. 

2.  The  great  splanchnic  nerve,  formed  by  the  union  of  branches  from 

the  fifth  to  the  tenth  thoracic  ganglia.     It  connects  these  gangha 
with  the  semilunar  ganglion. 

3.  The  small  splanchnic  nerve,  formed  by  the  union  of  branches  from 

the  ninth  and  tenth  thoracic  gangha.     It  connects  these  gangha 

wdth  the  solar  and  renal  plexuses. 

Distribution  of  the  Sympathetic  Fibers.— It  has  been  demon- 
strated by  histologic  and  physiologic  methods  of  investigation  that 
the  sympathetic  non-meduhated  fibers  which  have  their  origin  in 

receiving  a  motor  filament  from  the  motor  oculi  communis  and  a  sensory  filament 
from  the  nasal  branch  of  the  fifth.  12.  Spheno-palatine  ganglion.  13.  Otic  gang- 
lion. 14.  Lingual  branch  of  the  fifth  nerve.  15.  Submaxillary  ganghon.  16, 
17.  Superior  laryngeal  nerve.  18.  External  laryngeal  nerve.  19,  20.  Recurrent 
laryngeal  nerve.  21,  22,  23.  Anterior  branches  of  the  upper  four  cervical  nerves, 
sending  filaments  to  the  superior  cervical  sympathetic  ganglion.  24.  Anterior 
branches  of  the  fifth  and  sixth  cer\'ical  nerve,  sending  filaments  to  the  middle 
cer\dcal  ganglion.  25,  26.  Anterior  branches  of  the  seventh  and  eighth  cervical 
and  the  first  dorsal  nerves,  sending  filaments  to  the  inferior  cervical  ganglion. 
27.  Middle  cervical  ganghon.  28.  Cord  connecting  the  two  gangha.  29.  In- 
ferior cervical  ganglion.  30,  31.  Filaments  connecting  this  with  the  middle 
ganglion.  32.  Superior  cardiac  nerve,  t,^.  Middle  cardiac  nerve.  34.  Inferior 
cardiac  nerve.  35,35.  Cardiac  plexus.  36.  Ganglion  of  the  cardiac  plexus.  37. 
Nerve  following  the  right  coronary  artery.  38,  38.  Intercostal  nerves,  with  their 
two  filaments  of  communication  with  the  thoracic  ganglia.  39,  40,  41.  Great 
splanchnic  nerve.  42.  Lesser  splanchnic  nerve.  43,  43.  Solar  plexus.  44.  Left 
pneumogastric.  45.  Right  pneumogastric.  46.  Lower  end  of  the  phrenic  nerve. 
47.  Section  of  the  right  bronchus.  48.  Arch  of  the  aorta.  49.  Right  auricle. 
50.  Right  ventricle.  51,52.  Pulmonary  artery.  53.  Right  half  of  the  stomach 
54.  Section  of  the  diaphragm. — {Sappey.) 


58o 


TEXT-BOOK  OF  PHYSIOLOGY. 


Fig.  259. — Lumbar  and  Sacral  Portions  of  the  Sympathetic. — i.  Section  of  the 
diaphragm.  2.  Lower  end  of  the  esophagus.  3.  Left  half  of  the  stomach.  4. 
Small  intestine.  5.  Sigmoid  flexure  of  the  colon.  6.  Rectum.  7.  Bladder.  8. 
Prostate.  9.  Lower  end  of  the  left  pneumogastric.  10.  Lower  end  of  the  right 
pneumogastric.  11.  Solar  plexus.  12.  Lower  end  of  the  great  splanchnic  nerve. 
13.  Lower  end  of  the  lesser  splanchnic  nerve.  14,  14.  Last  two  thoracic  ganglia. 
15,  15.  The  four  lumbar  gangha.  16,  16,  17,  17.  Branches  from  the  lumbar 
ganglia.  18.  Superior  mesenteric  plexus.  19,  21,  22,  23.  Aortic  lumbar  plexus. 
20.  Inferior  mesenteric  plexus.  24,  24.  Sacral  portion  of  the  sympathetic.  25 
25,  26,  26,  27,  27.  Hypogastric  plexus.  28,  29,  30.  Tenth,  eleventh,  and  twelfth 
dorsal  nerves.  31, 32,  ^2>  34>  35'  3^>  37'  3^'  39-  Lumbar  and  sacral  nerves. — (Sappey.) 


THE  SYMPATHETIC  NERVE  SYSTEM.  581 

the  cells  of  the  sympathetic  ganglia,  vertebral,  pre-vertebral,  and 
peripheral,  are  distributed  ultimately  and  directly  to  but  two  struc- 
tures: viz.,  non-striated  muscle  and  secretor  epithelium.  Moreover, 
there  is  no  evidence  to  warrant  the  assumption  that  these  structures 
ever  receive  nerve  impulses  directly  from  the  spinal  or  cranial  nerves. 
All  nerve  impulses  which  influence  their  activities,  either  in  the  way 
of  augmentation  or  inhibition,  emanate  directly  though  not  origin- 
ally from  the  sympathetic  ganghon  cells.  Since  non-striated  mus- 
cles are  found  in  the  walls  of  blood-vessels,  in  the  walls  of  hollow 
viscera,  and  around  hair-follicles,  and  since  secretor  epithelium  is 
found  in  all  glands,  there  is  every  reason  to  believe  that  the  ganglia 
in  some  way  are  associated  with  vaso-motor  and  vaso-inhibitor, 
viscero-motor  and  viscero-inhibitor,  pilo-motor  and  secretor  phe- 
nomena. 

The  Anatomic  Relations  of  the  Sympathetic  and  Cerebro- 
spinal Systems. — The  sympathetic  gangha  are  connected  with  the 
spinal  nerves  by  two  branches,  one  white,  the  other  gray  in  color, 
and  known  respectively  as  the  white  and  gray  rami  communicantes. 
These  two  rami  differ  somewhat  in  their  topographic  distribution. 
The  white  rami  are  found  passing  only  from  those  spinal  nerves 
included  between  the  first  thoracic  and  second  or  third  lumbar  and 
their  corresponding  gangha.  The  gray  rami,  on  the  contrary,  are 
found  passing  from  the  ganglia  to  each  of  the  spinal  nerves.  In  the 
cervical  region,  where  the  gangha  do  not  correspond  in  number  with 
the  cervical  nerves,  each  ganglion  gives  off  two  or  more  gray  rami. 
In  man  the  superior  cervical  ganghon  sends  gray  rami  to  the  first 
four  cervical  nerves;  the  middle  and  inferior  gangha  apparently 
send  gray  rami  to  the  fifth  and  sixth,  the  seventh  and  eighth  nerves 
respectively. 

The  white  rami  are  composed  of  fine  meduUated  nerve-fibers 
which  arise  from  nerve- cells  situated  in  the  lateral  portion  of  the 
gray  matter  in  the  thoracic  and  lumbar  regions  of  the  spinal  cord. 
From  this  origin  they  pass  forward  into  the  ventral  roots  of  the 
spinal  nerves,  in  which  they  are  contained  until  the  spinal  nerve 
formed  by  the  union  of  the  ventral  and  dorsal  roots  divides  into  its 
anterior  and  posterior  divisions.  At  this  point  the  fine  meduUated 
nerve-fibers  leave  the  common  trunk  and  pass  forward  into  the  cor- 
responding vertebral  ganglion,  around  the  cell-bodies  of  which  some 
of  the  fibers  at  once  arborize.  Other  fibers,  however,  pass  through 
this  ganglion  and  ascend  or  descend  the  cord  for  a  variable  distance, 
and  arborize  around  the  cells  of  more  or  less  distant  ganglia;  others 
again  pass  forward  into  the  pre-vertebral  and  even  the  peripheral 
gangha  before  they  finally  terminate.  The  nerve-cells  in  the  spinal 
cord  are  thus  brought  into  relation  with  the  ganglia  of  all  three 
chains,  though   for  each   cell  there  is  but  one  ganglion  terminal, 


582  TEXT-BOOK  OF  PHYSIOLOGY. 

one  cell  station,  between  the  spinal  cord  and  the  tissues.  Though 
innervated  by  the  spinal  cord,  these  structures  receive  their  nerve 
impulses,  as  previously  stated,  not  directly  but  indirectly  through 
the  ganglion  cells.  The  meduUated  nerve-fibers  coming  from  the 
spinal  cord  are  known  as  pre-ganglionic  fibers;  the  non-medullated 
fibers,  passing  from  the  ganglia,  as  post- ganglionic  fibers. 

The  gray  rami  are  composed  of  non-medullated  nerve-fibers, 
axons  of  the  cells  in  the  vertebral  or  lateral  gangha.  After  their 
emergence  from  the  ganglia  they  take  a  backward  direction  and 
enter  the  spinal  nerve-trunks,  in  company  with  which  they  pass  to 
the  periphery,  to  be  finally  distributed  to  structures  in  the  skin:  viz., 
non-striated  muscles  of  blood-vessels,  non-striated  muscles  of  the 
hair-folhcles  and  epithehum  of  glands.  They  may  therefore  be 
regarded  as  having  vaso-motor,  pilo-motor,  and  secretor  functions. 

Afferent  Sympathetic  Fibers. — ^With  the  foregoing  groups  of 
efferent  fibers,  the  sympathetic  nerves,  in  the  thoracic  and  lumbar 
regions  more  especially,  contain  a  number  of  afferent  fibers  which 
when  stimulated  give  rise  to  sensations  of  pain  or  to  reflex  phe- 
nomena. The  routes  by  which  these  afferent  fibers  reach  the  spinal 
cord  lead,  on  the  one  hand,  into  and  through  the  gray  rami  to  the 
ganglia  on  the  posterior  roots,  where  they  have  their  cells  of  origin; 
and,  on  the  other  hand,  into  and  through  the  white  rami.  The 
number  of  afferent  fibers  in  any  trunk  in  comparison  with  the  effer- 
ent is  quite  small. 

FUNCTIONS  OF  THE  SYMPATHETIC  SYSTEM. 

The  view  according  to  which  the  sympathetic  system  is  to  be 
regarded  as  an  independent  apparatus  endowed  with  functions  of  its 
own  and  in  nowise  directly  dependent  for  its  activities  on  the  spinal 
cord,  is  at  the  present  time  discarded.  Peripheral  structures  cease 
to  exhibit  their  characteristic  functions  after  division  of  the  spinal 
nerves  in  connection  with  their  related  gangha.  This  does  not 
exclude  the  possibihty  of  the  sympathetic  cell-body,  in  virtue  of  the 
interchanges  between  it  and  the  blood  and  lymph  by  which  it  is 
surrounded,  maintaining  its  own  nutrition  and  exerting  a  favor- 
able influence  over  the  nutrition  of  the  peripheral  tissues  to  which 
its  efferent  branches  are  distributed. 

The  nerve-tissue  in  its  entirety  may  be  regarded  as  a  single 
system  which  may  be  functionally  divided  into  a  nerve  system  of 
animal  and  a  nerve  system  of  vegetative  life,  according  as  the  nerve 
energies  originating  in  and  emanating  from  the  central  nervous 
system  are  transmitted  directly  to  the  skeletal  muscles  or  indirectly, 
through  the  intervention  of  a  sympathetic  neuron,  to  visceral  muscles 
and  glands.     In  the  former  system  but  one  neuron,  the  spino-periph- 


THE  SYMPATHETIC  NERVE  SYSTEM.  583 

eric,  connects  the  spinal  cord  proper  with  the  muscle;  in  the  latter 
system  there  are  two,  the  spino-ganglionic  and  the  ganglio-peripheric. 

From  the  distribution  of  the  post-ganghonic  libers  it  may  be  in- 
ferred that  the  activities  of  the  vascular  and  visceral  muscles,  either 
in  the  way  of  augmentation  or  inhibition,  the  activities  of  the  muscles 
of  the  hair-foUicles,  and  of  the  epithehum  of  glands,  are  called  forth 
by  the  ganglia  in  consequence  of  the  arrival  of  nerve  impulses  coming 
from  the  spinal  cord  through  the  pre-ganghonic  fibers.  Experimental 
observations  show  this  to  be  true.  The  extent  to  which  these  different 
modes  of  activity  manifest  themselves  in  one  or  more  regions  of  the 
body  will  depend  to  some  extent  on  the  portion  of  the  sympathetic 
system  subjected  to  experimental  procedures. 

The  Functions  of  the  Cervical  Portion. — If  the  sympathetic 
cord  central  to  the  superior  cervical  ganglion  be  stimulated  with  the 
induced  electric  current,  among  the  resulting  phenomena  there  Avill  be 
observed  dilatation  of  the  pupil,  retraction  of  the  nictitating  mem- 
brane in  animals  possessing  it,  contraction  of  the  blood-vessels  of 
the  skin  and  mucous  membrane  in  different  parts  of  the  head  and 
face,  contraction  of  the  blood-vessels  of  the  sahvary  glands,  increase 
of  secretion  from  the  submaxillary  gland,  the  perspiratory  and 
mucous  glands,  erection  of  hairs  in  different  locahties  of  the  head  and 
neck,  and  in  the  dog  dilatation  of  the  blood-vessels  of  the  lips,  gums, 
and  hard  palate.  If  the  cervical  cord  be  divided,  opposite  effects 
will  be  observed:  viz.,  contraction  of  the  pupil,  dilatation  and  passive 
congestion  of  the  blood-vessels,  a  rise  in  temperature,  and  a  loss  of 
the  power  of  erecting  hairs.  Stimulation  of  the  peripheral  end  causes 
a  disappearance  of  the  latter  and  a  reappearance  of  the  former 
phenomena.  These  facts  indicate  that  the  cervical  portion  is  efferent 
in  function.  The  fibers  composing  it  are  medullated  nerve-fibers 
derived  from  the  thoracic  or  dorsal  nerves  from  the  first  to  the  fourth. 
From  the  several  sources  the  fibers  pass  via  the  white  rami  into  the 
vertebral  chain,  and  thence  without  interruption  to  the  superior 
cervical  ganglion,  in  and  around  the  cells  of  which  their  end-tufts 
arborize  in  their  characteristic  manner. 

That  the  superior  cervical  ganghon  is  the  cell  station  between  the 
spinal  cord  and  the  peripheral  organs  is  shown  by  the  fact  discovered 
and  applied  by  Langley  that  the  intravenous  injection  of  nicotin  or 
the  local  appHcation  of  it  to  the  ganghon  itself,  impairs  the  conductivity 
of  the  terminals  of  pre-ganghonic  fibers,  after  which  their  stimulation 
has  no  effect  on  the  ganglion  cells,  though  the  latter  retain  their 
activity,  as  shown  on  direct  stimulation.  Of  the  nerve-centers  in  the 
spinal  cord  which  through  pre-ganglionic  fibers  influence  peripheral 
structures,  some  appear  to  be  in  a  state  of  constant  activity:  e.  g., 
the  vaso-constrictor  centers  and  the  pupillo-dilatator  centers.  In  how 
far  this  action  is  automatic  or  autochthonic,  or  reflex,  is  uncertain. 


584  TEXT-BOOK  OF  PHYSIOLOGY. 

The  Functions  of  the  Thoracic  Portion. — The  phenomena 
which  follow  stimulation  of  this  portion  of  the  sympathetic  system 
resemble  in  a  general  way  those  observed  in  the  head  when  the  cervical 
portion  is  stimulated.  The  situation  of  the  resulting  phenomena 
will  vary  in  accordance  with  the  part  the  subject  of  the  experiment. 
For  an  understanding  of  the  results  of  experiment  the  origin  and 
distribution  of  the  following  nerve-branches  must  be  kept  in  view : 

(a)  The  cardiac  nerves  which  have  their  origin  in  the  first  thoracic 
ganglion.  From  this  point  they  pass  by  way  of  the  annulus  of 
Vieussens  to  the  inferior  cervical  ganglion  (from  which  they 
probably  receive  additional  fibers)  and  thence  to  the  heart. 
Stimulation  of  these  nerves  gives  rise  to  an  increased  frequency 
and  an  augmentation  in  the  force  of  the  heart-beat.  The  pre- 
ganglionic fibers  by  which  these  cells  are  excited  to  activity 
emerge  from  the  cord  by  the  first  and  second  thoracic  nerves. 

(b)  The  splanchnic  nerves  the  roots  of  which  emerge  from  the  fourth 
to  the  tenth  or  eleventh  thoracic  ganglia.  The  fibers  composing 
these  nerves  are  for  the  most  part  pre-ganglionic  and  derived 
from  the  corresponding  spinal  nerves.  The  cell  stations  of  the 
splanchnic  fibers  are  in  the  semilunar,  superior  mesenteric,  and 
renal  ganglia.  From  these  ganglia  non-medullated  post-gang- 
lionic  fibers  pass  peripherally  to  the  walls  of  the  intestines,  the 
blood-vessels  of  the  intestines,  liver,  kidneys,  spleen,  etc. 
Stimulation  of  the  great  splanchnic  produces  inhibition  of  the 
intestinal  movements,  a  marked  primary  contraction  of  the 
intestinal  blood-vessels  and  other  viscera,  followed  by  dilatation, 
coincidently  with  which  there  is  a  primary  rise  succeeded  by  a 
fall  of  blood-pressure  throughout  the  body.  Division  of  the 
nerve  is  followed  by  dilatation  of  the  intestinal  vessels  and  a  fall 
of  blood-pressure.  Stimulation  of  the  central  end  of  the  divided 
nerve  excites  the  activity  of  the  general  vaso-motor  center,  as 
shown  by  the  rise  of  the  general  blood-pressure.  Stimulation  of 
the  smaller  splanchnics  gives  rise  to  a  slight  primary  contraction 
of  the  blood-vessels,  soon  followed  by  a  marked  dilatation.  These 
facts  indicate  that  the  splanchnic  nerves  contain  visceral  nerves 
which  inhibit  intestinal  movements,  vaso-motor  fibers  both  aug- 
mentor  and  inhibitor.  The  presence  of  secretory  nerves  for 
the  intestinal  glands  is  disputed. 

(c)  The  cutaneous  nerves  for  the  trunk  leave  the  lateral  ganglia  by 

the  gray  rami,  enter  the  thoracic  spinal  nerves,  and  pass  in  com- 
pany with  them  to  their  terminations,  to  be  ultimately  distrib- 
uted to  the  walls  of  the  blood-vessels,  the  arrectores  pilorum 
muscles,  and  the  sweat-glands.  The  pre-ganglionic  fibers  come 
from  the  spinal  nerves  by  the  white  rami.  Their  functions  are 
vaso-motor,  pilo-motor,  and  secretor.     The   cutaneous   nerves 


THE  SYMPATHETIC  NERVE  SYSTEM.  585 

for  the  fore-limbs  have  their  origin  from  cells  in  the  stellate  gang- 
hon  (first  dorsal).  After  a  short  upward  course  they  enter 
the  trunks  of  the  nerves  composing  the  brachial  plexus.  The 
pre-ganglionic  fibers  come  from  the  white  rami  of  the  fourth  to  the 
ninth  thoracic  nerves.  After  entering  the  lateral  chain  they  take 
an  upward  direction  and  arborize  around  the  cells  of  the  stellate 
ganglion.  The  cutaneous  nerves  for  the  hind-limbs  are  derived 
from  the  lower  lumbar  and  the  upper  sacral  ganglia.  They  also 
enter  the  spinal  nerves  by  the  gray  rami  and  pass  to  the  blood- 
vessels and  glands  of  the  skin.  The  pre-ganglionic  fibers  come 
from  the  twelfth  thoracic  to  the  third  lumbar  nerves.  In  both 
the  brachial  and  sciatic  nerves  vaso-motor  fibers  (constrictors 
and  dilatators)  and  secretor  nerves  are  present,  as  shown  by 
experimental  methods  (see  page  345)- 

The  Functions  of  the  Lumbo-sacral  Portion. — From  the 
ganglia  of  the  lumbar  and  sacral  regions  gray  rami  enter  the  lumbar 
and  sacral  nerves  and  accompany  them  to  their  distribution.  In 
the  lumbar  region  the  vertebral  chain  contains  a  number  of  pre- 
ganglionic fibers  which  have  descended  from  the  thoracic  region  as 
well  as  fibers  which  have  come  into  the  chain  by  the  white  rami  from 
the  lumbar  nerves  themselves.  Many  of  these  fibers  pass  to  the 
inferior  mesenteric  ganglion,  in  which  they  find  their  cell  station. 
Fibers  from  the  sacral  cord  pass  into  the  hypogastric  plexus.  The 
course  and  distribution  of  the  individual  nerves  is  complicated  and 
involved.  In  a  general  way  it  may  be  said  that  these  two  regions  of 
the  lateral  chain  send  viscero-motor  and  viscero-inhibitor,  vaso-con- 
strictor  and  dilator  nerves  to  the  pelvic  viscera  and  to  the  external 
organs  of  generation.  Their  function  therefore  is  to  regulate  the 
activities  of  the  viscera  as  well  as  the  blood-supply  in  accordance 
with  functional  needs. 

The  Functions  of  the  Cephalic  Ganglia. — The  ganglia  situated 
in  the  head  are  usually  described  in  connection  with  and  as  con- 
stituent parts  of  the  cranial  nerve  system.  They,  however,  bear  the 
same  relation  to  the  cranial  nerves  that  the  ganglia  of  the  trunk 
bear  to  the  spinal  nerves.  They  consist  of  ganglion  cells  from  which 
post-ganglionic  fibers  pass  to  glands,  blood-vessels,  and  non-striated 
muscles,  and  to  which  pre-ganglionic  fibers  pass  from  the  cranial 
nerves.  Motor  and  sensor  nerves  pass  through  one  or  more  ganglia, 
though  they  have  no  anatomic  connection  with  them.  In  their 
structure,  distribution,  and  functions  they  closely  resemble  the  col- 
lateral ganglia  of  the  abdominal  sympathetic : 

I.  The  ciliary  or  ophthalmic  ganglion  is  situated  in  the  orbital  cavity 
posterior  to  the  eyeball.  It  is  small  in  size,  gray  in  color,  and 
consists  of  a  connective-tissue  stroma  containing  nerve-cells. 
From  these  cells  post-ganglionic  fibers  emerge  which,  after  a 


586  TEXT-BOOK  OF  PHYSIOLOGY. 

short  course  forward,  penetrate  the  eyeball  and  terminate  in  the 
circular  fibers  of  the  iris  and  the  ciliary  muscle.  Pre-ganglionic 
fibers  of  small  size,  and  similar  in  their  anatomic  features  to 
the  fibers  of  the  white  rami  of  the  spinal  nerves,  leave  the  motor 
oculi  by  a  short  root  from  the  inferior  division  and  arborize 
around  the  ganglionic  cells.  Stimulation  of  the  pre-ganglionic 
fibers  gives  rise  to  contraction  of  the  circular  fibers  of  the 
iris,  with  a  diminution  in  the  size  of  the  pupil,  and  contraction 
of  the  ciliary  muscle  with  accommodation  of  the  eye  for  near 
vision.  Division  of  these  fibers  is  followed  by  the  opposite 
results.  Post-ganglionic  fibers  from  the  superior  cervical  gang- 
lion which  come  through  the  cavernous  plexus  pass  through  the 
ciliary  ganglion  to  the  blood-vessels  of  the  iris  and  retina  which 
are  vaso-constrictor  in  function.  Sensor  fibers  from  the  per- 
ipheral division  of  the  fifth  nerve  pass  to  the  cornea  and  endow  it 
with  sensibiHty. 

2.  The  spheno- palatine  ganglion  is  situated  in  the  spheno-maxillary 

fossa.  Its  nerve-cells  send  non-medullated  post-ganglionic  fibers 
to  the  blood-vessels  and  glands  of  the  mucous  membrane  of  the 
nasal  and  oral  regions.  Stimulation  of  the  ganglion  gives  rise 
to  dilatation  of  the  blood-vessels  and  increase  of  secretion  in  this 
entire  region.  The  pre-ganglionic  fibers  are  derived  from  the 
seventh  or  facial  nerve  by  way  of  the  great  petrosal.  Sensor 
fibers  from  the  superior  maxillary  division  of  the  fifth  nerve  pass 
through  the  ganglion  to  the  same  regions. 

3.  The  otic  ganglion  is  situated  just  below  the  foramen  ovale  and 

internal  to  the  third  division  of  the  fifth  nerve.  The  post-gang- 
lionic fibers  pass  to  the  parotid  gland  by  w^ay  of  the  auriculo- 
temporal division  of  the  fifth  nerve,  and  to  the  blood-vessels  of  the 
lower  lip,  cheek,  and  gums.  The  pre-ganglionic  fibers  are  de- 
rived from  the  efferent  fibers  in  the  glosso-pharyngeal  or  ninth 
nerve,  by  way  of  Jacobsen's  nerve  and  the  small  petrosal.  Stimu- 
lation of  these  nerves  in  any  part  of  their  course  gives  rise  to 
vascular  dilatation  and  increase  of  secretion  in  the  region  of 
their  distribution.  Motor  fibers  from  the  small  or  motor  root 
of  the  fifth  nerve  pass  through  this  ganglion  to  the  tensor  tym- 
pani  muscle. 

4.  The  submaxillary  and  sublingual  ganglia  are  situated  close  to  the 

corresponding  glands.  Their  post-ganglionic  fibers  pass  to  the 
blood-vessels  and  gland-cells.  The  pre-ganglionic  fibers  are 
derived  from  the  seventh  or  facial  nerve  through  the  chorda 
tympani  branch.  Stimulation  of  the  chorda  or  of  the  ganglia 
themselves  gives  rise  to  marked  dilatation  of  the  blood-vessels 
and  an  increased  flow  of  saliva.  It  therefore  contains  vaso- 
dilatator and  secretor  fibers  for  these  glands.     Vaso-constrictor 


THE  SYMPATHETIC  NERVE  SYSTEM.  587 

and  a  few  secretor  nerves,  it  will   be    recalled,  come  to  these 

glands  from  the  superior  cervical  ganglion. 
Peripheral  Ganglia. — Among  the  peripheral  ganglia  may  be 
mentioned  those  in  the  heart  and  those  in  the  intestinal  walls.  The 
pre-ganglionic  fibers  are  contained  in  the  trunk  of  the  vagus  nerve. 
Stimulation  of  the  peripheral  end  of  the  divided  vagus  gives  rise  to 
inhibition  of  the  heart,  contraction  of  the  walls  of  the  stomach  and 
intestines,  secretion  from  the  gastric  and  perhaps  the  pancreatic 
gland. 


CHAPTER  XXIII. 
PHONATION;  ARTICULATE  SPEECH. 

Phonation,  the  emission  of  vocal  sounds,  is  accomplished  by  the 
vibration  of  two  elastic  membranes  which  cross  the  lumen  of  the 
larynx  antero-posteriorly  and  which  are  thrown  into  vibration  by  a 
blast  of  air  from  the  lungs. 

Articulate  speech  is  a  modification  of  the  voice  produced  by  the 
teeth  and  the  muscles  of  the  lips^and  tongue  and  employed  for  the 
expression  of  ideas. 

The  larynx,  the  organ  of  the  voice,  is  situated  in  the  forepart  of 
the  neck,  occupying  the  space  between  the  hyoid  bone  and  the  upper 
extremity  of  the  trachea.  In  this  situation  it  communicates  with  the 
cavity  of  the  pharynx  above  and  the  cavity  of  the  trachea  below. 
From  its  anatomic  relations  and  its  internal  structure — the  interpola- 
tion of  the  elastic  membranes — the  larynx  subserves  the  two  widely 
different  yet  related  functions,  respiration  and  phonation. 

THE  ANATOMY  OF  THE  LARYNX. 

The  larynx  consists  primarily  of  a  series  of  cartilages  united  one 
with  another  in  such  a  manner  as  to  form  a  more  or  less  rigid  frame- 
work, yet  possessing  at  its  different  joints,  a  certain  amount  of 
motion;  and,  secondarily,  of  muscles  and  nerves  which  conjointly 
impart  to  the  cartilages  the  degree  of  movement  necessary  to  the 
performance  of  the  laryngeal  functions.  It  is  covered  externally 
by  fibrous  tissue  and  lined  throughout  by  mucous  membrane  con- 
tinuous with  that  lining  the  pharynx  and  trachea. 

The  larynx  presents  a  superior  or  pharyngeal  and  an  inferior  or 
tracheal  opening.  The  pharyngeal  opening  is  triangular  in  shape, 
the  base  being  directed  forward,  the  apex  backward.  The  plane  of 
this  opening  in  the  living  subject  is  almost  vertical.  The  tracheal 
opening  is  circular  in  shape  and  corresponds  in  size  with  the  upper 
ring  of  the  trachea.  Viewed  from  above,  the  general  cavity  of  the 
larynx  is  seen  to  be  partially  subdivided  by  two  membranous  bands — 
the  vocal  bands  or  cords — which  run  from  before  backward  in  a  hori- 
zontal plane.  The  space  between  the  bands,  the  glottis,  varies  in 
size  and  shape  from  moment  to  moment  in  accordance  with  respira- 
tory and  phonatory  necessities.     The  average  width  of  the  glottis,  at 

588 


PHONATION;    ARTICULATE  SPEECH. 


589 


its  widest  part,  during  quiet  respiration  is  about  13.5  mm.  in  men 
and  1 1.5  mm.  in  women  (Semon).  With  the  advent  of  phonation 
the  vocal  membranes  are  at  once 
approximated,  and  to  such  an 
extent  that  the  glottic  opening  is 
reduced  to  a  mere  sht.  It  is  then 
spoken  of  as  the  rima  glottidis,  or 
chink  of  the  glottis. 

The  space  above  the  vocal  bands, 
the  supra-glottic  or  supra-rimal 
space,  is  triangular  in  shape  and 
extends  from  the  pharyngeal  open- 
ing to  the  plane  of  the  vocal  bands. 
The  mucous  membrane  lining  the 
walls  of  this  space,  presents  on  either 
side,  just  above  the  vocal  bands,  a 
crescentic  fold  which  runs  from 
before  backward,  and  is  known  as 
the  false  vocal  band  or  cord.  Be- 
tween the  true  and  false  bands 
there  is  a  cavity  or  space  prolonged 
upward  and  outward  for  some 
distance,  forming  What  is  known  as 
the  ventricle  of  the  larynx.  The 
space  below  the  vocal  bands,  the 
infra-glottic  or  infra-rimal  space,  is 
narrow  above  and  elongated  from 
before  backward,  but  wide  and 
circular  below,  corresponding  to  the 
lumen  of  the  trachea. 

The  Laryngeal  Cartilages, 
Articulations,  and  Ligaments. — 
The  cartilages  which  compose  the 
framework  of  the  larynx  are  nine 
in  number,  three  of  which  are 
single:  viz.,  the  cricoid,  the  thyroid, 
and  the  epiglottis,  while  six  occur 
in  pairs:  viz.,  the  arytenoids,  the 
cornicula  laryngis,  and  the  cunei- 
form. 

The  cricoid  cartilage  is  the 
foundation  cartilage,  and  affords 
support  to  the  remaining  cartilages 

and  the  structures  attached  to  them.     In  shape  it  resembles  a  signet- 
ring,  the   broad  quadrate  portion  of   which  is  directed  backward, 


Fig 


260. — Longitudinal  Section 
or  THE  Human  Larynx,  Show- 
ing THE  Vocal  Bands.  i. 
Ventricle  of  the  larynx.  2.  Supe- 
rior vocal  cord.  3.  Inferior  vocal 
cord.  4.  Arytenoid  cartilage.  5. 
Section  of  the  an,'tenoid  muscle. 
6,  6.  Inferior  portion  of  the  cavity 
of  the  larynx.  7.  Section  of  the 
posterior  portion  of  the  cricoid 
cartilage.  8.  Section  of  the  an- 
terior portion  of  the  cricoid  car- 
tilage. 9.  Superior  border  of  the 
cricoid  cartilage.  10.  Section  of 
the  thyroid  cartilage.  11,  11. 
Superior  portion  of  the  cavity  of 
the  larynx.  12,  13.  Arytenoid 
gland.  14,  16.  Epiglottis.  15,17. 
Adipose  tissue.  18.  Section  of 
the  hyoid  bone.  19,  19,  20. 
Trachea. — {Sappey) 


59° 


TEXT-BOOK  OF  PHYSIOLOGY. 


while  the  narrow  circular  portion  is  directed  forward.  It  rests  upon 
the  upper  ring  of  the  trachea,  to  which  it  is  firmly  attached  by 
fibrous  tissue.  The  posterior  upper  border  of  the  cj^uadrate  portion 
presents  on  either  side  an  oval  convex  facet  for  articulation  with  the 
arytenoid  cartilage.  The  long  axis  of  this  facet  is  directed  down- 
ward, outward,  and  forward. 


Fig .  2 6 1 . — -Laryngeal  Cartilages  and 
Ligaments,  Anterior  Surface. 
I.  Hyoid  bone.  2,  2,  3,  3.  Greater 
and  lesser  cornua.  4.  Thyroid- 
cartilage.  5.  Thyro-hyoid  mem- 
brane. 6.  Thyro-hyoid  hgaments. 
7.  Cartilaginous  nodule.  8.  Cri- 
coid cartilage.  9.  The  crico-thyroid 
membrane.  10.  The  crico-thyroid 
ligaments.  11.  Trachea. — {Sap- 
pey.) 


Fig.  262. — Laryngeal  Cartilages 
and  Ligaments.  Posterior  Sur- 
face. I,  I.  Thyroid  cartilage.  2. 
Cricoid  cartilage.  3,  3.  Arytenoid 
cartilages.  4,  4.  Crico-arytenoid 
articulations.  5,  5.  Crico-thyroid 
articulations.  6.  Union  of  the 
cricoid  cartilage  and  of  the  trachea. 
7.  Epiglottis.  8.  Ligament  uniting 
it  to  the  reentering  angle  of  the 
thyroid  cartilage. — (Sappey.) 


The  thyroid,  the  largest  of  the  laryngeal  cartilages,  is  composed 
of  two  fiat  quadrilateral  plates,  united  anteriorly,  at  an  angle  of 
about  90  degrees.  Each  plate  is  directed  backward  and  outward 
and  terminates  in  a  free  border,  which  is  prolonged  upward  and 
downward  for  some  distance,  terminating  in  two  processes,  the 
superior  and  inferior  cornua.  The  upper  border  of  the  thyroid  is 
deeply  notched  in  front.  The  inferior  border  overlaps  laterally 
the  cricoid. 


PRONATION;   ARTICULATE  SPEECH.  591 

The  epiglottis  is  a  leaf-shaped  piece  of  cartilage  attached  to  the 
thyroid  at  the  median  notch.  It  is  firmly  united  by  membranes  and 
ligaments  to  the  thyroid  and  arytenoid  cartilages  and  to  the  base  of 
the  tongue. 

The  arytenoid  cartilages  are  two  in  number  and  symmetric  in 
shape.  Each  cartilage  is  a  triangular  pyramid,  the  apex  of  which  is  re- 
curved, and  directed  backward  and  inward.  The  base  presents  three 
angles — an  anterior,  an  external,  and  an  internal.  The  anterior  angle 
is  long  and  pointed  and  projects  forward  in  a  horizontal  plane.  It 
serves  for  the  attachment  of  the  vocal  membranes  and  is  therefore 
termed  the  vocal  process.  The  external  angle  is  short,  rounded,  and 
prominent,  and  serves  for  the  attachment  of  muscles.  The  internal 
angle  affords  a  point  of  insertion  for  a  ligament.  The  inferior  surface 
of  the  arytenoid  is  concave  for  articulation  with  the  convex  surface 
of  the  cricoid  facet.  Its  long  axis,  however,  is  directed  from  before 
backward  and  almost  at  right  angles  to  the  long  axis  of  the  cricoid 
facet. 

The  cornicula  laryngis  and  the  cuneiform  cartilages  are  small 

■  nodules  of  yellow  elastic  cartilage  embedded  in  a  fold  of  membrane 

which  unites  the  arytenoid  and  the  epiglottis.     They  are  fragments 

of  a  ring  of  cartilage  which  in  some  animals — e.  g.,  anteater — extends 

between  these  two  cartilages. 

The  crico-thyroid  articulation  is  formed  by  the  apposition  of  the 
tip  of  the  inferior  cornu  of  the  thyroid  cartilage  and  an  articular 
facet  on  the  side  of  the  cricoid.  The  joint  is  provided  with  a  synovial 
membrane  and  enclosed  by  a  capsular  hgament.  The  movements 
permitted  at  this  joint  take  place  around  a  horizontal  axis  and  consist 
of  an  upward  and  downward  movement  of  both  the  thyroid  and 
cricoid,  combined  with  a  sliding  movement  of  the  latter  upward  and 
backward. 

The  crico-arytefioid  articulation  is  formed  by  the  apposition  of 
the  articulating  surfaces  of  the  cricoid  and  arytenoid  cartilages. 
This  joint  is  provided  with  a  synovial  membrane  and  enclosed  by 
a  loose  capsular  ligament  which  would  permit  of  an  extensive  sliding 
of  the  arytenoid  cartilage  downward  and  outward  were  it  not  pre- 
vented by  the  posterior  crico-arytenoid  ligament,  which  is  attached, 
on  the  one  hand,  to  the  cricoid,  and,  on  the  other,  to  the  inner  angle 
of  the  arytenoid.  The  movements  permitted  at  this  joint  are:  (i) 
Rotation  of  the  arytenoid  around  a  vertical  axis  which  lies  close  to 
its  inner  surface.  (2)  A  sliding  motion  inward  and  forward  with 
inward  rotation  of  the  vocal  process,  or  a  sliding  motion  outward  and 
backward  with  outward  rotation  of  the  vocal  process.  In  either  case 
the  process  describes  an  arc  of  a  circle.  (3)  A  sliding  movement 
towards  the  median  line  in  consequence  of  which  the  inner  surfaces 
of  the  arytenoids  are  brought  almost  in  contact. 


592  TEXT-BOOK  OF  PHYSIOLOGY. 

The  crico-thyroid  membrane  is  composed  mainly  of  elastic  tissue. 
It  may  be  divided  into  a  mesial  and  two  lateral  portions.  The 
mesial  portion  is  well  developed,  triangular  in  shape,  and  unites  the 
contiguous  borders  of  the  cricoid  and  thyroid  cartilages.  The  lateral 
portion  is  attached  below  to  the  superior  border  of  the  cricoid.  From 
this  attachment  it  passes  upward  and  inward  under  cover  of  the 
thyroid.  As  it  ascends  it  elongates  and  becomes  thinner,  and  is 
finally  attached  anteriorly  to  the  thyroid  near  the  median  line,  and 
posteriorly  to  the  vocal  process  of  the  arj^tenoid,  thus  constituting 
the  inferior  thyro- arytenoid  ligament.  It  is  covered  internally  by 
mucous  membrane  and  externally  by  the  internal  thyro-arytenoid 
muscle.  The  free  edge  of  this  ligament  forms  the  basis  of  the  true 
vocal  band.  A  superior  thyro-arytenoid  ligament  forms  the  basis 
of  the  false  vocal  band. 

The  thyro-hyoid  membrane,  composed  of  elastic  tissue,  unites 
the  superior  border  of  the  thyroid  to  the  hyoid  bone. 

The  mucous  membrane  lining  the  larynx  is  thin  and  pale.  As 
it  passes  downward  it  is  reflected  over  the  superior  thyro-arytenoid 
ligament,  and  assists  in  the  formation  of  the  false  vocal  band;  it 
then  passes  into  and  lines  the  ventricle,  after  which  it  is  reflected 
inward  over  the  superior  border  of  the  thyro-arytenoid  muscle  and 
hgament,  and  assists  in  the  formation  of  the  true  vocal  band  ;  it 
then  returns  upon  itself  and  passes  downward  over  the  lateral 
portion  of  the  crico-thyroid  membrane  into  the  trachea. 

The  thin,  free,  reduplicated  edge  of  the  mucous  membrane  con- 
stitutes the  true  vocal  band.  The  surface  of  the  mucous  membrane 
is  covered  by  cihated  epithehum  except  in  the  immediate  neighbor- 
hood of  the  vocal  bands. 

The  vocal  bands  are  attached  anteriorly  to  the  thyroid  cartilage 
near  the  receding  angle  and  posteriorly  to  the  vocal  processes  of  the 
arytenoid  cartilages.  They  vary  in  length  in  the  male  from  20  to  25 
mm.  and  in  the  female  from  15  to  20  mm. 

The  Muscles  of  the  Larynx. — The  muscles  which  have  a  direct 
action  on  the  cartilages  of  the  larynx  and  determine  the  position  of  the 
vocal  bands  both  for  respiratory  and  phonatory  purposes,  and  which 
regulate  their  tension  as  well,  are  nine  in  number  and  take  their 
names  from  their  points  of  origin  and  insertion:  viz.,  two  posterior 
crico-arytenoids,  two  lateral  crico-arytenoids,  two  thyro-arytenoids, 
one  arytenoid,  and  two  crico-thyroids  (Figs.  263  and  264). 

The  posterior  crico-arytenoid  muscle  lies  on  the  posterior  surface 
of  the  quadrate  plate  of  the  cricoid  cartilage,  on  either  side  of  the 
median  line,  from  which  it  takes  its  origin.  The  fibers  of  the 
muscle  pass  upward  and  outward  and  in  their  course  converge  to 
be  inserted  into  the  external  angle  of  the  arytenoid  cartilage.  The 
superior  and  more  horizontally  directed  fibers  rotate  the  arytenoid 


PRONATION;    ARTICULATE  SPEECH. 


593 


around  its  vertical  axis;  the  inferior  and  obliquely  directed  fibers 
draw  the  cartilage  downward  and  inward.  As  a  result  of  the  action 
of  the  muscle  in  its  entirety,  the  vocal  process  is  turned  upward  and 
outward,  and  as  the  vocal  band  is  carried  with  it  the  glottis  is  widened, 
a  condition  necessarv  to  the  free  entrance  of  air  into  the  lungs  (Fig. 


Fig.  263. — Posterior  View  of  the 
Muscles  of  the  Laryn:?^.  i. 
Posterior  crico-arytenoid  muscle. 
2,  3,  4.  Different  fasciculi  of  the 
arytenoid  muscle.  5.  Aryteno- 
epiglottidean  muscle. — (Sappey.) 


265).  Since  the  contraction  of  the 
crico-arytenoid  has  this  result,  it  is 
generally  spoken  of  as  the  abductor  or 
respiratory  muscle. 

The  lateral  crico-arytenoid  muscle 
arises  from  the  side  of  the  cricoid 
cartilage.  From  this  point  its  fibers 
are  directed  upward  and  backward  to 


Fig.  264. — Later.\l  View  of  the 
Muscles  of  the  Larynx,  i. 
Body  of  the  hyoid  bone.  2.  Verti- 
cal section  of  the  thyroid  cartilage. 
3.  Horizontal  section  of  the  th}Toid 
cartilage  turned  downward  to  show 
the  deep  attachment  of  the  crico- 
thyroid muscle.  4.  Facet  of  articu- 
lation of  the  small  cornu  of  the 
th)Toid  cartilage  with  the  cricoid 
cartilage.  5.  Facet  on  the  cricoid 
cartilage.  6.  Superior  attachment  of 
the  crico-thyroid  muscle.  7.  Pos- 
terior crico-arytenoid  muscle.  8, 
ID.  Arytenoid  muscle.  9.  Thyro- 
arytenoid muscle.  II.  Ar}teno- 
epiglottidean  muscle.  12.  Middle 
thyro-hyoid  ligament.  13.  Lateral 
thyro-hyoid     ligament. — (Sappey.) 


be  inserted  into  the  external  process 
of  the  arytenoid.     Its  action  is  to  draw  the  arytenoid  cartilage  for- 
ward and  inward,  thus  approximating  and  relaxing  the  vocal  band. 

The  thyro-arytenoid  muscle  arises  from  the  inferior  two-thirds 
of  the  inner  surface  of  the  thyroid  cartilage  just  external  to  the 
median  line.    From  this  origin  the  fibers  pass  backward  and  outward, 
38 


594 


TEXT-BOOK  OF  PHYSIOLOGY. 


to  be  inserted  into  the  anterior  surface  and  external  angle  of  the 
arytenoid  cartilage.  The  inner  portion  of  the  muscle  lies  close  to 
and  supports,  if  it  does  not  constitute  a  part  of,  the  vocal  band. 
The  action  of  the  thyro-arytenoid  muscle  in  conjunction  with  the 
lateral  crico-arytenoid  is  to  rotate  the  arytenoid  cartilage  around 
the  vertical  axis  and  to  drav^'  the  vocal  process  forward  and  inward, 
thus  carrying  the  vocal  cord  toward  the  median  line.  When  the 
muscles  of  the  two  sides  simultaneously  contract,  the  vocal  bands 
are  closely  approximated  and  the  space  between  them,  the  rima 
vocalis,  reduced  to  a  mere  slit,  one  of  the  conditions  essential  to 
phonation  (Fig.  266). 

The  arytenoid  muscle  consists  (i)  of  transversely  arranged  fibers 
which  arise  from  and  are  inserted  into  the  outer  surface  of  the  oppo- 


FlG 


265. — Glottis  Widely  Opened 
FROM  Simultaneous  Contraction 
OF  Both  Crico-arytenoid  Mus- 
cles, h.  Epiglottis.  rs.  False 
vocal  band.  ri.  True  vocal  band. 
ar.  Arytenoid  cartilages,  a.  Space 
between  the  arytenoids,  c.  Cunei- 
form cartilages,  ir.  Interarytenoid 
fold.  rap.  Aryepiglottic  fold.  cr. 
Cartilage  rings. — {Mandl.) 


Fig 


266. — Position  of  the  Vocal 
Bands  Due  to  the  Simultaneous 
Contraction  of  Both  Lateral 
Crico-arytenoid  Muscles  and 
Both  Thyro-arytenoid  Muscles. 
h.  Epiglottis,  rs.  False  vocal  band. 
ri.  True  vocal  band.  or.  Space  be- 
tween the  arytenoid  cartilages,  the 
glottis  respiratoria.  ar.  Arytenoid 
cartilages,  c.  Cuneiform  cartilages. 
rap.  Aryepiglottic  fold.  ir.  Interary- 
tenoid io\d.^{Mandl.) 


site  arytenoid  cartilages,  and  (2)  of  obliquely  directed  fibers  which 
arise  from  the  outer  angle  of  one  arytenoid  to  be  inserted  into  the 
apex  of  the  other.  In  their  course  they  decussate  in  the  median 
line.  The  action  of  this  muscle  is  to  approximate  the  arytenoid 
cartilages  and  thus  obliterate  that  portion  of  the  glottis  between  the 
vocal  processes,  the  rima  respiratoria,  and  so  direct  the  expiratory 
blast  of  air  toward  and  through  the  rima  vocalis. 

The  collective  actions  of  the  three  foregoing  muscles  is  to  close  or 
constrict  the  glottis,  and  for  this  reason  they  are  spoken  of  as  the 
adductor  or  phonatory  muscles. 

The  crico-thyroid  muscle  arises  from  the  side  and  front  of  the 
cricoid  cartilage  and  is  inserted  above  into  the  lower  border  of  the 
thyroid  cartilage.  The  action  of  this  muscle  is  to  draw  up  the  an- 
terior part  of  the  cricoid  cartilage  toward  the  thyroid,  which  remains 


PRONATION;    ARTICULATE  SPEECH.  595 

stationary,  and  to  swing  the  quadrate  plate  of  the  cricoid  and  the 
arytenoid  cartilages  downward  and  backward.  This  movement 
has  the  result  of  tensing  the  vocal  bands.  The  cricoid  is  at  the  same 
time  drawn  backward  by  the  action  of  the  more  longitudinally  dis- 
posed fibers. 

Nerves  of  the  Larynx. — The  nerves  which  innervate  the  muscles 
of  the  larynx  and  endow  the  mucous  membrane  with  sensibihty  are 
derived  from  the  vagus  trunk.  The  superior  laryngeal  is  for  the 
most  part  sensor  and  distributed  to  the  mucous  membrane,  though 
it  contains  motor  fibers  for  the  crico-thyroid  muscle.  The  inferior 
laryngeal  is  purely  motor  and  is  distributed  to  all  the  muscles  with 
the  exception  of  the  crico-thyroid. 

THE  MECHANISM  OF  PHONATION. 

Phonation,  the  production  of  vocal  sounds  in  the  larynx,  is  the 
result  of  the  vibration  of  the  vocal  bands  caused  by  an  expiratory 
blast  of  air  from  the  lungs.  That  a  sound  may  arise  it  is  essential 
that  the  glottis  be  approximately  closed  and  the  vocal  bands  be  made 
more  or  less  tense. 

The  closure  of  the  glottis — the  approximation  of  the  vocal  pro- 
cesses and  the  vocal  bands — is  accomplished,  it  will  be  recalled,  by 
the  contraction  of  the  lateral  crico-arytenoid,  the  arytenoid,  and  the 
thy ro- arytenoid  muscles.  The  increase  in  tension  is  accomphshed 
by  the  contraction  of  the  crico-thyroid  and  the  thyro-arytenoid 
muscles,  the  former  by  the  backward  displacement  of  the  cricoid  and 
arytenoid  cartilages,  the  latter  by  converting  the  natural  concave 
edge  of  the  vocal  band  to  a  straight  hne.  The  lengthening  and 
tensing  of  the  vocal  bands  by  the  crico-thyroid  muscle  is  regarded 
by  some  investigators  as  a  coarse  means,  the  approximation  of 
the  free  edges  by  the  thyro-arytenoid,  as  a  finer  means,  of  adjust- 
ment for  the  production  of  slight  changes  in  the  pitch  of  sounds. 
The  extent  to  which  the  glottis  is  closed  and  the  membranes  tensed 
will  depend,  however,  on  the  pitch  of  the  sound  to  be  emitted.  The 
appearance  presented  by  the  glottis  just  previous  to  the  emission 
of  a  note  of  medium  pitch,  as  determined  by  laryngologic  examina- 
tion, is  shown  in  Fig.  267.  When  the  foregoing  conditions  in 
the  glottis  are  reahzed,  the  air  stored  or  collected  in  the  lungs  is 
forced  by  the  contraction  of  the  expiratory  muscles,  through  the 
narrow  space  between  the  bands.  As  a  result  of  the  resistance 
offered  by  this  narrow  outlet  and  the  force  of  the  expiratory 
muscles  the  air  within  the  lungs  and  trachea  is  subjected  to  pressure, 
and  as  soon  as  the  pressure  attains  a  certain  level  the  vocal  bands 
are  thrown  into  vibrations,  which  in  turn  imp;  rt  to  the  column  of  air 
in  the  upper  air-passages  a  corresponding   s  ries  of  vibrations  by 


596 


TEXT-BOOK  OF  PHYSIOLOGY. 


which  the  laryngeal  vibrations  arc  reinforced.  The  degree  of  pres- 
sure to  which  the  air  in  the  lungs  and  trachea  is  subjected  was 
determined  by  Latour  to  vary  from  i6o  mm.  of  water  for  sounds  of 
moderate,  to  940  mm.  of  water  for  sounds  of  highest  intensity.  With 
the  escape  of  the  air  or  the  separation  of  the  vocal  bands  the  vibra- 
tion ceases  and  the  sound  dies  away. 

The  Characteristics  of  Vocal  Sounds. — In  common  with  the 
sounds  produced  by  all  other  music  instruments,  all  vocal  sounds 
are  characterized  by  intensity,  pitch  and  quahty,  tone  or  color. 

The  intensity  or  loudness  of  a  sound  depends  on  the  extent  or 
amplitude  of  the  up-and-down  vibration  or  the  extent  of  the  excur- 
sion of  the  vocal  band  on  either  side  of  the  position  of  equilibrium 
or  rest ;  and  this  in  turn  depends  on  the  force  with  which  the  blast  of 
air  strikes  the  band.  The  more  forceful  the  blast  of  air,  the  larger, 
other  things  being  equal,  will  be  the  primary  vibrations  of  the  bands, 


#r/  ^ 


Fig.  267. — Position  of  the  Vocal 
Bands  Previous  to  the  Emission 
OF  A  Sound,  b.  Epiglottis,  rs.  False 
vocal  band.  ri.  True  vocal  band. 
ar.  Ary'tenoid  cartilages. — (Mandl.) 


*L_Z 


Fig.  268. — Position  of  the  Vocal 
Bands  in  the  Production  of 
Notes  of  Low  Pitch.  /.  Epiglottis. 
or.  Glottis,  ns.  False  vocal  cord. 
ni.  True  vocal  cord.  ar.  Arytenoid 
cartilages. — (Mandl.) 


and  hence  the    secondary  vibrations  of  the   air  in  the   upper  air- 
passages. 

The  pitch  of  the  voice  depends  on  the  number  of  vibrations  in 
a  unit  of  time,  a  second.  This  will  be  conditioned  by  the  length  of 
the  bands  in  vibration  or  the  length  and  width  of  the  aperture  through 
which  the  air  passes  and  the  degree  of  tension  to  which  the  bands  are 
subjected.  In  the  emission  of  sounds  of  highest  pitch  the  tension  of 
the  vocal  bands  and  the  narrowing  of  the  glottis  attain  their  maxi- 
mum. In  the  emission  of  sounds  of  lowest  pitch  the  reverse  conditions 
obtain.  In  passing  from  the  lowest  to  the  highest  pitched  sounds  in 
the  range  of  the  voice  peculiar  to  any  one  individual,  there  is  a  pro- 
gressive increase  in  both  the  tension  of  the  vocal  bands  and  the 
narrowing  of  the  glottic  aperture.  In  the  production  of  low-pitched 
notes  of  men,  those  due  to  vibrations  lying  between  80  and  240  per 
second,    the  tension  is  regulated  by  the  crico- thyroid  muscle;  the 


PHONATION;    ARTICULATE  SPEECH. 


597 


aperture  of  the  glottis  during  this  time  being  elliptic  in  shape  and 
relatively  wide  (Fig.  268).  In  the  production  of  notes  due  to 
vibrations  lying  between  240  and  512  vibrations  per  second,  the 
anterior  fibers  of  the  crico-thyroid  muscle  relax  and  the  thyro-ary- 
tenoid  muscle  comes  into  play;  by  its  action  the  vocal  bands  are 
more  closely  approximated  and  the  vocal  aperture  reduced  to  a 
hnear  slit.  In  the  high-pitched  notes  emitted  by  soprano  singers  the 
vocal  bands  are  so  closely  apphed  to  each  other  that  only  a  very 
small  portion  in  front,  bounding  a  small  oval  aperture,  is  capable 
of  vibrating  (Fig.  269).  The  difference  in  the  pitch  of  the  voice 
in  men  and  women  is  due  largely  to  the 
greater  size  and  development  of  the  vocal 
bands  in  the  former  than  in  the  latter. 

The  quality  of  the  voice,  the  timbre  or 
color,  depends  on  the  form  combined  with 
the  intensity  and  pitch  of  the  vibration. 
As  with  sounds  produced  by  music  in- 
struments, the  primary  or  fundamental 
vibration  of  the  vocal  band  is  compli- 
cated by  the  superposition  of  secondary 
or  partial  vibrations  (overtones).  The 
form  of  the  vibration  will  therefore  be 
a  resultant  of  the  blending  of  a  number 
of  different  vibrations.  The  quality  of 
the  sound  produced  in  the  larynx  is, 
however,  modified  by  the  resonance  of 
the  mouth  and  nasal  cavities;  certain  of 
the  overtones  being  reinforced  by  changes 
in  the  shape  of  the  mouth  cavity  more 
especially,  thus  giving  to  the  voice  a 
somewhat  different  quahty. 

The  Varieties  of  Voice.  —  The 
region  of  the  music  scale,  comprising  all 
vibrations    between    32    and    2048    per 

second,  with  which  laryngeal  sounds  are  in  accord  will  vary  in  the  two 
sexes  and  in  dift'erent  individuals  of  the  same  sex.  It  is  customary 
to  classify  voices,  especially  those  of  singers,  into  bass,  baritone,  tenor, 
contralto,  mezzo-soprano,  and  soprano,  in  accordance  with  the  regions 
of  the  music  scale  with  which  they  correspond.  Thus  the  succession 
of  notes  characteristic  of  the  bass  voice  vary  in  pitch  from  F,  fa',  to 
C ,  do3,  or  from  87  to  256  vibrations  per  second;  those  of  the  baritone 
from  A,  la,  to  F',  fag,  or  from  106  to  341  vibrations  per  second; 
those  of  the  tenor  from  C,  dOg,  to  a',  kg,  or  from  128  to  435  vibrations 
per  second;  those  of  the  contralto  from  e,  mij,  to  C",  do^,  or  from  160 
to  512  vibrations  per  second;  those  of  the  mezzo-soprano  from  g,  S0I2, 


Fig.  269. — Glottis  Seen 
WITH  THE  Laryngo- 
scope DURING  THE  EMIS- 
SION OF  High-pitched 
Sounds,  i,  2.  Base  of 
the  tongue.  3,  4.  Epiglot- 
tis. 5,  6.  Pharynx.  7. 
Arytenoid  cartilages.  8. 
Opening  between  the  true 
vocal  cords.  9.  Aryteno- 
epiglottidean  folds.  10. 
Cartilage  of  Santorini.  1 1 . 
Cuneiform  cartilage.  12. 
Superior  vocal  cords.  13. 
Inferior  \'ocal  cords. — (Le 
Bon.) 


598  TEXT-BOOK  OF  PHYSIOLOGY. 

to  e",  mi^,  or  from  192  to  640  vibrations  per  second;  those  of  the 
soprano  from  b,  si2,  to  g",  sol^,  or  from  240  to  768  vibrations  per  second. 
The  range  of  the  voice  is  thus  seen  to  embrace  from  one  and  three- 
quarters  to  two  octaves.  Some  few  individual  singers  have  far  ex- 
ceeded this  range,  but  they  are  exceptional. 

Speech  is  the  expression  of  ideas  by  means  of  articulate  sounds. 
These  sounds  may  be  divided  into  vowel  and  consonant  sounds. 

The  vowel  sounds,  a,  e,  i,  0,  u,  are  laryngeal  sounds  modified  by 
the  superposition  and  reinforcement  of  certain  overtones  developed 
in  the  mouth  and  pharynx  by  changes  in  their  shapes.  The  number 
of  vibrations  underlying  the  production  of  each  vowel  sound  is  a 
matter  of  dispute.  According  to  Konig,  the  sound  of  a  is  the  result 
of  940  vibrations;  of  e,  1880  vibrations;  of  i,  3760  vibrations;  of  o, 
470  vibrations;  of  ou,   235   vibrations. 

Consonant  sounds  are  produced  by  the  more  or  less  complete  in- 
terruption of  the  vowel  sounds  during  their  passage  through  the  organs 
of  speech.     These  may  be  divided  into: 

1.  Labials,  p,  b,  m. 

2.  Labio-dentals,  /,  v. 

3.  Linguo-dentals,  s,  z. 

4.  Anterior  hnguo-palatals,  t,  d,  I,  n. 

5.  Posterior  linguo-palatals,  k,  g,  h,  y,  r. 

The  names  of  these  different  groups  of  consonants  indicate  the 
region  of  the  mouth  in  which  they  are  produced  and  the  means  by 
which  the  air  blast  is  interrupted. 

THE  NERVE  MECHANISM  OF  THE  LARYNX. 

The  nerve  mechanism  by  which  the  musculature  of  the  larynx 
is  excited  to  action  and  coordinated  so  as  to  subserve  both  res- 
piration and  phonation  involves  the  fibers  contained  in  the  superior 
and  inferior  laryngeal  nerves  (both  branches  of  the  vagus)  and  their 
related  nerve-centers  in  the  central  nerve  system. 

For  respiratory  purposes  it  is  essential  that  the  lumen  of  the 
glottis  shall  be  sufficiently  large  to  permit  the  entrance  and  exit  of  air 
without  hindrance.  Laryngoscopic  examination  of  the  larynx  in  the 
human  being  shows  that  during  quiet  respiration  the  vocal  bands  are 
widely  separated  and  almost  stationary,  moving  but  slightly  during 
either  inspiration  or  expiration.  At  this  time,  according  to  the  in- 
vestigations of  Semon,  the  area  of  the  glottis  is  approximately 
160  sq.  mm.,  somewhat  less  than  the  area  of  either  the  supraglottic 
or  infraglottic  regions,  which  is  about  200  sq.  mm.  This  condition 
of  the  glottis  is  maintained  by  the  steady  continuous  contraction 
of  the  posterior  crico-arytenoid  muscles,  the  abductors  of  the  vocal 
bands. 


PRONATION;    ARTICULATE  SPEECH.  599 

For  phonatory  purposes  it  is  essential  that  the  respiratory  function 
be  temporarily  suspended  and  the  vocal  bands  closely  approx- 
imated. This  is  accomplished  by  the  contraction  of  the  remaining 
muscles  of  the  larynx,  with  the  exception  of  the  crico-thyroid, 
which  are  collectively  known  as  the  adductors  of  the  vocal  bands. 
During  phonation  the  adductor  muscles  overcome  the  activity  of  the 
abductors.  With  the  cessation  of  phonation  the  abductors  immedi- 
ately restore  the  vocal  bands  to  their  former  respiratory  position. 

The  activities  of  these  two  antagonistic  groups  of  muscles  are  under 
the  control  of  the  central  nerve  system.  The  only  pathway  for 
the  excitatory  nerve  impulses  is  through  the  fibers  of  the  inferior  or 
recurrent  laryngeal  nerve.  The  relation  of  these  nerve-fibers  both 
centrally  and  peripherally,  as  well  as  their  physiologic  action,  has  been 
the  subject  of  much  experimentation.  The  results  have  not  always 
been  in  accord,  owing  to  the  choice  of  animal,  the  use  of  anes- 
thetics, strength  of  stimulus,  etc. 

As  the  outcome  of  many  investigations  it  is  beheved  that  each 
muscle  group  is  innervated  by  its  own  bundle  of  nerve-fibers,  both 
of  which  are  contained  in  the  inferior  laryngeal,  though  coming 
from  two  separate  centers  in  the  medulla  oblongata.  Russell  suc- 
ceeded in  separating  the  fibers  for  the  abductors  from  the  fibers 
for  the  adductors  in  the  inferior  laryngeal,  and  in  tracing  them 
to  their  terminations.  So  completely  was  this  done  that  it  became 
possible  to  produce  at  will,  through  stimulation,  either  abduction  or 
adduction,  without  contraction  of  the  muscle  of  opposite  function. 

The  laryngeal  respiratory  center  was  located  by  Semon  and 
Horsley,  in  the  cat,  in  the  upper  part  of  the  floor  of  the  fourth  ven- 
tricle. Stimulation  of  this  area  during  etherization  was  followed  by 
abduction  of  the  vocal  bands.  The  efferent  fibers  of  this  center  are 
beheved  by  some  investigators  to  leave  the  central  nerve  system  in 
the  spinal  accessory  nerve,  by  others  in  the  lower  roots  of  the  vagus. 

From  the  continuous  activity  of  the  abductor  muscle,  and  the 
stationary  position  of  the  vocal  bands,  it  is  probable  that  the  medul- 
lary center  is  in  a  state  of  continuous  activity  or  tonus,  the  result 
probably  of  reflex  influences. 

A  cortical  representation  for  laryngeal  respiratory  movements 
has  been  determined  by  Semon  and  Horsley  in  different  classes  of 
animals.  In  the  cat  especially,  stimulation  of  the  border  of  the 
olfactory  sulcus  gives  rise  to  complete  abduction  of  the  vocal  bands 
on  both  sides.     The  representation  is  therefore  bilateral. 

The  phonatory  center  was  located  by  the  same  investigators  in 
the  medulla  near  the  ala  cinerea  and  the  upper  border  of  the  calamus 
scriptorius.  Stimulation  of  this  area  was  invariably  followed  by 
bilateral  adduction  of  the  vocal  bands  and  closure  of  the  glottis. 

A   cortical  representation  for  phonatory  movements   also  was 


6oo  TEXT-BOOK  OF  PHYSIOLOGY. 

located  in  the  lower  portion  of  the  precentral  convolution,  near  the 
anterior  border.  Stimulation  of  this  area  gives  rise  to  marked  ad- 
duction of  both  vocal  bands,  indicating  that  the  representation  is 
also  bilateral. 

Faradic  stimulation  of  the  inferior  laryngeal  nerve  during  slight 
ether  anesthetization  gives  rise  to  closure  of  the  glottis;  the  same 
stimulation,  however,  during  deeper  anesthetization  gives  rise  to 
opening  or  dilatation  of  the  glottis,  a  fact  indicating  that  either 
the  adductor  muscles  or  their  nerve  terminals  are  depressed  by  the 
action  of  the  ether  before  the  muscles  and  nerves  of  opposite  function. 
The  superior  laryngeal  nerves  contain  motor  fibers  for  the  crico- 
thyroid muscles.  Stimulation  of  the  nerve  gives  rise  to  contraction 
of  the  muscle  and  increased  tension  of  the  vocal  bands.  It  is  believed 
that  these  fibers  are  derived  originally  from  the  efferent  fibers  of  the 
glosso-pharyngeal  nerve.  The  remaining  fibers  of  the  superior 
laryngeal  endow  the  upper  portion  of  the  larynx  with  extreme  sen- 
sibility which  to  a  certain  extent  protects  the  air-passages  against  the 
entrance  of  foreign  bodies.  Irritation  of  the  terminal  filaments  of 
this  nerve  by  particles  of  food,  solid  or  liquid,  gives  rise  to  marked 
reflex  spasm  of  the  adductor  muscles  and  closure  of  the  glottis,  fol- 
lowed by  a  strong  expiration  blast  of  air  from  the  lungs  by  which  the 
offending  particles  are  removed.  Division  of  this  nerve  on  both 
sides  is  followed  by  a  paralysis  of  the  crico-thyroid  muscles,  a  lower- 
ing of  the  tension  of  the  vocal  bands,  and  a  loss  of  sensibility  of  the 
laryngeal  mucous  membrane. 


CHAPTER  XXIV. 
THE  SPECIAL  SENSES. 

It  is  one  of  the  functions  of  the  nerve  system  to  bring  the 
individual  into  conscious  relation  with  the  external  world.  This  is 
accomphshed  in  part  through  the  intermediation  of  afferent  nerves, 
connected  peripherally,  with  highly  specialized  terminal  organs  and 
centrally,  with  specialized  areas  in  the  cerebral  cortex. 

Excitation  of  the  terminal  organs  by  material  changes  in  the 
environment  develops  nerve  impulses  which,  transmitted  to  the 
cortical  areas,  evoke  sensations.  These  sensations,  differing  in 
character  from  those  vague  ill-defined  sensations — e.  g.,  fatigue, 
well-being,  discomfort,  etc. — caused  by  material  changes  occurring 
within  the  body,  are  termed  special  sensations — e.  g.,  touch;  pres- 
sure ;  pain  ;  temperature;  taste;  smell;  light  and  its  varying  quali- 
ties, intensity,  hue,  and  tint;  sound  and  its  var}-ing  qualities,  intensity, 
pitch,  and  timbre. 

The  terminal  organs  which  receive  the  impress  of  the  external 
world  are  the  skin,  tongue,  nose,  eye,  and  ear,  and  collectively  con- 
stitute the  special  sense-organs.  The  physiologic  mechanisms  which 
underlie  and  develop  these  special  sensations  are  known  respectively 
as  the  tactile,  gustatory,  olfactory,  optic,  and  auditory.  Each 
mechanism  responds  to  but  a  single  form  of  stimulus  and  to  no 
other.  Thus,  the  stimulus  for  the  skin  is  mechanic  pressure; 
for  the  tongue,  soluble  organic  and  inorganic  matter;  for  the  nose, 
volatile  or  gaseous  matter;  for  the  eye,  ether  vibrations;  for  the 
ear,  atmospheric  undulations.  These  stimuli  alone  are  adequate 
to  the  physiologic  excitation  of  the  different  mechanisms. 

The  factors  involved  in  the  production  of  the  sensations  include 
(i)  a  special  physical  stimulus;  (2)  a  specialized  terminal  organ; 
(3)  an  afferent  nerve  pathway;  and  (4)  a  specialized  receptive 
sensor  cell  in  the  cerebral  cortex. 

Though  the  resulting  sensations  in  each  instance  differ  widely  in 
their  characteristics,  it  is  difficult  to  present  a  satisfactory  explanation 
for  these  differences.  If  it  be  assumed  that  the  nerve  impulses  which 
ascend  the  different  nerves  of  special  sense  are  ahke  in  quality,  then 
it  must  be  admitted  that  the  character  of  the  sensation  is  the  ex- 
pression of  a  specialization  and  organization  of  the  cortical  area. 
If,  on  the  other  hand,  specialization  of  the  cortex  is  denied,  then 
there  must  be  admitted  a  specialization  of  the  peripheral  organ — with 

601 


6o2  TEXT-BOOK  OF  PHYSIOLOGY. 

a  resulting  difference  in  quality  or  rapidity  of  the  nerve  impulses 
which  would  impress  or  excite  the  non-specialized  cortex  in  such  a 
way  as  to  call  forth  the  characteristic  sensation.  It  is  possible,  how- 
ever, that  neither  supposition  is  wholly  correct,  and  that  the  char- 
acter of  the  sensation  depends  on  the  construction  and  adaptation 
of  the  entire  sense  apparatus  to  the  character  of  the  stimulus. 

Whatever  the  conditions  for  their  origin  and  whatever  their 
characteristics,  sensations  in  themselves  do  not  constitute  knowledge ; 
they  are  but  elementary  states  of  consciousness,  raw  materials  out 
of  which  the  mind  elaborates  conceptions  and  forms  judgments  as 
to  the  character  of  any  given  object  in  comparison  with  former 
experiences. 

THE   SENSE   OF   TOUCH. 

The  physiologic  mechanism  involved  in  the  sense  of  touch  in- 
cludes the  skin  and  the  mucous  membrane  lining  the  mouth,  the 
afferent  nerves,  their  cortical  connections,  and  nerve-cells  in  the 
cortex  of  the  parietal  lobe  and  the  gyrus  fornicatus  (Figs.  226,  227). 

Peripheral  excitation  of  this  mechanism  develops  nerve  impulses 
which,  transmitted  to  the  cortex,  evoke  sensations  of  touch  and 
temperature.  To  the  skin,  therefore,  is  ascribed  a  touch  sense 
and  a  temperature  sense.  Of  the  touch  sensations  two  kinds  may 
be  distinguished:  viz.,  pressure  sensations  and  place  sensations. 
With  the  contact  of  an  external  body  there  arises  the  perception 
not  only  of  the  pressure,  but  also  the  perception  of  the  place  or 
locality  of  the  contact.  In  accordance  with  this,  it  is  customary  to 
attribute  to  the  skin  a  pressure  sense  and  a  location  sense. 

The  specific  physiologic  stimuli  to  the  terminal  organs  in  the  skin 
and  oral  mucous  membrane  are  mechanic  pressure  and  thermic 
vibrations. 

The  Skin. — The  skin,  which  constitutes  the  basis  for  the  sense 
of  touch,  covers  and  closely  invests  the  entire  body.  It  varies  in 
thickness  and  delicacy  in  different  regions,  though  its  structure  is 
everywhere  essentially  the  same.  As  the  physiologic  anatomy  of  the 
skin  has  elsewhere  been  detailed  (page  453),  it  is  only  necessary  to 
state  here  that  it  is  divided  into  a  deep  and  a  superficial  layer.  The 
former,  known  as  the  derma,  consists  of  an  inner  layer  of  rather 
loose  connective  tissue  and  an  outer  layer  of  condensed  connective 
tissue.  The  latter,  known  as  the  epidermis,  consists  of  an  inner 
layer  of  pigment  cells  and  a  thick  outer  layer  of  epithelial  cells. 
The  derma  is  characterized  by  the  presence  of  elevations  (papillae) 
which  are  everywhere  extremely  abundant.  Throughout  the  derma 
ramify  blood-vessels  and  nerves. 

The  Peripheral  or  Terminal  Organs. — Between  the  contact 
surface  and  the  afferent  nerves  specialized    structures  are  found 


THE  SENSE  OF   TOUCH. 


603 


>^3ri;J^- 


^.-:...X^..^-^:.^^:g--0^ 


which  serve  as  intermediates  between  the  stimulus,  on  the  one  hand, 
and  the  afferent  nerves,  on  the  other  hand.  By  virtue  of  their  struc- 
ture they  are  far  more  irritable  than  the  nerve-fibers  and  hence 
respond  more  quickly  to  the  physiologic  stimulus  than  the  nerve- 
fiber  itself.  To  these  specialized  organs,  found  not  only  in  the 
skin  but  in  other  sense-organs  as  well,  the  term  peripheral  or  terminal 
organ  is  given.  It  is  these  structures  that  are  primarily  excited  to 
activity  by  the  physiologic  stimulus,  and  that  in  turn  arouse  the 
nerve  to  activity.  Peripheral  organs  are  to  be  regarded  as  special 
modes  of  termination  of  afferent  nerves  and  adapted  for  the  impress 
of  a  specific  stimulus.  The  peripheral  organs  of  afferent  nerves 
found  in  the  skin  and  oral  mucous  membrane  present  a  variety  of 
forms,  some  of  which  are  as  follows: 

1.  Free  Endings. — These  are  pointed  or  club-shaped  processes,  the 

ultimate  terminations  of  af- 
erent  nerve-fibrils,  found  in 
and  among  epidermic  cells. 

2.  Tactile  Cells. — These  are  oval 

nucleated  bodies  found  in 
the  deeper  layers  of  the 
epidermis.  They  rest  upon 
or  are  embraced  by  a  cres- 
centic  shaped  body,  the 
tactile  meniscus,  which  in 
turn  is  directly  connected 
with  the  nerve-fibril  and 
probably  a  modification  of  it 
(Fig.  270). 

3.  The  Corpuscles   of    Meissner 

and  Wagner. — In  the  papillae 

of  the  derma,  especially  in  the  palm  of  the  hand  and  in  the 
finger-tips,  are  found  elliptical  bodies  consisting  of  a  connective- 
tissue  capsule  containing  a  number  of  tactile  discs  with  which  the 
nerve-fibrils  are  connected.  If  the  afferent  nerve  is  traced  to 
the  capsule,  it  is  found  to  lose  both  its  neurilemma  and  medulla, 
after  which  the  naked  fibril  penetrates  the  capsule,  breaks  up 
into  a  number  of  branches,  and  after  pursuing  a  more  or  less 
spiral  course  becomes  connected  with  the  tactile  discs  (Fig.  271). 

4.  Hair  Wreaths. — Just  below  the  openings  of  the  sebaceous  gland 

the  hair  follicles  are  surrounded  by  naked  axis-cylinder  fibrils 
in  the  form  of  a  wTeath,  which  in  all  probability  terminate  in 
the  cells  of  the  external  root-sheath.  These,  too,  are  to  be 
regarded  as  part  of  the  touch  apparatus. 

5.  Corpuscles  0}  Vater  or  Pacini. — These  are  oval-shaped  structures 

found  along  the  nerves  distributed  to  the  palms  of  the  hands  and 


Fig.  270. — Tactile  Cells  from  Snout 
OF  Pig.  a.  Tactile  cell.  m.  Tactile 
disc.    «.  Nerve-fiber. — {Stirling.) 


6o4 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  soles  of  the  feet,  on  the  nerves  distributed  to  the  external 
genital  organs,  to  joints  and  other  structures.  They  consist  of 
a  thick  capsule  of  lamellated  connective  tissue  in  the  interior 
of  which  is  a  bulb  resembling  granular  protoplasm.  The  axis- 
cylinder  of  the  nerve-fiber  enters  the  capsule  and  becomes  con- 
nected with  the  bulb  (Fig.  272). 
Other  forms  of  peripheral  organs  are  found  in  special  regions  of 
the  skin  as  well  as  in  different  animals. 

Touch  Sense. — The  area,  stimulation 
of  which  evokes  sensations  of  touch,  is 
coextensive  with  the  skin  and  that  limited 
portion  of  the  mucous  membrane  lining 
the  mouth.  Careful  stimulation  of  the 
skin  by  means  of  a  fine  stiff  bristle  has 
revealed  the  fact,  however,  that  the  touch 
area  is  not  continuous,  but  discrete,  pre- 
senting itself  under  the  form  of  small  areas 
or  spots,  separated  by  relatively  large  areas 
insensitive  to  the  same  agent.  Stimulation 
of  these  spots  always  calls  forth  a  sensation 
of  touch.  For  this  reason  they  are  known 
as  "touch  spots."  The  number  of  such 
spots  in  any  given  area  of  skin  varies 
considerably.  Thus,  in  the  skin  of  the  calf 
fifteen  such  spots  have  been  counted  in  a 
square  centimeter.  In  the  palm  of  the 
hand,  from  forty  to  fifty  have  been  counted 
in  an  area  of  the  same  extent.  They  are 
also  especially  abundant  in  the  immediate 
neighborhood  of  the  hair  follicles. 

The  peripheral  end-organ  associated 
with  the  touch  spots  in  the  neighborhood 
of  a  hair  follicle  is  in  all  probability  the 
wreath  of  nerve-fibrils  surrounding  the 
follicle.  In  regions  devoid  of  hairs  the 
end-organ  is  the  Meissner  corpuscle,  for 
in  the  palmar  surface  of  the  last  phalanx 
of  the  index-finger,  where  the  touch  sense  is  quite  acute,  about  20 
corpuscles  are  present  in  each  square  millimeter  of  surface.  The 
specific  stimulus  necessary  to  evoke  the  sensation  of  touch  is  a 
deformation  of  the  skin;  and  the  greater  this  is  within  physiologic 
limits,  the  more  pronounced  is  the  sensation. 

Pressure  Sense. — The  contact  of  an  external  body  is  attended 
by  a  certain  amount  of  pressure,  which,  however,  must  attain  a 
certain  degree  before  the  sensation  can  be  evoked.     This  is  known 


Fig.  271. — Touch  Corpus- 
cle OF  Meissner  and 
Wagner,  b.  Papilla  of 
cutis,  d.  Nerve-fiber  of 
touch-corpuscle.  e,  j. 
Nerve-fiber  in  touch- 
corpuscle,  g.  Cells  of 
Malpighian  layer. — 

(From  Stirling.) 


THE  SENSE  OF   TOUCH. 


605 


as  the  threshold  value,  or  the  degree  of  liminal  intensity.  Since 
the  sensations  are  the  result  of  pressure,  they  are  termed  pressure 
sensations,  and  their  intensity  may  be  expressed  in  terms  of  pressure. 

The  sensitivity  of  the  skin  as  determined  by  the  pressure  sense 
varies  in  different  regions  of  the  body  and  in  accordance  with  the 
size  of  the  area  pressed.  Thus,  the  liminal  intensity  of  a  stimulus 
for  an  area  of  nine  square  millimeters  for  the  skin  of  the  forehead  is 
0.002  gram;  for  the  flexor  aspect  of  the  forearm,  0.003  gram;  and 
for  the  hips,  thigh,  and  abdomen,  0.005  g^^am;  for  the  palmar  surface 
of  the  finger,  0.019  gra^i;  for  the  heel,  i  gram.  The  delicacy  of  the 
sense  of  touch  is  measured  by  the  slight 
increase  or  decrease  in  the  intensity  of  the 
stimulus  that  v^^ill  produce  an  appreciable 
change  in  the  intensity  of  the  sensation. 
Not  all  changes  in  the  stimulus,  however, 
are  attended  by  a  change  in  the  sensation. 
It  has  been  determined  that  the  latter  will 
change  only  when  the  former  changes  in  a 
definite  ratio,  which  for  the  volar  surface  of 
the  third  phalanx  of  the  index-finger  is  as  29 
is  to  30.  Thus,  other  things  being  equal, 
a  sensation  caused  by  a  given  weight  will 
only  change  with  moderate  stimulation  when 
one-thirtieth  of  the  weight  is  either  added 
or  subtracted.  The  ratio  of  change,  how- 
ever, varies  in  different  regions  of  the  body : 
thus,  for  the  back  of  the  hand  the  ratio 
varies  from  one-tenth  to  one-twentieth;  for 
the  tongue,  one-thirtieth  to  one-fortieth. 
The  difference  of  stimulus  necessary  to 
evoke  a  sensation  is  known  as  the  threshold 
difference.  It  seems  to  be  a  law  not  only 
for  the  skin,  but  for  other  senses  as  well, 
that  a  change  in  the  intensity  of  a  sensation, 

to  an  appreciable  extent,  will  occur  only  when  the  objective  stimulus 
changes  in  a  definite  ratio.  This  ratio,  however,  will  vary  not  only 
in  different  regions  of  the  skin,  in  different  individuals,  but  with  the 
sense-organ  investigated. 

Place  Sense. — The  sensation  evoked  by  stimulation  of  the  skin 
is  always,  under  normal  conditions,  referred  to  the  place  stimulated. 
This  holds  true  not  only  for  two  or  more  points  near  or  widely  sepa- 
rated on  the  same  side,  but  also  for  corresponding  points  on 
opposite  sides  of  the  body,  even  when  the  stimuli  have  the  same 
intensity  and  are  simultaneously  applied.  The  cause  for  this  localiz- 
ing power  is  to  be  found  in  a  difference  in  the  quality  of  the  sensation 


Fig. 


272 .  —  Pacinian 
Corpuscles,  c.  Cap- 
sules, d.  Endothelial 
lining  separating  the 
latter,  n.  Nerve.  /. 
Funicular  sheath  of 
nerv-e.  m.  Central 
mass.  n'.  Terminal 
fiber;  and  a.  Where 
it  splits  up  into  finer 
fibrils. — {Stirling.) 


6o6  TEXT-BOOK  OF  PHYSIOLOGY. 

related  in  some  way  to  the  part  stimulated.  Each  cutaneous  area 
is  supposed  to  give  to  the  tactile  sensation  a  quality  or  local  sign 
by  virtue  of  which  the  mind  is  enabled  to  localize  the  point  of  contact. 

Each  cutaneous  area  which  has  a  local  sign  of  its  own  is  known 
as  a  sensory  circle,  for  the  reason  that  the  mind  does  not  refer  the 
sensation  to  a  point,  but  to  an  area  more  or  less  circular  in  outline. 
The  skin  may  therefore  be  regarded  as  composed  of  myriads  of 
such  circles  varying  in  size  in  different  regions  of  the  body. 

The  delicacy  of  the  localizing  power  in  any  part  of  the  skin  is 
determined  by  testing  the  power  which  the  part  possesses,  of 
distinguishing  the  sensations  produced  by  the  contact  of  the 
points  of  a  pair  of  compasses  placed  close  together.  The  distance 
to  which  the  points  must  be  separated  in  order  to  evoke  two  separate 
recognizable  sensations  is  a  measure  of  the  diameter  of  the  sensory 
circle.  Within  this  circle  the  two  sensations  become  fused  into  one 
sensation.  The  discriminative  sensibihty  of  different  regions  as 
determined  by  compass  points  is  shown  in  the  following  table;  the 
numbers  represent  the  distances  at  which  two  sensations  are  recog- 
nized : 

Tip  of  tongue, i.i 

Palmar  surface  of  third  phalanx  of  index-finger, 2.2 

Red  surface  of  Ups, 4.5 

Palmar  surface  of  first  phalanx  of  finger, 5.5 

Tip  of  nose, 6.8 

Palm  of  hand, 8.9 

Lower  part  of  forehead, 22.6 

Dorsum   of  hand, 31.6 

Dorsum  of  foot, 40.6 

Middle  of  the  back,   67.7 

The  discriminative  sensibility  of  any  portion  of  the  body  is  a 
function  of  its  mobility.  This  is  shown  by  the  fact  that  it  increases 
rapidly  from  the  shoulders  to  the  fingers  and  from  the  hips  to  the 
toes. 

The  Temperature  Sense. — The  sensations  of  heat  and  cold 
which  are  experienced  from  time  to  time  are  caused  by  changes  in 
the  temperature  of  the  skin  produced  in  a  variety  of  ways.  As 
these  sensations  are  specifically  different  from  those  of  touch,  as 
well  as  different  from  each  other,  it  is  highly  probable  that  for  each 
sensation  there  are  special  nerve-endings  distributed  throughout  the 
skin.  Investigations  have  shown  that  all  over  the  skin  there  are 
innumerable  spots  of  varying  size  which  if  stimulated  evoke 
sensations  of  heat  or  cold.  Such  points  are  termed  heat  and 
cold  spots.  Each  responds  to  but  one  kind  of  stimulus.  A  warm 
object  applied  to  a  heat  spot  will  evoke  a  sensation  of  warmth. 
It  will  have  no  effect  on  the  cold  spot.  The  reverse  is  also  true. 
Between  the  cold  and   heat  spots   there  are  areas  that  are  neutral 


THE  SENSE  OF   TOUCH. 


607 


insensitive  to  either  heat  or  cold.     The  cold  spots  are  more  numerous 
than  the  heat  spots  in  almost  all  regions  of  the  body. 

-'The  sensitivity  of  the  skin  to  temperature  changes  is  ver}'  acute, 
as  shown  by  the  fact  that  even  0.05°  C.  is  readily  appreciable.  This 
holds  true,  however,  only  when  the  temperature  of  the  object  lies 
between  27°  and  2,^°  C.  This  capability  varies  in  different  regions 
of  ''the  skin,  and  depends  on  the  number  of  heat  and  cold  spots 
present,  the  thickness  of  the  epidermis,  the  thermal  conductivity 
of  the  object  touching  it,  and  the  extent  to  which  it  is  habitually 
exposed  or  protected. 

The  physiologic  stimulus  to  the  thermic  end-organs  is  the  passage 


nil 


■Ji  ihii'^^^  lll•....•■••■ 
'••ili'  '--'ii  111'  •- 


ii'ii'i  1111 


ji'ji' 


„.-.  n,-: 


Fig.  273. — Cold  and  Hot  Spots  from  the  Anterior  Surface  of  the  Forearm. 
a.  Cold  spots,  b.  Hot  spots.  The  dark  parts  are  the  most  sensitive,  the  hatched 
the  medium,  the  dotted  the  feebly,  and  the  vacant  spaces  the  non-sensitive. — {Lan- 
dois  and  Stirling.) 

of  heat  through  the  skin  from  the  interior  of  the  body  to  the  sur- 
rounding air.  If  the  radiation  is  continuous  and  uniform,  the  end- 
organs  soon  adapt  themselves  to  the  temperature  of  the  surrounding 
air  and  the  sensation  of  heat,  under  physiologic  conditions,  is  not 
evoked.  If  there  is  a  sudden  rise  in  the  external  temperature  caused 
by  natural  or  artificial  means,  which  diminishes  the  radiation,  the 
temperature  of  the  skin  will  at  once  rise,  the  end-organs  will  be 
stimulated,  and  a  sensation  of  warmth  developed.  If,  on  the  other 
hand,  there  is  a  sudden  fall  in  temperature  and  an  increased  radia- 
tion, the  temperature  of  the  skin  will  fall,  the  end-organs  will  be 
stimulated,  and  a  sensation  of  cold  developed.  Experiment  also 
teaches  that  the  intensity  of  a  warm  or  cold  sensation  will  depend 


6o8  TEXT-BOOK  OF  PHYSIOLOGY. 

on  the  existing  temperature  of  the  skin,  and  not  upon  the  absolute 
temperature  of  the  object.  Thus,  water  at  20°  C.  will  evoke  a 
sensation  of  heat  or  cold  according  as  the  skin  has  previously  been 
cooled  below  or  warmed  above  this  temperature. 

The  Muscle  Sense. — As  a  result  of  the  activities  of  the  muscula- 
ture of  the  body  or  even  of  its  individual  parts,  there  arises  in  con- 
sciousness a  series  of  sensations,  which  are  termed  muscle  sensations. 
These  sensations  give  rise  to  the  perception — 

1.  Of  the  direction  and  duration  of  both  passive  (due  to  external 

causes)  and  active  movements  (due  to  internal,  volitional  efforts) 
which  take  place  without  hindrance ; 

2.  Of  the  resistance  offered  to  movements  by  external  bodies;  and — 

3.  Of  the  posture  of  the  body  or  of  its  individual  parts. 

As  to  the  seat  of  the  physiologic  processes  which  precede  and 
underHe  the  development  of  the  sensations  two  views,  at  least,  may 
be  advanced,  viz. : 

1.  That  the  processes  are  central  in  origin  and  partake  of  the  nature 

of  a  discharge  of  nerve  impulses  from  the  nerve-cells  through 
the  motor  nerves  to  the  muscles,  the  entire  process  being  accom- 
panied by  sensation.     This  is  known  as  the  innervation  theory. 

2.  That  the  processes  are  peripheral  in  origin,  initiated  by  stimulation 

of  specialized  end-organs  in  the  muscles  and  tendons  which  are 
connected  through  the  intermediation  of  afferent  nerves  with 
nerve-cells  in  the  cerebral  cortex. 
The  physiologic  mechanism  subserving  the  muscle  sense,  accord- 
ing to  the  second  theory,  now  held  by  many  physiologists,  thus  in- 
volves peripheral  end-organs,  afferent  nerves,  their  cortical  connec- 
tions and  nerve-cells  in  the  cerebral  cortex  at  or  near  the  junction 
of  the  superior  and  inferior  parietal  convolutions. 

The  End-organs. — These  are  small  fusiform  structures  found  in 
and  among  the  muscle  bundles  of  all  the  muscles  of  the  body  with 
the  exception  of  the  diaphragm  and  eye  muscles.  In  the  muscles  of 
the  arm  and  in  the  small  muscles  of  the  hand  they  are  especially 
abundant.  From  their  shape  they  are  known  as  muscle  spindles. 
They  vary  in  length  from  2  to  12  mm.  and  in  breadth  from  0.15 
to  0.4  mm.  Each  spindle  (Fig.  274)  consists  of  a  connective-tissue 
capsule  containing  from  two  to  ten  longitudinally  arranged  striated 
muscle  fibers  of  fine  diameter.  In  the  middle  or  equatorial  region 
of  these  intra-jusal  fibers  there  is  frequently  found  a  quantity  of 
non-striated  protoplasmic  matter.  The  spindle  is  supplied  with  both 
sensor  and  motor  nerves.  The  sensor  fiber  loses  its  external  invest- 
ments as  it  approaches  the  capsule.  The  naked  axis-cylinder  then 
penetrates  the  capsule,  and  after  dividing  several  times  terminates  in 
a  ribbon-hke  or  spiral  manner  around  the  intra-fusal  muscle  fiber. 
This  ending  was  described  by  and  is  known  as  Ruffini's  "annulo- 


THE  SENSE  OF   TOUCH. 


609 


spiral  ribbon."  The  motor  nerve  also  penetrates  the  capsule  and 
terminates  in  the  polar  extremities  of  the  intra-fusal  fiber.  Sensor 
end-organs  supposed  to  be  connected  with  the  muscle  sense  are  also 
found  in  the  tendons  of  muscles. 

Afferent  Nerves. — That  muscles  are  abundantly  supplied  with 
afferent  nerves  has  been  proved  by  different  methods  of  investigation. 
With  histologic  methods  Sherrington  has  traced  afferent  fibers  from 
the  muscle  spindles  directly  into  the  spinal  nerve  ganglia.  The  con- 
tractions of  muscles  from  electric  stimulation  as  well  as  the  con- 
tractions known  as  muscle  cramp,  due  to  unknown  agents,  give  rise 
to  sensations  of  pain,  a  fact  which  indicates  the  presence  in  muscles 
of  afferent  or  sensor  nerves. 

Cortical  Area. — Pathologic  findings  have  shown  that  an  im- 
pairment or  a  loss  of  the  muscle  sense  is  associated  with  de- 
structive   lesions    of    perhaps    the    superior    and    inferior    parietal 


Fig.  274. — A  Neuro-muscle  Spindle  of  a  Cat.  (Ruffini.)  c.  Capsule,  pr.  e. 
Primary  ending.  5.  e.  Secondary  ending,  pi.  e.  Plate  ending.  (All  these  are 
probably  sensor  in  function.) — {Starling's  "Physiology.") 


convolutions  (Fig.  226).  In  a  case  reported  by  Starr  the  removal 
of  a  small  tumor  in  the  pia  mater  situated  over  the  junction 
of  the  superior  and  inferior  parietal  lobules  was  followed  by  a 
loss  of  the  muscle  sense  and  marked  ataxia  in  the  right  hand 
for  a  period  of  six  weeks,  after  which  recovery  took  place.  These 
symptoms  were  attributed  to  injury  of  the  cortex  from  unavoidable 
surgical  procedures. 

The  muscle  sensations,  as  stated  in  foregoing  paragraphs,  form 
the  basis  of  the  perception  not  only  of  the  direction  and  the  duration 
of  a  body  movement  and  the  resistance  experienced,  but  also  of  the 
position  and  the  tension  of  the  muscle  groups.  The  latter  fact 
more  especially  makes  it  possible  for  the  mind  to  direct  the  muscles 
and  to  graduate  the  energy  necessary  to  the  accomphshment  of  a 
definite  purpose. 

Active  Touch. — x^ctive  touch  or  the  apphcation  of  the  fingers 
to  the  surfaces  of  external  objects  implies  the  cooperation  of  the 
skin  and  the  muscles.  The  sensations  which  are  evoked  are  combina- 
39 


6io 


TEXT-BOOK  OF  PHYSIOLOGY. 


tions  of  contact  and  muscle  sensations.  The  union  of  these  sensa- 
tions forms  the  basis  of  the  perception  of  hardness,  softness,  smooth- 
ness, and  roughness  of  bodies. 


THE   SENSE   OF   TASTE. 

The  physiologic  mechanism  involved  in  the  sense  of  taste  includes 
the  tongue,  the  gustatory  nerves  (the  chorda  tympani  and  the  glosso- 
phar}'ngeal),  their  cortical  connections  and  nerve-cells  in  the  gray 
matter  of  the  fourth  temporal  convolutions.  The  peripheral 
excitation  of  this  apparatus  gives  rise  to  nerve  impulses  v^hich 
transmitted  to  the  brain  evoke  the  sensations  of  taste.  The 
specific  physiologic  stimulus  is  matter,  organic  and  inorganic,  in  a 
state  of  solution. 

The  Tongue. — The  tongue  consists  of  both  intrinsic  and  extrinsic 
muscles,  in  virtue  of  which  it  is  susceptible 
of  both  a  change  in  shape  and  position. 
The  movements  of  the  tongue,  though  not 
essential  to  taste,  are  made  use  of  in  the 
finer  discrimination  of  tastes. 

The  tongue  is  covered  over  by  mucous 
membrane  continuous  with  that  lining  the 
oral  cavity.  The  dorsum  of  the  tongue 
presents  a  series  of  papillae  richly  suppHed 
with  blood-vessels  and  nerves.  Of  these 
there  are  three  varieties,  the  filiform,  the 
fungiform,  and  the  circum vallate  (Fig.  275). 

1.  The  filiform  papillcB,  the  most  numerous, 
cover  the  anterior  two -thirds  of  the 
tongue;  they  are  conical  or  filiform  in 
shape  and  covered  with  homy  epithe- 
lium which  is  often  prolonged  into 
filamentous  tufts. 

2.  The  fungiform  papillce,  found  chiefly  at 
the  tip  and  sides  of  the  tongue,  are 
less  numerous  but  larger  than  the  pre- 

ceding and  of  a  deep  red  color. 

3.  The  circumvallate  papillce,  from  eight  to  ten  in  number,   are 

situated  at  the  base  of  the  tongue  arranged  in  the  form  of  the 

letter  V.     They  consist  of  a  central  projection  surrounded  by 

a  wall  or  circumvallation  from  which  they  take  their  name. 

The  Peripheral  End-organs.    The  Taste-buds. — Embedded 

in  the  epithelium  covering  the  mucous  membrane  not  only  of  the 

tongue  but  of  the  palate  and  posterior  surface  of  the  epiglottis  are 

small  ovoid  bodies  which  from  their  relation  to  the  gustatory  nerves 


Fig.  275. — The  Tongue 
I.  Papillae  circumval- 
latae.  2.  Papillae  fungi 
formes. 


THE  SENSE  OF   TASTE. 


6ii 


^;%!^^V  > 


Fig. 


are  regarded  as  their  peripheral  end-organs  and  known  as  taste-buds 
or  taste-beakers.  Each  bud  is  ovoid  in  shape  (Fig.  276).  Its  base 
rests  on  the  tunica  propria ;  its  apex  comes  up  to  the  epithehum,  where 
it  presents  a  narrow  funnel-shaped  opening,  the  taste-pore.  The  wall 
of  the  bud  is  composed  of  elongated  curved  epithelium.  The 
interior  contains  narrow  spindle-shaped  neuro-epithelial  cells  pro- 
vided at  their  outer  extremity  with  stiff  hair-like 
filaments  which  project  into  the  taste-pore. 

The  neuro-epithelial  cells  are  in  physiologic 
relation  with  the  nerves  of  taste.  The  terminal 
branches,  after  entering  the  bud  at  its  base, 
develop  fine  tufts  which  come  into  contact  with 
the  cells.  That  the  taste-buds  are  connected  with 
the  nerves  of  taste  is  rendered  probable  from  the 
fact  of  their  degeneration  after  division  of  the 
nerves. 

The  Taste  Area.— The  taste  area,  though 
confined  for  the  most  part  to  the  tongue,  extends 
itself  in  different  individuals  to  the  mucous  mem- 
brane of  the  hard  palate,  to  the  anterior  sur- 
face of  the  soft  palate,  to  the  uvula,  the  anterior 
and  posterior  half  arches,  the  tonsils,  the  posterior 
wall  of  the  pharynx,  and  the  epiglottis. 

The  Taste  Sensations.  —  The  sensations 
which  arise  in  consequence  of  impressions  made 
by  different  substances  on  the  peripheral  appara- 
tus of  this  area  are  in  so  many  instances  com- 
binations of  taste,  touch,  temperature,  and  smell 
that  they  are  extremely  difficult  of  classification. 
Nevertheless  four  primary  tastes  can  be  recog- 
nized: viz.,  bitter,  sweet,  acid  or  sour,  salt  or  sahne. 
Though  the  contact  of  any  bitter,  sweet,  acid, 
or  salt  substance  with  any  part  of  the  tongue 
will,  if  the  substance  be  present  in  sufficient 
quantity  or  concentration,  develop  a  corresponding 
sensation,  some  regions  of  the  tongue  are  more  sensitive  and  respon- 
sive than  others.  Thus,  the  posterior  portion  is  more  sensitive  to 
bitter  substances  than  the  anterior;  the  reverse  is  true  for  sweet 
substances  and  perhaps  for  acids  and  salines. 

The  intensity  of  the  resulting  sensation  in  any  given  instance 
will  depend  on  the  degree  of  concentration  of  the  substance,  while 
its  massiveness  will  depend  on  the  area  affected. 


276. — Taste- 
bud  FROM  ClR- 
cumvallate 
Papilla  of  a 
Child.  The 
oval  structure  is 
limited  to  the 
epithehum  (e) 
Uning  the  fur- 
row, encroach- 
ing slightly 
upon  the  adja- 
cent connective 
tissue  .  (/) ;  o, 
taste-pore 
through  which 
the  taste  -  cells 
communicate 
with  the  mucous 
surface. —  {After 
Piersol.) 


6i2  TEXT-BOOK  OF  PHYSIOLOGY. 

THE   SENSE  OF   SMELL. 

The  physiologic  mechanism  involved  in  the  sense  of  smell  in- 
cludes the  nasal  fossae,  the  olfactory  nerves,  the  olfactory  tracts,  and 
nerve-cells  in  those  areas  of  the  cortex  known  as  the  uncinate  con- 
volution and  anterior  part  of  the  gyrus  fornicatus.  Peripheral  stimu- 
lation of  this  mechanism  develops  nerve  impulses  which,  transmitted 
to  the  cortex,  evoke  the  sensations  of  odor.  The  specific  physiologic 
stimulus  is  matter  in  the  gaseous  or  volatile  state. 

The  Nasal  Fossae. — The  nasal  fossae  are  irregularly  shaped 
cavities  separated  by  a  vertical  septum  formed  by  the  perpendicular 
plate  of  the  ethmoid  bone,  the  vomer,  and  the  triangular  cartilage. 
The  outer  wall  presents  three  recesses  separated  by  the  projection 
inward  of  the  turbinated  bones.  Each  fossa  opens  anteriorly  and 
posteriorly  by  the  anterior  and  posterior  nares,  the  latter  com- 
municating with  the  pharynx.  Both  fossae  are  lined  throughout  by 
mucous  membrane.  The  upper  part  of  the  fossa  is  known  as  the 
olfactory,  the  lower  portion  as  the  respiratory  region.  In  the  former, 
the  mucous  membrane  over  the  septum  and  superior  turbinated 
bone  is  somewhat  thicker  than  elsewhere  and  covered  with  a  neuro- 
epithelium  which  constitutes — 

The  Peripheral  End-organ.^ — This  consists  of  a  basement 
membrane  supporting  two  kinds  of  cells,  the  olfactory  and  the 
sustentacular.  The  olfactory  cells  are  bipolar  nerve-cells,  the  center 
of  which  contains  a  large  spheric  nucleus.  The  peripheral  pole 
is  cylindric  or  conic  in  shape  and  provided  at  its  extremity  with 
several  hair-like  processes.  The  central  pole  becomes  the  axon 
process  and  passes  directly  to  the  olfactory  bulb. 

The  sustentacular  cells  are  epithelial  in  character  and,  as  their 
name  implies,  support  or  sustain  the  olfactory  cells. 

For  the  appreciation  of  odorous  particles  the  air  must  be  drawn 
through  the  nasal  fossae  with  a  certain  degree  of  velocity.  If  the 
particles  are  widely  diffused  in  the  air,  they  must  be  drawn  not  only 
more  quickly  but  more  forcibly  into  contact  with  the  olfactory 
hairs,  as  in  the  act  of  sniflfing,  the  result  of  short  energetic  inspira- 
tions. To  many  substances  the  olfactory  apparatus  is  extremely 
sensitive.  Thus,  it  has  been  shown  that  a  particle  of  mercaptan 
the  actual  weight  of  which  was  calculated  to  be  ^,;^-^,jn„  of  a  milli- 
gram gave  rise  to  a  distinct  sensation. 

The  Olfactory  Sensations. — The  sensations  which  arise  in 
consequence  of  the  excitation  of  the  olfactory  apparatus  are  very 
numerous  and  their  classification  is  extremely  difficult.  For  this 
reason  it  is  customary  to  divide  them  into  two  groups:  viz.,  agreeable 
and  disagreeable,  in  accordance  with  the  feelings  they  excite  in  the 
individual.     As  the  olfactory  sensations  give  rise  to  feelings  rather 


THE  SENSE  OF  SMELL.  613 

than  ideas,  this  sense  plays  in  man  a  subordinate  part  in  the  acquisi- 
tion of  knowledge.  In  lower  animals  this  sense  is  employed  for  the 
purpose  of  discovering  and  securing  food,  for  detecting  enemies  and 
friends,  and  for  sexual  purposes.  In  land  animals  the  entire  olfac- 
tory apparatus  is  well  developed  and  the  sense  keen ;  in  some  aquatic 
animals,  as  the  dolphin,  whale,  and  seal,  the  apparatus  is  poorly 
developed  and  the  sense  dull. 


CHAPTER  XXV. 
THE  SENSE  OF  SIGHT. 

The  physiologic  mechanism  involved  in  the  sense  of  sight  in- 
cludes the  eyeball,  the  optic  nerve,  the  optic  tracts,  their  cortical 
connections,  and  nerve-cells  in  the  cuneus  and  adjacent  gray  matter. 
Peripheral  stimulation  of  this  mechanism  develops  nerve  impulses 
w^hich  transmitted  to  the  cortex  evoke  (i)  the  sensation  of  light  and 
its  different  qualities — colors;  (2)  the  perception  of  light  and  color 
under  the  form  of  pictures  of  external  objects;  and  (3)  in  connection 
vi^ith  the  ocular  muscles,  the  production  of  muscle  sensations  by 
which  the  size,  distance,  and  direction  of  objects  may  be  judged. 

The  specific  physiologic  stimulus  to  the  terminal  end-organ, 
the  retina,  is  the  impact  of  ether  vibrations.  In  general,  it  may 
be  said  that,  at  least  for  the  same  color,  the  intensity  of  the  objective 
vibration  determines  the  intensity  of  the  sensation. 

THE  PHYSIOLOGIC  ANATOMY  OF  THE  EYEBALL. 

The  eyeball  is  situated  at  the  fore  part  of  the  orbit  cavity,  and 
in  such  a  position  as  to  permit  of  an  extensive  range  of  vision.  It 
is  loosely  held  in  position  by  a  fibrous  membrane,  the  capsule  of 
Tenon,  which  is  attached,  on  the  one  hand,  to  the  eyeball  itself, 
and,  on  the  other,  to  the  walls  of  the  orbit  cavity.  Thus  suspended, 
the  eyeball  is  susceptible  of  being  moved  in  any  direction  by  the 
contraction  of  the  muscles  attached  to  it. 

The  ball  is  spheroid  in  shape,  measuring  about  24  millimeters 
in  its  antero-posterior  diameter  and  a  little  less  in  its  transverse 
and  vertical  diameters.  When  viewed  in  profile,  it  is  seen  to  con- 
sist of  the  segments  of  two  spheres,  of  which  the  posterior  is  the 
larger,  occupying  five-sixths,  and  the  anterior  is  the  smaller,  occupy- 
ing one-sixth  of  the  ball.  It  is  composed  of  several  concentrically 
arranged  membranes  enclosing  various  refracting  media  essential  to 
vision. 

The  membranes,  enumerating  them  from  without  inward,  are 
as  follows:  the  sclera  and  cornea,  the  chorioid  and  iris,  and  the 
retina.  The  refracting  media  are  the  aqueous  humor,  the  crystal- 
line lens,  and  the  vitreous  humor. 

The  Sclera  and  Cornea. — The  sclera  is  the  thick  opaque  mem- 
brane covering  the  posterior  five-sixths  of  the  ball.     It  is  composed 

614 


THE   SENSE   OF   SIGHT.  615 

of  layers  of  connective  tissue  which  are  arranged  transversely  and 
longitudinally.  It  is  pierced  posteriorly  by  the  optic  nerve  about 
3  or  4  millimeters  internal  to  the  optic  axis.  By  virtue  of  its  firmness 
and  density  the  sclera  gives  form  to  the  eyeball,  protects  delicate 
structures  enclosed  by  it,  and  serves  for  the  attachment  of  the  muscles 
by  which  the  ball  is  moved.  The  cornea  is  the  transparent  mem- 
brane forming  the  anterior  one-sixth  of  the  ball.  It  is  nearly  circular 
in  shape,  measuring  in  its  horizontal  meridian  12  mm.,  in  its  vertical 
meridian  11  mm.  The  curvature  is  therefore  sharper  in  the  latter 
than  in  the  former.  The  radius  of  curvature  of  the  anterior  surface 
at  that  central  portion  ordinarily  used  in  vision  is  7.829  mm.;  that 
of  the  posterior  surface  about  6  mm. 

The  substance  of  the  cornea  is  made  up  of  thin  layers  of  delicate 
transparent  fibrils  of  connective  tissue  continuous  with  those  found 
in  the  sclera.  Lymph-spaces  are  present  throughout  the  cornea,  in 
which  are  to  be  found  l}TTiph-corpuscles.  The  anterior  surface  of 
the  cornea  is  covered  with  several  layers  of  nucleated  epithelium 
supported  by  a  structureless  membrane,  the  anterior  elastic  lamina. 
The  posterior  surface  also  is  covered  by  a  layer  of  epithelium  sup- 
ported by  a  similar  membrane,  the  membrane  of  Descemet,  which 
at  its  periphery  becomes  continuous  with  the  iris.  At  the  junction 
of  the  cornea  and  sclera  there  is  a  circular  groove,  known  as  the 
canal  of  Schlemm. 

The  Chorioid,  Iris,  Ciliary  Muscle,  and  Ciliary  Processes. — 
The  chorioid  is  the  dark  brown  membrane  which  extends  forward 
nearly  to  the  cornea,  where  it  terminates  in  a  series  of  folds,  the 
cihary  processes.  Posteriorly,  it  is  pierced  by  the  optic  nerve.  It 
is  composed  largely  of  blood-vessels,  arteries,  capillaries,  and  veins, 
supported  by  connective  tissue.  Externally  it  is  loosely  connected 
to  the  sclera ;  internally  it  is  fined  by  a  layer  of  hexagonal  ceUs  con- 
taining black  pigment  which,  though  usually  described  as  a  part  of 
the  chorioid,  is  now  known  to  belong,  embryologicly  and  physio- 
logicly,  to  the  retina.  Lying  within  the  outer  layer  of  arteries  and 
veins  there  is  a  thick  layer  of  small  arterioles  and  capillaries,  known 
as  the  chorio-capillaris.  The  chorioid  with  its  contained  blood-vessels 
bears  an  important  relation  to  the  nutrition  and  function  of  the  eye. 
It  provides  a  free  supply  of  lymph  and  presents  a  uniform  temperature 
to  the  retina  in  contact  with  it  (Fig.  277). 

The  iris  is  the  circular,  variously  colored  membrane  in  the  anterior 
part  of  the  eye  just  behind  the  cornea.  It  presents  a  fittle  to  the 
nasal  side  of  the  center  a  circular  opening,  the  pupil.  The  outer  or 
circumferential  border  is  united  by  connective  tissue  to  the  cornea, 
sclera,  and  ciliary  muscle;  the  inner  border  forms  the  boundary  of 
the  pupil.  The  iris  consists  of  a  framework  of  connective  tissue  sup- 
porting blood-vessels,  muscle-fibers,  and  pigmented  connective-tissue 


6x6 


TEXT-BOOK  OF  PHYSIOLOGY 


5^ 


cells.  The  anterior  surface  is  covered  by  a  layer  of  cells  continuous 
with  those  covering  the  posterior  surface  of  the  cornea.  The  pos- 
terior surface  is  formed  by  a  thin  structureless  membrane  supporting 
a  layer  of  pigment  cells  continuous  with  those  lining  the  chorioid. 
The  color  which  the  iris  presents  in  different  individuals  depends  on 
the  relative  amount  of  pigment  in  the  connective-tissue  corpuscles. 
In  blue  eyes  the  pigment  is  wanting.  In  gray,  brown,  and  black 
eyes  the  pigment  is  present  in  progressively  increasing  amounts.     The 

blood  -  vessels  are 
connected  with 
those  of  the  chori- 
oid coat. 

The  muscle- 
fibers  are  of  the 
non-striated  variety 
and  arranged  in 
two  sets,  one  cir- 
cularly, the  other 
radially,  disposed. 

The  circular 
fibers  are  found 
close  to  the  pupil 
near  the  posterior 
surface  of  the  iris. 
Contraction  of  this 
band  of  fibers  di- 
minishes, relaxation 
increases,  the  size 
of  the  pupil.  This 
muscle  is  known 
as  the  sphincter 
pupilla  or  sphincter 
iridis. 

The  radial 
fibers  form  a  more 
or  less  continuous 
layer  in  the  posterior  part  of  the  iris,  extending  from  the  margin 
of  the  pupil,  where  they  blend  with  the  circular  fibers,  to  the  outer 
border.  Contraction  of  the  fibers  enlarges  the  size  of  the  pupil. 
The  muscle  is  known  as  the  dilatator  pupilla. 

The  nerves  exciting  the  circular  fibers  to  action  are  the  ciliary 
nerves,  axons  of  nerve-cells  located  in  the  ciliary  or  ophthalmic  gan- 
glion. Stimulation  Of  these  fibers  gives  rise  to  contraction  of  the 
sphincter  and  diminution  in  the  size  of  the  pupil.  The  nerves  excit- 
ing the  dilatator  fibers  are  axons  of  nerve-cells  located  in  the  superior 


Fig.  277. — Chorioid  Coat  of  the  Eye. — i.  Optic 
nerve.  2,  2,  2,  2,  3,  3,  3,  4.  Sclerotic  coat  divided 
and  turned  back  to  show  the  chorioid.  5,  5,  5,  5. 
The  cornea,  divided  into  four  portions  and  turned 
back.  6,  6.  Canal  of  Schlemm.  7.  E.xternal 
surface  of  the  chorioid,  traversed  by  the  ciliary 
nerves  and  one  of  the  long  ciliary  arteries.  8. 
Central  vessel  into  which  open  the  vasa  vorticosa. 
9,  9,  10,  10.  Chorioid  zone.  11,  11.  Ciliary 
nerves.  12.  Long  ciliary  artery.  13,  13,  13,  13. 
Anterior  ciliary  arteries.  14.  Iris.  15,  15.  Vas- 
cular circle  of  the  iris.     16.  Pupil. — (Sappey.) 


THE  SENSE  OF  SIGHT. 


617 


cervical  ganglion.  They  reach  the  iris  by  way  of  the  cervical  sympa- 
thetic, the  ophthalmic  division  of  the  fifth,  and  the  long  ciHary  nerve. 
Stimulation  of  these  nerves  is  followed  by  dilatation  of  the  pupil. 
Both  the  cihary  and  superior  cervical  ganglia  are  in  relation  with 
pre-ganglionic  fibers  coming  from  the  central  nerve  system.  (See 
page  551.) 

The  ciliary  muscle  is  a  gray  circular  band  about  two  millimeters 
in  width,  consisting  of  non-striated  muscle-fibers.  The  majority  of 
these  fibers  pursue  a  radial  or  meridional  direction.  Taking  their 
origin  from  the  junction  of  the  sclera,  cornea,  and  iris,  they  pass 
backward  to  be  inserted  into  the  chorioid  coat  opposite'  the  ciliary 
processes.  The  inner  portion  of  the  muscle  is  interrupted  by  bundles 
of  fibers  which  pursue  a  circular  direction.  They  collectively  con- 
stitute the  annular  or  ring  muscle  of  Muller.     The  ciliary  muscle  in 


Fig.  2 78. — Section  through  the  Ciliary  Region  of  the  Human  Eye.  a.  Radi- 
ating bundles  of  the  ciliary  muscle,  b.  Deeper  bundles,  c.  Circular  network. 
d.  Annular  muscle  of  Muller.  e.  Tendon  of  ciliary  muscle.  /.  Muscle-fibers 
on  posterior  side  of  the  iris.  g.  Muscles  on  the  cihary  border  of  the  same.  h. 
Ligamentum  pectinatum. — {After  Iwanoff.) 


common  with  the  circular  fibers  of  the  iris  receives  its  nerve-supply 
direct  from  the  nerve- cells  in  the  cihary  ganghon.  Contraction  of 
the  cihary  muscle  tenses  the  chorioid  coat,  and  for  this  reason  it  is 
frequently  termed  the  tensor  chorioidecE. 

The  Retina. — The  retina  is  the  internal  coat  of  the  eye,  extending 
forward  almost  to  the  cihary  processes,  where  it  terminates  in  an 
indented  border,  known  as  the  ora  serrata.  In  the  living  condition 
it  is  clear,  transparent,  and  pink  in  color.  After  death  it  becomes 
opaque.  The  retina  is  abundantly  supplied  with  blood-vessels,  de- 
rived from  the  arteria  centralis  retince,  a  branch  of  the  ophthalmic, 
which  pierces  the  optic  nerve  near  the  sclera,  runs  forward  in  its 
center,  to  the  retina,  in  which  its  terminal  branches  are  distributed. 
The  veins  arising  from  the  capillary  plexus  leave  the  retina  by  the 
same  route. 


6i8 


TEXT-BOOK  OF  PHYSIOLOGY. 


In  the  posterior  portion  of  the  retina,  at  a  point  corresponding 
with  the  axis  of  vision,  there  is  a  small  oval  area  about  2  mm.  in 
its  transverse  and  about  0.8  mm.  in  its  vertical  diameter.     From  the 

fact  that  it  presents  a 
Pigment-layer  (not  shown).  ycllow  appearance,it  is 
known  as  the  macula 
lutea.  This  area  pre- 
sents in  its  center  a 
depression  with  slop- 
ing sides,  known  as 
the  jovea  centralis. 
About  3.5  mm.  to  the 
nasal  side  of  the 
macula  is  the  point  of 
entrance  of  the  optic 
nerve. 

The  retina  is  re- 
markably complex  in 
structure,  presenting 
an  appearance  when 
viewed  microscopic- 
ally, something  like 
that  represented  in 
Fig.  279,  indicating 
that  it  is  composed  of 
different  cellular  ele- 
have  been  named,  from  behind 


Fig. 


279. — Vertical   Section   of 
— (Schaper.) 


2.  Layer  of  rods  and  cones- 


3.   External  limiting  membrane 


4.  Outer  nuclear  layer, 


5.  Outer  molecular  layer. 

6.  Inner  nuclear  layer. 

7.  Inner  molecular  layer. 

8.  Layer  of  ganglion  cells. 

9.  Layer  of  nerve-fibers. 

Human   Retina. 


ments  arranged  in  layers.     These 
forward,  as  follows : 

1.  The  layer  of  pigment  cells. 

2.  The  layer  of  rods  and  cones,  or  Jacobson's  layer. 
The  external  limiting  membrane. 
The  outer  nuclear  or  granular  layer. 
The  outer  molecular  or  reticular  layer. 
The  inner  nuclear  or  granular  layer. 
The  inner  molecular  or  reticular  layer. 

8.  The  layer  of  gangUon  cells. 

9.  The  layer  of  nerve-fibers. 

Modern  histologic  methods  of  research  have  made  it  possible  to 
reduce  the  retina,  exclusive  of  the  pigment  cells,  to  three  successive 
layers  of  nerve-cells,  supported  by  a  highly  developed  neurogha, 
forming  what  has  been  termed  the  fibers  of  Miiller.  These  nerve- 
cells  are  as  follows : 

1.  The  visual  cells. 

2.  The  bipolar  cells. 

3.  The  ganglion  cells. 


THE  SENSE  OF  SIGHT. 


619 


The  relation  of  these  nerve-cells  one  to  another  and  to  the  supporting 
neuroglia  tissue  and  the  manner  in  which  they  unite  to  form  the 
above-mentioned  layers  are  schematicly  shown  in  Fig.  280. 

The  pigment  layer  is  composed  of  hexagonal  cells.  Though 
formerly  described  as  forming  a  part  (the  inner  layer)  of  the  chorioid, 
these  cells  belong  embryologicly  to  the  retina.  From  their  retinal 
surface  dehcate  pigmented  processes  extend  into  and  between  the 
rods  and  cones.  On  expo- 
sure to  light  these  processes 
elongate  and  push  themselves 
between  the  rods.  In  the 
dark  they  retract  and  with- 
draw into  the  cell-body. 

The  visual  cells  which 
form  the  layer  of  rods  and 
cones  are  of  two  varieties,  the 
rod-shaped  and  the  cone- 
shaped. 

The  rod-shaped  visual  cells 
consist  of  a  straight  elongated 
cylinder  extending  through  the 
entire  thickness  of  Jacobson's 
membrane  and  a  fine  fiber 
containing  a  nucleus,  which, 
after  piercing  the  external 
limiting  membrane,  passes 
into  the  outer  molecular  layer, 
where  it  terminates  in  a  spheric 
enlargement.  The  outer  por- 
tion of  the  rod  is  clear  and 
homogeneous,  though  contain- 
ing a  pigment  known  as  visual 
purple  or  rhodopsin ;  the  inner 
portion  of  the  rod  is  slightly 
granular. 

The  cone  -  shaped  visual 
cells  also  consist  of  two  por- 
tions, a  conic  portion  situated  in  Jacobson's  membrane  between  the 
rods,  and  a  fine  fiber,  containing  a  nucleus,  which,  after  piercing  the 
external  limiting  membrane,  passes  into  the  outer  molecular  layer, 
where  it  terminates  in  a  fine  tuft.  The  inner  portion  of  the  cone  is 
thicker  than  the  rod  and  rests  on  the  hmiting  membrane;  the  outer 
portion  tapers  to  a  fine  point  and  is  known  as  the  cone-style.  The 
cones,  as  a  rule,  are  shorter  than  the  rods.  The  proportion  of  rods 
to  cones  varies  in  different  parts  of  the  retina,  though  there  are  on 


Fig 


280. — Cross-section  of  the  Retina 
FROM  A  Mammal.  A.  Layer  of  rods 
and  cones.  B.  Visual  cells  (outer  gran- 
ules). C.  Outer  molecular  layer.  E. 
Bipolar  cells  (inner  granules).  F.  In- 
ner molecular  layer.  G.  Ganglion  cells. 
H.  Layer  of  nerve-fibers,  a.  Rods.  b. 
Cones,  e.  Bipolar  rod.  f.  Bipolar 
cone.  r.  Lower  ramification  of  a  bi- 
polar rod.  f.  Lower  ramification  of  a 
bipolar  cone,  g,  h,  i,  j,  k.  Ganglion 
cells  in  various  stages,  branching  from 
F.  X,  z.  Bipolar  contact  of  rods  and 
cones,  t.  Miiller's  supporting  fibers. 
S.  Centrifugal  nerve-fibers. — {After  Ra- 
mon y  Cajal.) 


620  TEXT-BOOK  OF  PHYSIOLOGY. 

the  average  about  fourteen  rods  to  one  cone.  In  the  macula  the  rods 
are  almost  entirely  absent,  cones  alone  being  present. 

The  layer  of  visual  cells  together  with  the  neuroglia  constitute  the 
first  three  layers  of  the  retina  proper.  The  external  limiting  mem- 
•brane  is  formed  by  the  blending  of  the  ends  of  neuroglia  cells. 

The  bipolar  cells  consist  of  a  central  portion,  found  in  the  inner 
nuclear  layer,  from  which  are  given  off  two  processes  which  pass  in 
opposite  directions,  one  toward  the  visual  cells,  the  other  toward  the 
ganglion  cells.  The  former  terminate  in  tufts  which  arborize  around 
the  tufts  and  spheric  enlargements  of  the  visual  cells,  and  assist  in 
the  formation  of  the  outer  molecular  layer;  the  latter  terminate  in 
similar  tufts  in  the  inner  molecular  layer. 

The  ganglion  cells  are  arranged  in  a  single  layer,  as  a  rule.  They 
are  large  and  nucleated.  From  the  inner  side  of  each  cell  there  is 
given  off  a  single  axon  which  passes  toward  the  center  of  the  retina 
(forming  the  nerve-fiber  layer),  where  it  enters  and  assists  in  forming 
the  optic  nerve.  From  the  outer  side  of  the  ganglion  cell  dendrites 
pass  into  and  assist  in  forming  the  inner  molecular  layer.  These  den- 
drites come  into  physiologic  relation  with  those  of  the  inner  pro- 
cesses of  the  bipolar  cells. 

Horizontally  disposed  nerve-cells  are  also  present  in  the  outer 
molecular  layer  in  relation  with  the  visual  cells.  Spongioblasts  or 
amacrine  cells  are  also  present  at  the  border  of  and  in  the  inner  molec- 
ular layer. 

From  the  relation  of  the  ganglion  cells,  from  which  the  optic  nerve- 
fibers  take  their  origin,  to  the  visual  cells  and  the  bipolar  cells,  the 
latter  may  be  regarded  as  the  terminal  visual  organ,  the  intermediate 
between  the  ether  vibrations  and  the  ganglion  cell.  The  visual  cells 
are  directed  toward  the  chorioid,  away  from  the  entering  light, 
dipping  into  the  pigment  cells.  They,  with  the  pigment  layer,  are 
the  elements  by  which  the  ether  vibrations  are  transformed  into  ner\'e 
energy. 

In  the  fovea  most  of  the  retinal  elements  are  wanting  or  are 
reduced  in  thickness.  The  cones  alone  are  present.  The  cone-fibers 
with  their  nuclei  are  directed  obliquely  upward  and  outward  along 
the  slope  of  the  fovea,  to  end  in  tufts  which  come  into  physiologic 
relation  with  the  dendrites  of  the  ganglion  cells  which  at  the  top  of 
the  fovea  are  generally  increased  in  number  (Fig.  281). 

It  is  estimated  that  the  optic  nerve  contains  about  500,000  nerve- 
fibers,  and  that  for  each  fiber  there  are  about  7  cones,  100  rods,  and 
7  pigment  cells.  In  accordance  with  this  estimate  there  would  be 
about  3,500,000  cones,  50,000,000  rods,  and  3,500,000  pigment  cells. 
The  distance  between  the  centers  of  two  adjacent  cones  in  the  fovea 
is  4  micromillimeters. 


THE  SENSE  OF  SIGHT.  621 

The  vitreous  humor  is  the  largest  of  the  refracting  media  and 
occupies  by  far  the  largest  portion  of  the  interior  of  the  eyeball.  From 
its  position  it  gives  support  to  the  retina.  Anteriorly  it  presents  a 
concavity,  in  which  the  crystalline  lens  is  lodged.  The  vitreous  humor 
consists  of  water  (97  per  cent.),  organic  matter  and  salts,  enclosed  in  a 
transparent  membrane,  the  tunica  hyaloidea.  The  mass  of  the  vitre- 
ous humor  is  penetrated  by  a  species  of  connective  tissue. 

The  aqueous  humor  is  small  in  amount  in  comparison  with  the 
vitreous  and  is  found  in  the  space  bounded  by  the  cornea,  the  ciliary 
body,   the  suspensory  ligament,  and  the  lens.     The  projection  of 


:^ 


®^«e 


Fig.  281. — Horizontal  Section  through  the  Macula  and  Fovea  of  a  Man 
Sixty  Years  Old.  The  section  is  not  through  the  exact  center  of  the  fovea, 
for  there  are  only  cone  visual  cells  and  no  remnants  of  the  confluence  of  the  inner 
granule  and  ganglion  cell  layers  are  present,  i.  Cones.  2.  External  limiting 
membrane.  3.  Outer  nuclear  layer.  4.  Henle's  fiber  layer.  5.  Outer  molec- 
ular or  reticular  layer.  6.  Inner  nuclear  layer.  7.  Inner  molecular  or  reticular 
layer.  8.  Layer  of  ganglion  cells.  9.  Nerve-fiber  layer. — {After  Schaper, 
Stohr's  "Histology") 

the  iris  into  this  space  partially  divides  into  an  anterior  and  posterior 
portion  or  chamber.  The  aqueous  humor  is  a  clear,  w^atery,  alkaline 
fluid  derived  from  or  secreted  by  the  capillary  blood-vessels  of  the 
ciliary  body.  From  this  origin  it  passes  through  the  pupil  into  the 
anterior  chamber.  It  serves  to  keep  the  cornea  tense  and  smooth. 
The  ocular  tension  partly  depends  on  the  presence  of  this  fluid  in 
the  eyeball.  There  is  every  reason  for  believing  that  there  is  a 
constant  stream  of  fluid  from  the  blood-vessels  into  the  eye  and 
from  the  eye  through  the  spaces  of  Fontana  at  the  base  of  the  iris 
into  the  canal  of  Schlemm,  and  so  into  the  blood.  Any  interference 
with  the  exit  of  this  fluid  rapidly  increases  the  ocular  tension. 


622 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  lens  is  the  transparent  biconvex  body  situated  just  behind 
the  iris,  in  the  concavity  of  the  vitreous.  The  thickness  of  the  lens 
is  3.6  mm.,  the  diameter  about  9  mm.  It  consists  of  a  transparent 
capsule  containing  elongated  hexagonal  fibers  which,  having  their 
origin  near  the  anterior  central  portion  of  the  lens,  pass  out  toward 
the  margin,  where  they  bend  around  to  terminate  in  a  triradiate 
figure  on  the  opposite  side.  Chemicly  the  lens  consists  of  water, 
a  globuhn  body  (crystalhn),  and  salts. 


Fig.  282. — Horizontal   Section   of    the   Eyeball. — i.  Sclera.  2.   Cornea.     3. 

Chorioid.     4.  Iris.     5.  Ciliary  muscle.     6.  Retina.      7.  Lens.  8.   Suspensory 

ligament.     9.   Canal  of    Schlemm.      10.   Canal  of   Petit.     11.  Optic  [nerve. — 
(Deaver.) 


The  Suspensory  Ligament. — The  lens  is  held  in  position  by  the 
suspensory  ligament,  formed  in  part  by  the  hyaloid  membrane  and 
in  part  by  fibers  derived  from  the  ciliary  processes.  The  former  be- 
comes attached  to  the  posterior  surface,  the  latter  to  the  anterior 
surface  of  the  lens  near  the  equator.  The  space  between  the  two 
layers  of  the  ligament  is  the  canal  of  Petit.  The  anterior  surface  of 
the  ligament  presents  a  series  of  plications  conforming  to  correspond- 
ing plications  on  the  surface  of  the  ciliary  processes. 

The  relations  of  all  the  parts  entering  into  the  structure  of 
the  eye  are  shown  in  Fig.  282. 


THE  SENSE  OF  SIGHT.  623 


THE  PHYSIOLOGY  OF  VISION. 


The  Retinal  Image. — The  general  function  of  the  eye  is  the 
formation  of  images  of  external  objects  on  the  free  ends  of  the  per- 
cipient elements  of  the  retina,  the  rods  and  cones.  The  existence  of 
an  image  on  the  retina  can  be  readily  seen  in  the  excised  eye  of  an 
albino  rabbit,  when  placed  between  a  lighted  candle  and  the  eye  of 
an  observer.  Its  presence  in  the  human  eye  can  be  demonstrated 
with  the  ophthalmoscope.  It  is  this  image,  composed  of  focal  points 
of  luminous  rays,  which  stimulates  the  rods  and  cones,  which  is 
the  basis  of  our  sight-perceptions,  and  out  of  which  the  mind  con- 
structs space-relations  of  external  objects.  In  only  two  essential 
respects  does  the  image  on  the  retina  differ  from  the  object,  aside 
from  the  fact  that  the  object  has  usually  three,  the  image  only  two, 
dimensions — viz.,  in  size  and  relative  arrangement  of  its  parts.  What- 
ever the  distance,  the  image  is  generally  smaller  than  the  object;  it 
is  also  reversed,  the  upper  part  of  the  object  becoming  the  lower 
part  of  the  image,  and  the  right  side  of  the  object  the  left  of  the 
image. 

The  Dioptric  Apparatus. — The  formation  of  an  image  is  made 
possible  by  the  introduction  of  a  complex  refracting  apparatus  con- 
sisting of  the  cornea,  aqueous  humor,  lens,  and  vitreous  humor. 
Without  these  agencies  the  ether  vibrations  would  give  rise  only  to 
a  sensation  of  diffused  luminosity.  Rays  of  light  emanating  from 
any  one  point — that  is,  homocentric  rays — arriving  at  the  eye  must 
traverse  successively  the  different  refracting  media.  In  their  passage 
from  one  to  the  other,  they  undergo  at  the  surfaces  changes  in  direc- 
tion before  they  are  finally  converged  to  a  focal  point.  In  order  to 
mathematically  follow  the  rays  in  all  their  deviations  through  the 
media,  to  determine  their  focal  points  and  to  construct  an  image,  a 
knowledge  of  the  form  of  the  refracting  surfaces,  the  refractive  indices 
of  the  different  media,  and  the  distances  of  the  surfaces  from  one 
another  must  be  known. 

The  following  constants  are  now  accepted :  The  radius  of  curvature 
of  that  portion  of  each  refracting  surface  used  for  distinct  vision  is 
for  the  cornea  7.829  mm.,  for  the  anterior  and  posterior  surfaces  of 
the  lens  10  and  6  mm.,  respectively.  The  indices  of  refraction  of  the 
different  media  are  as  follows:  cornea  and  aqueous  humor,  1.3365; 
lens,  1. 437 1 ;  vitreous  body,  1.3365.  The  distance  from  the  vertex  of 
the  cornea  to  the  lens  is  3.6  mm.;  the  thickness  of  the  lens,  3.6  mm.; 
the  distance  from  the  posterior  surface  of  the  lens  to  the  retina,  15  mm. 
As  the  two  surfaces  of  the  cornea  are  practically  parallel,  and  as 
the  index  of  refraction  of  the  aqueous  humor  is  the  same  as  that  of 
the  cornea,  they  may  be  regarded  as  but  one  medium.  The  refracting 
surfaces  may  therefore  be  reduced  to  the  anterior  surface  of  the 


624  TEXT-BOOK  OF  PHYSIOLOGY. 

cornea,  the  anterior  surface  of  the  lens,  and  the  posterior  surface  of 
the  lens.* 

Parallel  rays  of  light  entering  the  eye  pass  from  air,  with  an  index 
of  refraction  of  1.00025,  ^^^^  the  cornea,  with  an  index  of  refraction 
of  1.3365.  In  passing  from  the  rarer  into  the  denser  medium  they 
undergo  refraction  in  accordance  with  the  laws  of  optics  and  are 
rendered  somewhat  convergent.  The  extent  of  this  first  refraction 
and  convergence  is  sufficiently  great  to  bring  parallel  rays,  if  con- 
tinued, to  a  focus  about  10  mm.  behind  the  retina.  This  would  be 
the  condition  in  aphakia  whether  the  lens  is  congenitally  absent  or 
has  been  removed  by  surgical  procedures.  Perfect  vision,  however, 
requires  that  the  convergence  of  the  light  must  be  great  enough  to 
bring  the  focal  point,  the  image,  on  the  retina.  This  is  accomplished 
by  the  introduction  of  an  additional  refracting  body,  the  lens.  On 
entering  the  lens  the  rays  are  for  the  same  reason — i.  e.,  the  passage 
from  a  rarer  into  a  denser  medium — again  refracted  and  converged, 

and  if  continued  would 
come  to  a  focus  about  6.5 
mm.  behind  the  retina.  On 
passing  from  the  lens  into 
the  vitreous — i.  e.,  from  a 
denser  into  a  rarer  medium 
— the  rays  are  once  more 
converged  and  to  an  extent 
sufficient  to  focahze  them 
Fig.  283.— Refraction  of  Homocenteic  Rays  on  the  retina  (Fig.  283). 
AND  THE  Formation  of  an  Image.  While  it  is  thus  possible 

to  geometricly  follow  the 
rays  through  these  media  by  means  of  the  above-mentioned  factors, 
the  procedure  is  attended  with  many  difficulties.  Moreover,  as  the 
relations  all  change  when  rays  enter  the  eye  from  objects  situated 
progressively  nearer  the  eye,  a  separate  calculation  is  necessitated  for 
each  distance  for  the  determination  of  the  size  of  the  image. 

A  method  by  which  these  difficulties  are  much  reduced  was  sug- 
gested by  Gauss  and  developed  by  Listing.  It  was  demonstrated  by 
Gauss  that  in  every  complicated  system  of  refracting  media  separated 
by  centered  spheric  surfaces  there  may  be  assumed  certain  ideal 
or  cardinal  points,  to  which  the  system  may  be  reduced,  and  which, 
if  their  relative  position  and  properties  be  known,  permit  of  the  de- 


*  Strictly  speaking,  the  posterior  surface  of  the  cornea  is  not  parallel  to  the 
anterior  surface,  and  the  index  of  refraction  of  the  cornea  is  a  triiie  greater  than  that 
of  the  aqueous  humor,  viz.,  1.377.  But  as  the  increase  in  the  corneal  refraction  due 
to  a  higher  index  is  almost  exactly  counteracted  by  a  decrease  in  refraction,  due  to 
the  higher  curvature  of  the  posterior  corneal  surface,  the  usual  assumptions  furnish 
quite  accurate  results. 


THE   SENSE    OF   SIGHT.  625 

termination,  either  by  calculation  or  geometric  construction,  of  the 
path  of  the  refracted  ray,  and  the  position  and  size  of  the  image  in 
the  last  medium,  of  the  object  in  the  first. 

Every  dioptric  system  can  be  replaced,  as  Gauss  showed,  by  a 
single  system  composed  of  six  cardinal  points  and  six  planes  per- 
pendicular to  the  common  axis — e.  g.,  two  focal  points,  two  principal 
points,  two  nodal  points,  two  focal  planes,  two  principal  planes,  and 
two  nodal  planes. 

Properties  of  the  Cardinal  Points. — The  first  focal  point,  F^,in 
Fig.  284,  has  the  property  that  every  ray  wdiich  before  refraction  passes 
through  it,  after  refraction  is  parallel  to  the  axis. 

The  second  focal  point,  F^,  has  the  property  that  every  ray  which 
before  refraction  is  parallel  to  the  axis,  passes  after  refraction 
through  it. 

The  second  principal  point,  H^,  is  the  image  of  the  first,  H^ ;  that 
is,  rays  in  the  first  medium  which  go  through  the  first  principal  point 
pass  after  the  last  refraction  through  the  second.     Planes  at  right 


yl  )r  M, 


ff,A  U      ^     ~     ^ 


Fig.  284. — Diagram  showing  the  Position  and  Relation  of  the  Cardinal 

Points. 

angles  to  the  axis  at  these  points  are  principal  planes.  The  second 
principal  plane  is  the  image  of  the  first.  Every  point  in  the  first 
principal  plane  has  its  image  after  refraction  at  a  corresponding  point 
in  the  second  principal  plane  at  the  same  distance  from  the  axis  and 
on  the  same  side. 

The  second  nodal  point,  iV,,  is  the  image  of  the  first,  N^:  a  ray 
which  in  the  first  medium  is  directed  to  the  first  nodal  point  passes 
after  refraction  through  the  second  nodal  point,  and  the  directions  of 
the  rays  before  and  after  refraction  are  parallel  to  each  other.  In 
Fig.  284  let  A  B  represent  the  axis.  The  distance  of  the  first  focal 
point,  F^,  from  the  first  principal  plane,  H^,  is  the  anterior  focal 
distance.  The  distance  of  the  posterior  focal  point,  F^,  from  the 
second  principal  plane,  H^,  is  the  posterior  focal  distance.  The  dis- 
tance of  the  first  nodal  point,  N^,  from  the  first  focal  point  is  equal 
to  the  second  focal  distance.  The  distance  of  the  second  nodal 
point,  N^,  from  the  posterior  focal  point  is  equal  to  the  anterior  focal 
distance.  It  is  evident,  therefore,  that  the  distance  of  the  correspond- 
ing principal  and  nodal  points  from  each  other  is  equal  to  the  differ- 
40 


626 


TEXT-BOOK  OF  PHYSIOLOGY. 


ences  between  the  two  focal  distances.  Also  the  distance  of  the  two 
principal  points  from  each  other  is  equal  to  the  distance  of  the  two 
nodal  points  from  each  other.  Finally,  the  focal  distances  are  pro- 
portional to  the  refractive  indices  of  the  first  and  last  media.  Planes 
passing  through  the  focal  points  vertically  to  the  axis  are  known  as 
focal  planes. 


Fig.  285. — Diagram  to  Find  the  Image  in  Last  Medium  of  a  Luminous  Point 

IN  THE  First. 


From  these  properties  of  the  cardinal  points  the  position  of  an 
image  in  the  last  medium  of  a  luminous  point  in  the  first  may  be 
determined,  and  the  course  of  a  refracted  ray  in  the  last  medium  be 
constructed  if  its  direction  in  the  first  be  given  according  to  the  fol- 
lowing rules : 

I .  To  find  the  image  in  the  last  medium  of  a  luminous  point  in  the 
first:  Let  A  (Fig.  285)  be  this  given  point.  Draw  A  B  parallel  to 
the  axis  until  it  meets  the  second  principal  plane  in  B;  then  B  F.^ 
will  be  this  ray  after  refraction.     Draw  a  second  ray  from  A  to 


//,  //, 


Fig.  286. — Diagram  to  Find  the  Refracted  Ray  in  the  Last  Medium  of  a 
Given  Ray  in  the  First  Medium. 

the  first  nodal  point;  then  draw  another  ray,  D  E,  from  the 
second  nodal  point  parallel  to  A  C.  This  will  be  the  refracted 
ray  in  the  last  medium.  Where  the  two  refracted  rays,  BF^  and 
D  E,  intersect,  the  image  of  A  will  be  ^j.* 
2.  To  find  the  refracted  ray  in  the  last  medium  of  a  given  ray  in  the 
first  medium :  Let  A  B  (Fig.  286)  be  the  given  ray.    Continue  this 

*  If  the  point  A  is  infinitely  far  from  the  eye,  all  the  rays  striking  the  eye  will  be 
parallel  to  each  other.  The  nodal  ray  must  therefore  be  drawn,  and  the  point 
where  this  nodal  ray  meets  the  second  focal  plane  will  be  the  image  of  ^  =  A^, 
where  all  rays  parallel  to  the  nodal  ray  will  meet. 


THE  SENSE  OF  SIGHT. 


627 


ray  until  it  meets  the  first  principal  plane  in  C.     Draw  C  D 

parallel  to  the  axis.     Now  assume  any  point,  such  as  E,  in  the 

given  ray,  and  find  its  image  E^  by  the  Rule  i.     Then  D  E^ 

becomes  the  course  of  the  refracted  ray. 

The  Schematic  Eye. — x\ccepting  the  system  of  cardinal  points. 

Listing,  Bonders,  and  v.  Helmholtz  have  constructed  "schematic" 

eyes  to  be  substituted  for  the  refracting  system  of  the  natural  eye. 

For  this  purpose  it  is  necessary  to  make  use  of  the  various  esti- 
mates of  thp  indices  of  refraction  of  the  different  media,  of  the  radii 
of  curvatures  of  the  different  refracting  surfaces,  and  of  the  distances 


Fig.  287. — Diagram  showing  the  Position  of  the  Cardinal  Points  in  the 
"Schematic  Eye."  The  continuous  lines  in  the  upper  half  of  the  figure  show 
their  position  in  the  passive  emmetropic  eye.  The  dotted  lines  indicate  the 
change  in  their  position  in  an  eye  accommodated  for  the  object  A  at  the  distance 
a  from  the  cornea,  or  152  mm.  The  lower  half  of  the  figure  shows  the  formation 
of  a  distinct  image  on  the  retina  of  an  eye  accommodated  for  the  object  A  at  the 
distance  a  from  the  cornea. 


separating  them,  to  deduce  an  average  eye  as  a  basis  for  calculation. 
The  most  widely  accepted  attempt  is  that  of  v.  Helmholtz.  The  data 
he  assumed  are  as  follows :  The  refractive  index  of  air  =  i ;  of  the 
cornea  and  aqueous  humor,  1.3365;  of  the  lens,  1.4371;  of  the 
vitreous  humor,  1.3365;  the  radius  of  curvature  of  the  cornea,  7.829 
mm.;  of  the  anterior  surface  of  the  lens,  10  mm.;  of  the  posterior 
surface,  6  mm. ;  the  distance  from  the  apex  of  the  cornea  to  the  ante- 
rior surface  of  the  lens,  3.6  mm.;  thickness  of  lens,  3.6  mm.  From 
the  above-mentioned  data  v.  Helmholtz  calculated  the  position  of 
the  cardinal  points  for  the  eye  as  follows  (see  Fig.  287):   The  first 


628  TEXT-BOOK  OF  PHYSIOLOGY. 

focal  point  is  situated  13.745  mm.  before  the  anterior  surface  of  the 
cornea;  the  posterior  focal  point  is  situated  15.619  mm.  behind  the 
posterior  surface  of  the  lens;  the  first  principal  point,  1.753  mm. 
behind  the  cornea;  the  second  principal  point,  2.106  mm.  behind  the 
cornea;  the  first  and  second  nodal  points,  6.968  and  7.321  mm. 
behind  the  apex  of  the  cornea,  respectively.  The  anterior  focal 
distance  of  this  schematic  eye,  the  distance  between  F^  and  H ^, 
therefore  amounts  to  15.498  mm.,  and  the  posterior  focal  distance, 
H^  to  F^,  to  20.713  mm. 

When  the  eye,  however,  is  accommodated  for  near  vision,  the 
relations  of  the  cardinal  points  are  changed  and  will  be  as  follows, 
if  the  point  accommodated  for,  lies  152  mm.  from  the  cornea: 
Anterior  focal  distance,  13.990  mm.;  posterior  focal  distance,  18.689 
mm. ;  distance  from  cornea  of  the  first  and  second  principal  points, 
1.858  and  2.257  mm.  respectively;  distance  of  the  posterior  focus, 
20.955  mm.  from  cornea.  Given  this  schematic  eye  in  the  accom- 
modated state,  the  course  of  the  rays  and  the  determination  of  the 

position  of  an  image  in  the  last  medium 
of  a  luminous  point  in  the  first  can 
easily  be  determined  by  the  rules  already 
given. 

The  Reduced  Eye.— As  suggested  by 
Listing,  this  schematic  eye  may  be  yet 
further  simplified  or  reduced  to  a  single 
Fig.  288.— The  Reduced  Eye.    refracting  surface  bounded  anteriorly  by 

air  and  posteriorly  only  by  aqueous  or 
vitreous  humor.  Without  introducing  any  noticeable  error  in  the 
determination  of  the  size  of  the  retinal  image,  the  anterior  principal 
and  the  anterior  nodal  points  may  be  disregarded,  owing  to  the  minute- 
ness of  the  distances  (0.39  mm.)  separating  the  two  systems  of  points. 
There  is  thus  obtained  one  principal  point  and  one  nodal  point, 
which  latter  becomes  the  center  of  curvature  of  the  single  refracting 
surface.  The  dimensions  of  this  "reduced"  eye  are  as  follows  (see 
Fig.  288).  From  the  anterior  surface  of  the  cornea,  corresponding 
to  the  principal  plane  H ,  to  the  nodal  point  N,  5.215  mm.,  from  the 
anterior  focal  point  F^,  to  the  principal  plane  H,  i.  e.,  the  anterior 
focal  distance  /,'  15.498  mm;  from  the  principal  plane  H  to  the 
posterior  focal  point  Fy,  i.  g.,  the  posterior  focal  distance  /,"  20.713 
mm;  the  index  of  refraction  is  1.3365.  There  is  thus  substituted 
for  the  natural  eye  a  single  refracting  surface  with  a  radius  of  curva- 
ture, r,  of  5.215  mm.  In  such  an  eye  luminous  rays  emanating  from 
the  anterior  focal  point  are  parallel  to  the  axis  after  refraction  in  the 
interior  of  the  eye.  Also  rays  parallel  to  the  axis  before  refraction 
unite  at  the  posterior  focal  point. 

By  means  of  this  reduced  eye  the  construction  of  the  refracted  ray, 


THE  SENSE  OF  SIGHT.  629 

the  various  calculations  as  to  the  size  of  the  image,  the  size  of  diffusion 
circles,  etc.,  are  greatly  facihtated:  e.  g., 

In  Fig.  289  let  A  B  represent  an  object.  From  A  homocentric  rays 
fall  on  the  single  refracting  surface.  One  of  the  rays,  the  nodal 
ray,  falling  on  the  surface  perpendicularly,  passes  unrefracted  through 
the  single  nodal  point,  N,  to  the  posterior  focal  plane.  The  remain- 
ing rays,  partially  represented  in  the  figure,  falling  on  this  surface 
under  varying  degrees  of  incidence,  undergo  corresponding  degrees  of 
refraction,  by  which  they 
form  a  converging  cone 
of  rays  which  unite  at  a 
point  situated  on  the 
nodal  ray.  These  two  - 
points,  A,  a,  are  known 
as  conjugate  foci.  The 
same  holds  true  for  ho-     "  ^**«=*»*^ 

mocentnc   rays    emanat-  Fig.  289. — The  Formation  of  an  Image  in  the 
ing  from  B  or  any  other  Reduced  Eye. 

point  of  the  object. 

The  Size  of  the  Retinal  Image. — The  size  of  the  retinal  image, 
/,  (in  Fig.  289  ab)  may  now  be  easily  calculated,  when  the  size  of  the 
object,  O,  (in  fig.  289  A  B)  and  its  distance,  D,  from  the  refracting 
surface  with  radius  of  curvature,  r,  are  known,  by  the  following  formula : 

O  :  I  =  D  -^  r  :  f"  —  r. 
For,  as  the  triangles  A  N  B  and  a  N  b  are  similar,  we  have 

A  B  :ab  =fN  :  N  g,  or  ab  ^  — ^^T^"^. 

Independent  of  the  foregoing  method,  the  size  of  the  retinal 
image  may  be  calculated  if  it  is  remembered  that  the  eye,  like  any 
optic  system,  has  a  point  of  such  a  quality  that  a  ray  of  light  which 
before  entering  the  eye  was  directed  toward  it,  after  refraction  con- 
tinues as  if  it  came  from  this  point.  In  other  words,  there  is  in  the 
eye  a  point  which  allows  a  ray  of  light  to  pass  unrefracted.  This 
point,  termed  the  nodal  point  of  the  eye,  determines  the  size  of  the 
image ;  for  if  a  line  be  drawn  from  both  the  upper  and  lower  ends  of 
an  object  through  this  nodal  point,  it  is  clear  that  the  images  of  the 
respective  points  must  lie  on  these  two  rays  where  they  intersect  the 
retina.  The  distance  of  this  nodal  point  from  the  retina  is  15.498 
mm.  It  is  clear,  therefore,  that  the  size  of  the  object  is  to  the  size 
of  the  image,  as  the  distance  of  the  object  from  the  nodal  point  is 
to  the  distance  of  the  nodal  point  from  the  retina ;  or,  in  other  words, 
to  find  the  size  of  the  retinal  image:  multiply  the  size  of  the  object 
by  15.5  mm.  and  divide  by  the  distance  of  the  object  from  the  eye. 

The  Visual  Angle. — The  angle  included  between  the  lines  coming 
from  the  opposite  extremities  of   an    object    and   crossing   at    the 


630  TEXT-BOOK  OF  PHYSIOLOGY. 

nodal  point  is  termed  the  visual  angle.  The  size  of  this  angle  in- 
creases with  the  nearness  and  decreases  with  the  remoteness  of  an 
object.  The  retinal  image  correspondingly  increases  or  decreases 
in  size.  The  acuteness  of  vision  depends  on  the  power  of  the  emme- 
tropic eye  to  distinguish  the  smallest  retinal  image  or  the  smallest 
distance  between  two  points  on  the  retina.  It  has  been  experiment- 
ally determined  that  the  retina  can  not  distinguish  two  points  unless 
their  images  are  separated  by  a  distance  of  0.004  mm.  corresponding 
to  a  visual  angle  of  60  seconds.  If  the  distance  is  less  than  this  the 
two  sensations  fuse  into  one.  The  reason  assigned  for  this  is,  that 
the  distance  between  the  centers  of  two  adjoining  cones  in  the 
macula  is  0.004  rnni-  With  a  visual  angle  of  60  seconds  the  two 
foci  fall  on  separate  cones;  with  a  smaller  visual  angle  the  two 
foci  fall  on  and  excite  but  a  single  cone,  and  hence  there  arises 
the  sensation  of  but  a  single  point. 

In  ophthalmic  practice  it  is  customary  in  testing  the  acuteness 
of  vision  to  employ  test  type  of  a  certain  size  for  specified  distances. 


%:^=^  D=6  D=1S  D-60 

Fig.  290. — Visual  Angle  of  5  Minutes. — {After  Hansell  and  Sweet.) 

Though  the  entire  letter  is  embraced  in  an  angle  of  5  minutes, 
the  strokes  are  included  within  an  angle  of  60  seconds  or  one  minute 
(Fig.  290). 

Accommodation. — In  a  normal  or  emmetropic  eye,  homocentric 
parallel  rays  of  hght  (Fig.  291,  a,  h)  after  passing  through  the  optic 
media  are  converged  and  brought  to  a  focus  on  the  retina,  /.  Rays, 
however,  which  come  from  a  luminous  point  situated  near  the  eye, 
P,  and  are  therefore  divergent,  passing  through  the  optic  media  at 
the  same  time,  are  intercepted  by  the  retina  before  they  are  focused, 
and  give  rise  to  the  formation  of  diffusion-circles  and  indistinctness 
of  vision.  The  reverse  is  also  true.  When  the  eye  is  adjusted  for 
the  refraction  and  focusing  of  divergent  rays  (Fig.  291,  P)  parallel 
rays  will  be  brought  to  a  focus  before  reaching  the  retina,  and, 
again  diverging,  will  form  diffusion-circles.  It  is  evident,  there- 
fore, that  it  is  impossible  to  simultaneously  focus  both  parallel 
and  divergent  rays,  and  to  see  distinctly  at  the  same  time,  two 
objects  which  are  situated  at  different  distances.  The  eye  must  be 
alternately  adjusted  first  to  one  object  and  then  to  another.  The 
capabihty  which  the  eye  possesses  of  adjusting  itself  to  vision  at 
different  distances  is  termed  accommodation. 


THE  SENSE  OF  SIGHT. 


t>3i 


The  following  table  of  Listing  shows  the  size  of  the  diffusion- 
circles  formed  of  objects  situated  at  different  distances  when  the 
accommodative  power  is  suspended  in  an  emmetropic  eye : 


Distance  of  Luminous  Point. 

00 

65     m. 

25 

12      " 

6 

3 

1.500  " 
0.750  " 

0-375  " 

0.188  " 

0.094  " 

0.088  " 


Distance  of  the  Focal 

Point  behind  the  Posterior 

Surface  of  the  Retina. 

Diameter  of  the  Diffusion-circle. 

0.0      mm. 

0.0        mm. 

0.005     " 

O.OOII       " 

0.012     " 

0.0027     " 

0.025     " 

0.0056     " 

0.050     " 

0.0112     " 

O.IOO      " 

0.0222     " 

0.20       " 

0.0443     " 

0.40       " 

0.0825     " 

0.80 

o.i5i6     " 

1.60       " 

0.3122     " 

3.20 

0.5768     " 

3-42       " 

0.6484     " 

The  normal  eye  when  adjusted  for  distant  vision  is  in  a  passive 
condition,  and  hence  vision  of  distant  objects  is  unattended   with 


Fig.  291. — The  Refraction  of  Parallel  and  Divergent  Rays  in  the  Emme- 
tropic Eye  in  the  Passive  and  in  the  Active  or  Accommodated  Condition, 

fatigue.  In  the  act  of  adjustment,  however,  for  near  vision  the 
eye  passes  into  an  active  state,  the  result  of  a  muscle  effort,  the 
energy  of  which  is  proportional  to  the  nearness  of  the  object  toward 
which  the  eye  is  directed. 

From  the  foregoing  table  it  is  evident  that  between  infinity  and 
65  meters,  the  diffusion-circles  are  so  sHght  that  no  perceptible 
accommodative  effort  is  required  to  ehminate  them.  From  65  meters 
to  6  meters  the  diffusion-circles  gradually  become  larger,  though  they 
are  yet  so  faint  as  to  require  for  their  correction  an  accommodative 
effort  which  is  scarcely  measurable.  From  6  meters  up  to  6  centi- 
meters, however,  a  progressive  increase  in  accommodative  power 
is  demanded  for  distinct  vision. 


632  TEXT-BOOK  OF  PHYSIOLOGY. 

Mechanism  of  Accommodation. — Inasmuch  as  neither  the 
corneal  curvature  nor  the  shape  of  the  eyeball  undergoes  any  change 
during  accommodation,  the  necessary  change,  whatever  it  may  be, 
is  to  be  sought  for  in  the  lens.  As  to  the  character  of  the  changes 
in  this  body,  two  views  are  held,  based  largely  on  the  fact  and  its 
interpretation,  that  images  of  a  luminous  point  reflected  from  the 
anterior  surface  of  the  cornea,  the  anterior  and  posterior  surfaces 
of  the  lens,  change  their  relative  positions  during  accommodation. 

Thus,  if  in  a  darkened  room  a  lighted  candle  be  placed  in  front 
of  and  to  the  side  of  an  individual  whose  eye  is  directed  to  a  distant 
object,  an  observer  placed  in  the  same  relative  position  as  the 
candle  will  observe  three  images  in  the  eye,  one  at  the  surface  of  the 
cornea,  two  at  the  pupillary  margin  (Fig.  292).  Of  the  two  latter, 
one  is  quite  large  and  situated  apparently  in  front  of  the  third,  which 
is  faint,  small,  and  inverted.     The  middle  image 

Ois  reflected  from  the  convex  surface  of  the  lens, 
the  last  from  the  concave  surface.  These  images 
of  reflection  are  known  as  catoptric  images.  If 
now  the  individual  be  directed  to  fix  the  gaze  on  a 
near  object,  the  second  image  changes  its  position, 
,     !  advances  toward  the  corneal  image    and  at  the 

a     b    c  same  time  becomes  smaller,  a  change  which,  in 

Fig.  292.— Catop-  accordance  with  the  laws  of  optics,  could  only  be 
TRIG  Images  in  ^^g  ^q  ^^^  increase  in  the  convexity  of  the  anterior 
Upright  image  surface  of  the  lens.  A  slight  displacement  of  the 
of  reflection,  third  image  sometimes  observed  indicates  a  pos- 
frotn  the  cornea.     ^{\^\q  increase  in   the  convexity  of  the  posterior 

o.  Upright  image  J  tr 

from    the    ante-      SUrfacC  lens. 

rior  surface  of  According  to  Helmholtz,  during  accommoda- 

verteT^  image'     ^^^^  ^^^  entire  anterior  surface  of  the  lens  becomes 
from  the  poste-     more  convex,  while  at  the  same  time  it  slightly 
nor  surface  of     advances,  possibly  as  much  as  0.4  mm.  in  extreme 
(Helmholtz.')         efforts.    This  change  is  represented  in  Fig.  293. 
According  to  Tscherning,  the  increase  in  convexity 
of  the  anterior  surface  is  confined  to  the  central  portion,  the  re- 
mainder of   the  surface  becoming  somewhat  flattened.     There   is, 
moreover,  no  evidence  that  there  is  any  advance  of  the  surface  or 
any  increase  in   the   thickness  of   the    lens.     A  series  of   new  and 
ingenious  experiments  lend  support    to    Tscherning's    view.      The 
radius  of  curvature  in  either  case  approximates  6  mm.  in  extreme 
efforts  of  accommodation.     The  increase  in  convexity  naturally  in- 
creases the  refracting  power. 

Whichever  view  is  accepted,  the  nearer  the  object, — that  is,  the 
greater  the  degree  of  divergence  of  the  light  rays, — the  more  pro- 
nounced must  be  the  increase  in  convexity  in  order  that  they  may  be 
sufficiently  converged  and  focalized  on  the  retinal  surface.    Changes  in 


THE  SENSE  OF  SIGHT. 


633 


the  convexity  of  the  lens,  either  of  increase  or  decrease,  are  attended 
by  changes  in  the  distinctness  of  images.  Coincident  with  the 
lens  change,  the  pupillary  margin  advances  and  the  pupil  itself 
becomes  smaller.  By  this  means  an  indistinctness  of  the  image  is 
prevented  by  cutting  off  the  rays  which  would  give  rise,  owing  to 
the  angle  at  which  they  fall  on  the  surface,  to  dift'usion  circles,  from 
spheric  aberration. 

The  Function  of  the  Ciliary  Muscle. — Though  it  is  generally 
admitted  that  the  increase  in  the  convexity  of  the  lens  is  caused  by 
the  contraction  of  the  ciliary  muscle,  the  exact  manner  in  which  this 
is  accomplished  is  not  clearly  understood.  According  to  Helmholtz, 
when  the  eye  is  in  repose  and  directed  to  a  distant  object  the  lens  is 
somewhat  flattened  from  a  traction  exerted  by  the  suspensory  liga- 
ment. When  the  eye  is  directed  to  a  near  object,  the  ciliary  muscle 
contracts,  thereby  relaxing  the  ligament,  as  a  result  of  which  the  lens, 
by  virtue  of  an  inherent  elasticity,  bulges  forward  and  becomes  more 
convex.     In  consequence  of  this  latter  fact  the  refracting  power  is 


Cornell  propei" 
\Sescemef  Jfemir^^jte 


r— ^>_--. SpCncterJridu 


CAssuj  (Xiutria 

Fig.  293. — The  Left  Half  Represents  the  Eye  in  a  State  of  Rest. 
Right  Half  in  State  of  Accommodation. 


The 


proportionally  increased.  In  extreme  efforts  of  accommodation  it  is 
believed  by  some  observers  that  the  circularly  arranged  fibers,  the 
so-called  annular  muscle,  contract  and  exert  a  pressure  on  the  periph- 
ery of  the  lens  and  thus  aid  other  mechanisms  in  relaxing  the  ligament 
and  in  increasing  the  convexity.  This  view  appears  to  be  supported 
by  the  fact  that  in  hypermetropia,  where  a  constant  effort  is  re- 
quired to  obtain  a  distinct  image  of  even  distant  objects,  the  annular 
muscle  becomes  very  much  hypertrophied,  thus  reinforcing  the 
meridional  fibers.  In  myopia,  on  the  contrary,  where  the  accommo- 
dative effort  is  at  a  minimum,  the  entire  muscle  possesses  less  than 
its  average  size  and  development. 

According  to  Tscherning,  a  different  explanation  of  the  action  of 
the  cihary  muscle  must  be  given.  Thus,  when  it  contracts,  the  antero- 
internal  angle,  that  portion  in  close  relation  with  the  suspensory 
ligament,  recedes  and  exerts  on  the  ligament  a  pressure  which  in 
turn  exerts  a  traction  on  the  peripheral  portions  of  the  anterior 
surface  of  the  lens,  which  produces  the  deformation  observed.      At 


634  TEXT-BOOK  OF  PHYSIOLOGY. 

the  same  time  the  postero-external  portion  of  the  muscle  exerts 
traction  on  the  chorioid,  thus  sustaining  the  vitreous  and  indirectly 
the  lens. 

The  reason  for  the  flattening  of  the  periphery  of  the  lens  from 
zonular  compression  and  the  sharpening  of  the  central  convexity  is 
to  be  found  in  the  fact  that  the  convexity  of  the  more  solid  central 
portion,  the  nucleus,  is  greater  than  that  of  the  lens  itself.  Hence  it 
is  easily  understood  why  a  zonular  traction  vv^ould  give  rise  to  periph- 
eral flattening. 

There  is,  however,  one  point  which  seems  difficult  to  harmonize 
with  Tscherning's  view;  that  is,  the  fact  that  during  accommodation 
the  lens  appears  to  be  sHghtly  tremulous,  thus  showing  relaxation, 
and  not  increased  tension,  of  the  suspensory  ligament. 

Range  of  Accommodation. — It  has  been  stated  that  rays  of 
light  coming  from  a  luminous  point  situated  at  any  distance  beyond 
65  meters  are  so  nearly  parallel  that  no  accommodative  effort  is  re- 
quired for  their  focalization.  So  long  as  the  luminous  point  remains 
between  infinity  and  65  meters,  the  eye,  directed  toward  it,  remains 
completely  relaxed.  The  point  at  which  the  object  can  be  distinctly 
seen  without  accommodation  is  termed  the  far  point  or  the  punctum 
remotum.  This  for  the  normal  eye  is  at  a  distance  of  65  meters  or 
beyond.*  If  the  luminous  point  gradually  approaches  the  eye  from 
a  point  65  meters  distant,  the  accommodative  power  comes  into 
play  and  gradually  increases  until  it  attains  its  maximum.  The 
nearest  point  up  to  which  the  eye  is  able  to  form  distinct  images  of 
objects  is  called  its  near  point  or  punctum  proximum.  This  near  point 
in  a  healthy  boy  of  twelve  years  will  lie  at  2§  inches  from  the  eye, 
while  the  same  point  lies  only  at  8  inches  or  20  cm.  in  a  man  of  forty  years. 
Of  objects  which  lie  nearer  than  the  punctum  proximum  the  eye 
cannot  form  distinct  images.  The  distance  between  the  punctum  remo- 
tum and  the  punctum  proximum  is  termed  the  range  of  accommodation. 

Force  of  Accommodation. — The  increase  in  curvature  of  the 
lens  necessary  to  focalize  rays  when  the  eye  is  directed  from  the  far 
to  the  near  point  necessitates  the  expenditure  of  energy  on  the  part 
of  the  ciliary  muscle.  The  energy  expended  in  the  act  of  accommo- 
dation may  be  measured  by  a  lens,  the  refracting  power  of  which  is 
such  as  to  enable  it  to  produce  the  same  result — that  is,  to  give  the 
diverging  rays  coming  from  the  near  point,  e.  g.,  20  cm.,  a  parallel 
direction.  A  lens,  therefore,  which  has  for  a  near  point  a  focal  dis- 
tance of  20  cm.  would  be  a  measure  of  the  force  expended,  for  such  a 
lens  placed  in  front  of  the  crystalline  lens,  when  in  a  state  of  repose, 
would,  with  the  assistance  of  the  latter,  bring  diverging  rays  coming 

*  In  practical  ophthalmic  work  a  point  six  meters  distant  is  taken  as  the  far  point 
for  the  reason  that  the  rays  at  this  distance  are  practically  parallel. 


THE  SENSE  OF  SIGHT.  635 

from  the  near  point  to  a  focus  on  the  retina.     A  lens  of  this  character 
is  said  to  have  a  refracting  power  of  5  dioptrics. 

Since  lenses  of  the  same  curvature  made  from  different  materials 
have  different  refracting  powers,  it  becomes  necessary  to  have,  for 
purposes  of  comparison,  some  unit  of  measurement.  The  unit  now 
accepted  is  the  refracting  power  of  a  glass  lens  which  is  sufficient 
to  focalize  parallel  rays  at  a  distance  of  100  cm.  or  i  meter.  This 
amount  of  refracting  power  is  termed  a  dioptry.  Lenses  which 
would  focalize  parallel  rays  at  a  distance  of  50,  20,  or  10  cm.  are 
said  to  have  a  refractive  power  of  2,  5,  or  10  dioptrics,  respectively, 
obtained  by  dividing  into  100  cm.  the  focal  distance.  The  refracting 
power  of  a  biconcave  lens  is  determined  by  prolonging  backward  in 
the  direction  the  parallel  rays  have  come,  the  rays  which  have  been 
rendered  divergent  by  the  lens. 

The  refracting  media  of  the  human  eye  in  repose  have  collectively 
a  refracting  power  of  about  64  dioptrics,  the  reciprocal  of  its  focal 
length.  The  refracting  power  of  the  corneal  surface  alone  is  equiva- 
lent to  42  dioptrics.  The  crystalline  lens  could  in  the  schematic  eye 
be  replaced  by  a  lens  of  about  13  dioptrics  in  front  of  the  eye,  as  is 
done  after  the  extraction  of  a  cataract.  But  owing  to  its  position  in 
a  medium  denser  than  air,  it  has  been  calculated  that  its  refracting 
power  is  about  20  dioptries. 

The  capabiUty  of  the  lens  to  increase  its  refraction  during  accom- 
modative efforts  beyond  the  20  dioptries  varies  considerably  at 
different  periods  of  hfe.  At  ten  years  the  increase  is  14  dioptries, 
as  the  near  point  is  7  cm. ;  at  thirty  years  the  increase  is  but  7  diop- 
tries, as  the  near  point  is  14  cm. ;  at  sixty  the  increase  is  but  i  dioptry, 
and  the  near  point  100  cm.;  at  seventy  it  is  zero.  From  youth  to  old 
age,  the  elasticity  of  the  lens  steadily  dechnes,  and  the  range  of  accom- 
modation diminishes  from  the  recession  of  the  near  point. 

Convergence  of  the  Eyes  during  Accommodation.— In  binocu 
lar  vision  of  near  objects  the  eyes  are  turned  inward  and  the  optic 
axis  of  each — a  line  passing  through  the  center  of  the  cornea  and 
the  center  of  the  eye — turned  toward  the  median  line  during  accom- 
modation. So  long  as  the  eyes  are  directed  toward  the  far  point,  65 
meters  or  beyond,  the  optic  axes  are  parallel.  When  the  eyes  are 
directed  to  any  point  within  65  meters  the  optic  axes  are  converged, 
the  convergence  increasing  steadily  as  the  near  point  is  approached. 
In  this  way  the  fovea  of  each  eye  is  directed  to  the  same  point  and 
single  vision  made  possible.  Were  this  not  the  case,  double  vision 
would  result. 

Functions  of  the  Iris. — For  purposes  of  distinct  vision  it  is  essen- 
tail  that  the  quantity  of  light  entering  the  interior  of  the  eye  shall 
be  so  adjusted  that  the  formation  and  subsequent  perception  of  the 
image  shall  be  sharp  and  distinct.     This  is  accomplished  by  the  iris, 


636  TEXT-BOOK  OF  PHYSIOLOGY. 

the  circular  fibers  of  which  alternately  contract  and  relax  with  in- 
creasing and  decreasing  intensities  of  the  light.  The  size  of  the  pupil, 
therefore,  through  which  the  light  passes,  will  vary  from  moment 
to  moment  and  in  accordance  with  variation  in  the  light  intensity. 
The  quantity  of  light  necessary  to  distinct  vision  is  thus  regulated. 

In  the  total  absence  of  light  the  sphincter  pupillse  muscle  is 
relaxed  and  the  pupil  widely  dilated.  With  the  appearance  of  light 
and  an  increase  in  its  intensity  the  muscle  again  contracts  and  the 
pupil  progressively  narrows.  With  a  given  intensity  in  the  light,  the 
sphincter  contraction  is  greater  when  the  hght  falls  directly  into  the 
fovea.  Contraction  of  this  muscle  also  occurs  as  an  associated  move- 
ment in  the  convergence  of  the  eyes  during  accommodation  and  in 
consensus  with  the  other  eye. 

In  addition  to  this  function  of  the  iris,  it  constitutes,  by  virtue  of 
the  sphincter  muscle  contraction,  an  important  corrective  apparatus. 
Being  non-transparent,  it  serves  as  a  diaphragm  intercepting  those 
rays  which  would  otherwise  pass  through  the  peripheral  portions  of 
the  lens  and  by  spheric  aberration  give  rise  to  indistinctness  of  the 
image.  The  movements  of  the  iris  by  which  the  size  of  the  pupil  is 
determined  are  caused  by  the  contractions  and  relaxations  of  the 
sphincter  piipillcB  and  dilatator  pupiUce  muscles.  The  contraction  of 
the  sphincter  is  entirely  reflex  and  involves  those  structures  necessary 
to  the  performance  of  any  reflex  act,  viz.:  a  sentient  surface,  the 
retina;  an  afferent  nerve,  the  optic;  a  central  emissive  center  situated 
in  the  gray  matter  beneath  the  aqueduct  of  Sylvius;  and  an  efferent 
nerve,  the  motor  oculi.  The  stimulus  requisite  to  the  excitation  of 
this  mechanism  is  the  impact  of  light  waves  or  ether  vibrations  on 
the  rods  and  cones.  According  to  the  intensity  of  these  vibrations 
will  be  the  resulting  contraction  of  the  muscle.  The  contraction  of 
the  dilatator  pupillae  muscle  is  determined  by  the  activity  of  a  con- 
tinuously active  nerve-center  in  the  medulla  oblongata  which  trans- 
mits its  nerve  impulses  through  the  spinal  cord,  along  the  first  and 
second,  dorsal  nerves  to  the  superior  cervical  ganglion,  and  thence  to 
the  iris  by  way  of  the  fifth  nerve.  (See  Fig.  245,  page  534.)  These 
two  muscles  appear  to  bear  an  antagonistic  relation  to  each  other,  for 
section  of  the  motor  oculi  is  followed  by  relaxation  of  the  sphincter 
muscle  and  dilatation  of  the  pupil.  Stimulation  of  the  sympathetic 
is  followed  by  a  more  pronounced  dilatation.  The  size  of  the  pupil 
is  the  resultant  of  a  balancing  of  these  two  forces. 

OPTIC  DEFECTS. 

Presbyopia. — This  is  a  condition  of  the  eye  characterized  by  a 
defective  or  diminished  accommodative  power.  As  age  advances  the 
lens  loses  its  elasticity  and  the  power  to  increase  its  refraction,  and 
vision  at  the  normal  reading  distance  becomes  impossible.     The  near 


THE  SENSE  OF  SIGHT. 


637 


point  therefore,  advances  toward  the  far  point,  or  recedes  from  the  indi- 
vidual. The  range  of  accommodation  is  also  diminished.  At  forty 
years  the  near  point  is  about  22  cm.;  at  forty-five  years  it  has  receded 
to  28  cm.  This  would  indicate  that  the  lens  in  these  five  years  has 
lost  I  dioptry  of  refracting  power;  at  fifty  years  the  near  point 
recedes  to  43  cm.,  and  at  sixty  to  200  cm.,  indicating  a  loss  in  refract- 
ing power  on  the  part  of  the  lens  of  2  and  4  dioptrics  respectively. 
Convex  lenses  placed  before  the  eyes  having  a  refracting  power  of 
1,2,  and  4  dioptrics  would  in  the  three  instances  return  the  near  point 
to  its  normal  position.  At  the  age  of  seventy  the  lens  is  incapable 
of  any  increase  during  an  accommodative  eftort.  A  lens  of  4  diop- 
trics would  therefore  be  required  by  such  a  man,  for  near  vision  at 
10  inches. 

Myopia. — This  is  a  condition  of  the  eye  characterized  by  an 
increase  in  the  antero-posterior  diameter  or  a  hypernormal  refracting 
power  of  the  lens.  The  former  is  the  usual  condition.  Parallel  rays 
of  light  brought  to  a  focus  in  front  of  the  retina  again  diverge,  giving 


Fig.  294. — Myopia.  Parallel  rays 
focus  at  F,  cross  and  form  diffu- 
sion-circles; divergent  rays  from 
A  focus  on  the  retina. — {Hansell 
and  Sweet.) 


Fig.  295. — Correction  of  Myopia 
BY  A  Concave  Lens.— (iJZ^awse// 
and  Sweet.) 


rise  to  diffusion-circles  and  indistinctness  of  the  image.  Divergent 
rays  alone  are  capable  of  being  focahzed  on  the  retina  in  its  new 
position.  The  punctum  remotum  is  always  at  a  definite  distance, 
but  approaches  the  eye  as  the  myopia  increases.  The  near  point 
is  usually  much  nearer  the  eye  than  20  cm.  For  this  reason  the 
condition  is  termed  near  sight. 

The  increase  in  the  length  of  the  antero-posterior  diameter  may 
range  from  a  fraction  of  a  millimeter  up  to  10  mm.  With  an  increase 
of  0.16  mm.  the  far  point  is  but  200  cm.  distant;  and  with  an  increase 
of  3.2  mm.  it  is  but  10  cm.  distant.  Inasmuch  as  only  divergent 
rays  can  be  focahzed  by  the  myopic  eye  normal  vision  can  be  restored 
by  the  use  of  a  biconcave  lens  with  a  diverging  power  in  the  first 
instance  of  0.5  dioptry  and  the  second  of  10  dioptrics. 

Hypermetropia. — This  is  a  condition  of  the  eye  characterized  by 
decrease  of  the  normal  antero-posterior  diameter  or  by  a  subnormal 
refracting  power  of  the  lens.  The  former  is  the  usual  condition. 
Parallel  rays  of  light  do  not,  therefore,  come  to  a  focus  when  the 


638 


TEXT-BOOK  OF  PHYSIOLOGY. 


accommodation  is  suspended.  Falling  on  the  retina  previous  to 
focalization,  they  give  rise  to  diffusion-circles  and  indistinctness  of  the 
image.  As  no  object  can  be  seen  distinctly  no  matter  how  remote, 
there  is  no  positive  far  point.  The  near  point  is  abnormally  distant — 
sometimes  as  far  as  200  cm.  For  this  reason  the  condition  is  termed 
jar  sight.  A  hypermetropic  eye  w^ithout  accommodative  effort  can 
focahze  only  converging  rays  on  the  retina.  If  rays  of  light  vv^ere  to 
come  from  the  retina  of  such  an  eye,  they  would,  on  emerging,  take 


Tig.  296. — The  Hypermetropic  Eye.  Parallel  rays  (.4,  B)  can  be  focused  only  at  a 
point  behind  the  eye,  as  at  /;  rays  of  light  coming  from  the  retina  take,  on 
emerging  from  the  eye,  a  divergent  direction,  C,  D.  K.  The  negative  punctum 
remotum. 


a  divergent  direction,  as  shown  in  Fig.  206,  dotted  line  C  and  D. 
If  these  same  rays  were  to  be  prolonged  backward,  they  would  meet 
at  the  point  K,  which  is  the  punctum  remotum;  and  as  it  is  behind 
the  eye,  it  is  termed  negative.  Since  rays  coming  from  the  retina 
take  a  divergent  direction  on  emerging  from  the  eye,  it  is  evident 
that  only  converging  rays  can  be  focahzed  by  a  passive  hyperme- 


FiG.  297. — Hypermetropia.  Par- 
allel Rays  Focused  behind 
THE  Retina.  —  (Hansell  and 
Sweet.) 


Fig.  298. — Correction  of  Hyper- 
metropia BY  A  Convex  Lens. 
{Hansell  and  Sweet.) 


tropic  eye.  As  there  are  no  convergent  rays  in  nature,  it  is  necessary 
for  distinct  vision  that  all  rays,  parallel  and  divergent,  shall  be  given 
a  convergent  direction  before  entering  the  eye.  This  is  done  by 
placing  before  the  eye  convex  lenses  the  converging  power  of  which 
is  proportional  to  the  degree  of  hypermetropia  (Figs.  297,  298). 

Astigmatism. — This  is  a  condition  of  the  eye  characterized  by 
an  inequality  of  curvature  of  its  refracting  surfaces  in  consequence  of 
which  not  all  of  a  homocentric  bundle  of  rays  are  brought  to  the 


THE  SENSE  OF  SIGHT. 


639 


same  focus.     The  inequality  may  be  either  in  the  cornea  or  lens,  or 
both,  though  usually  in  the  cornea. 

In  the  normal  cornea  the  radius  of  curvature  in  the  vertical 
meridian  is  a  trifle  shorter,  7.6  mm.,  than  that  of  the  horizontal,  7.8 
mm.,  and  hence  its  focal  distance  is  shghtly  shorter.  The  difference, 
however,  in  the  focal  distances  is  so  shght  that  the  error  in  the  forma- 
tion of  the  image  is  scarcely  noticeable.  A  transection  of  a  cone  of 
light  coming  from  the  cornea  is  practically  a  circle.  If,  however,  the 
vertical  curvature  exceeds  the  normal  to  any  marked  extent,  the  rays 
passing  through  this  meridian  will  be  more  sharply  refracted  and 
brought  to  a  focus  much  sooner  than  the  rays  passing  through  the 
horizontal  meridian.  The  result  will  be  that  the  cone  of  light  will 
be  no  longer  circular,  but  more  or  less  elliptic.  The  variations  of  the 
shape  of  this  cone  are  shown  in  Fig.  299,  which  represents  the 
appearances  presented  on  cross-section  both  before  and  after  focaliza- 
tion  of  each  set  of  rays.     Though  the  vertical  meridian  has  usually 


Fig.  299. — Refraction  by  an  Astigmatic  Surface. — {Hansell  and  Sweet.) 

the  sharper  curvature,  it  not  infrequently  happens  that  the  reverse 
is  true.  For  the  reason  that  the  rays  from  one  point  do  not  all 
come  to  the  same  focus  or  point,  the  condition  is  termed  astigmatism. 
Spheric  Aberration. — When  the  ra}'s  of  hght  which  emanate 
from  a  point  fall  upon  a  spheric  lens,  they  do  not  after  passing  through 
it  reunite  at  one  point  because  of  the  fact  that  the  more  peripheral 
rays  have  a  shorter  focus  than  the  central  rays.  To  this  condition 
the  term  spheric  aberration  is  given.  Spheric  aberration  can  be  dem- 
onstrated in  the  human  eye.  That  this  condition  is  present  to  but 
a  slight  extent  in  the  nomial  eye  is  due  to  the  presence  of  the  iris, 
which  intercepts  those  rays  which  would  otherwise  pass  through  the 
marginal  portions  of  the  refracting  media.  In  widely  dilated  eyes 
the  spheric  aberration  of  the  peripheral  parts  may  amount  to  as  much 
as  4.5  dioptries. 

.  Chromatic  Aberration. — When  a  beam  of  white  light  is  made 
to  pass  through  a  prism,  it  is  decomposed  into  the  primary  colors 
owing  to  a  difference  in  the  refrangibility  of  the  rays.  In  passing 
through  the  refracting  media  of  the  eye  the  different  rays  composing 


640  TEXT-BOOK  OF  PHYSIOLOGY. 

white  light  also  undergo  unequal  refraction  and  those  rays  which 
give  rise  to  one  color  are  brought  to  a  focus  at  a  point  somewhat 
different  from  those  which  give  rise  to  other  colors.  If  the  eye  is 
accommodated  for  one  set  of  rays,  it  is  not  for  another,  and  the  result 
is  a  fringe  of  colors  around  the  image.  This  defect  in  the  normal 
eye  is  so  slight  that  the  mind  fails  to  take  cognizance  of  it.  That  the 
eye  is  incapable  of  simultaneously  focalizing  rays  of  widely  different 
refrangibility,  as  those  which  give  rise  to  the  blue  and  red  colors, 
is  shown  by  the  following  experiment:  The  eye  being  directed  to  a 
luminous  point,  a  plate  of  cobalt-glass  is  placed  between  the  light  and 
the  observer  close  to  the  eye.  This  substance  has  the  property  of 
intercepting  all  rays  but  the  red  and  the  blue  and  hence  these  alone 
will  be  seen.  The  center  of  the  image  produced  will  be  red  and  clearly 
defined,  the  periphery  blue  and  ill  defined.  The  reason  for  this  is 
clear.  The  eye  more  readily  accommodates  itself  for  the  red  rays, 
and  hence  their  focal  point  is  distinct.  The  blue  rays,  having  a 
higher  degree  of  refrangibility,  come  to  a  focus,  cross  and  diverge, 
and  give  rise  to  diffusion-circles.  If  a  biconcave  glass  be  placed  before 
the  cobalt,  the  blue  rays  can  be  focahzed  on  the  retina,  while  the  red 
will  fall  on  the  retina  without  focalization.  The  image  will  now  be 
blue  and  distinct  in  the  center,  the  periphery  red  and  ill  defined. 
With  the  removal  of  the  minus  glass  the  reverse  condition  again 
obtains. 

Imperfect  Centering. — From  a  purely  physical  point  of  view, 
the  eye  is  not  a  perfect  optic  instrument.  In  addition  to  the  defects 
noticed  in  the  foregoing  paragraphs,  there  is  yet  another,  viz.:  an 
imperfect  centering  of  the  refracting  surfaces.  In  first-class  optic 
instruments  the  lenses  are  centered — that  is,  their  exact  centers  are 
situated  on  the  same  axis.  In  viewing  an  object  through  such  a 
system  the  visual  line  corresponds  with  the  axis  of  the  lens  system. 
This  is  not  the  case  with  the  refracting  system  of  the  eye.  A  line 
passing  through  the  center  of  the  cornea  and  the  center  of  the  eye, 
the  optic  axis  (O  A  in  Fig.  300),  does  not  pass  exactly  through  the 
center  of  the  lens  and  does  not  fall  into  the  point  for  most  distinct 
vision,  the  fovea.  This  has  led  to  the  recognition  of  other  lines  the 
relations  of  which  must  be  kept  in  mind  in  all  optic  discussions,  viz. : 

1.  The  visual  axis  or  visual  line  {V  L),  the  line  connecting  the  point 

viewed,  the  nodal  point  and  the  fovea  centralis. 

2.  The  line  of  fixation  or  line  of  regard  {V  C),  the  line  connecting  the 

point  viewed  with  the  center  of  rotation,  the  latter  being  situated 
6  mm.  behind  the  nodal  point  of  the  eye  and  9  mm.  before  the 
retina.  The  relations  of  these  lines  and  certain  angles  connected 
with  them  are  shown  in  Fig.  300.  The  angle  included  between 
the  line  D  D  (the  major  axis  of  the  corneal  elhpse)  and  the  visual 
line  is  the  angle  alpha,  amounting  on  the  average  to  5°.     The 


THE  SENSE  OF  SIGHT. 


641 


angle  included  between  the  optic  axis  and  the  line  of  fixation  or 
regard  is  the  angle  gamma,  while  the  angle  between  the  optic 
axis  and  the  line  of  vision  is  the  angle  beta.     In  emmetropia  the 
angle  alpha  is  about  5°.     In  hypermetropia  it  is  greater,  amount- 
ing to  7°  or  8°,  giving  to  the  eye  an  appearance  of  divergence. 
In  myopia  it  is  much  smaller — 2° — or  in  extreme  cases  may  be 
abolished,  the  line  of  vision  corresponding  with  the  optic  axis 
or  even  passing  beyond  it.    The  angle  gamma  is  of  value  in  de- 
termining the  actual  deviation  of  the  eye  in  squint. 
Functions  of  the  Retina. — Of  all  the  layers  of  the  retina,  the 
rods  and  cones  appear  to  be  the  most  essential  to  vision.     It  is  only 
this  layer  that  is  capable  of  receiving  the  light  stimulus  and  of  trans- 
forming it  into  some  specific  form  of  energy,  which  in  turn  arouses 


Jem^or-aZ  Su^ 


JVhsaZ  SuZe. 


Fig.  300. — Diagram  showing  the  Corneal  Axis  D  D,  the  Optic  Axis  O  A,  the 
Visual  Axis  V  L,  and  the  Line  of  Fixation  V  C;  also  the  Three  Angles, 


in  the  fibers  of  the  optic  nerve  the  characteristic  nerve  impulses. 
A  ray  of  light  entering  the  eye  passes  entirely  through  the  various 
layers  of  the  retina,  and  is  arrested  only  upon  reaching  the  pigmentary 
epithehum  in  which  the  rods  and  cones  are  embedded.  As  to  the 
manner  in  which  the  objective  stimuli — light  and  color,  so  called — 
are  transformed  into  nerve  impulses,  but  little  is  known.  It  is  prob- 
able that  the  ether  vibrations  are  transformed  into  heat,  which 
excites  the  rods  and  cones.  These,  acting  as  highly  specialized  end 
organs  of  the  optic  nerve,  start  the  impulses  on  their  way  to  the  brain, 
where  the  seeing  process  takes  place.  As  to  the  relative  function  of 
the  rods  and  cones,  it  has  been  suggested,  from  the  study  of  the  facts 
of  comparative  anatomy,  that  the  rods  are  impressed  only  by  differ- 
ences in  the  intensity  of  light,  while  the  cones,  in  addition,  are  im- 
pressed by  qualitative  differences  in  color.     The  nerve-fibers  them- 


41 


642  TEXT-BOOK  OF  PHYSIOLOGY. 

selves  are  insensible  to  the  impact  of  the  ether  vibrations,  and  require 
for  their  excitation  some  intermediate  form  of  energy.  That  this  is 
the  case  was  shown  by  Bonders,  who  reflected  a  beam  of  Hght  on 
the  optic  nerve  at  its  entrance  without  the  individual  experiencing  any 
sensation  of  light.  This  region,  occupied  only  by  the  optic-nerve 
fibers  and  devoid  of  any  special  retinal  elements,  is  therefore  an 
insensitive  or  blind  spot.  The  diameter  of  this  spot  is  about  1.5  mm., 
and  occupies  in  the  field  of  vision  a  space  of  about  6°.  It  is  situated 
about  3.5  mm.  to  the  nasal  side  of  the  visual  axis.  Its  existence  can 
be  demonstrated  by  the  famihar  experiment  of  Mariotte,  which  con- 
sists in  placing  before  the  eye  two  objects  having  the  relation  to 
each  other  as  in  Fig.  301.  With  the  left  eye  closed  and  the  right  eye 
directed  to  the  cross,  both  objects  may  be  visible.  But  by  moving 
the  figure  away  from  or  toward  the  eye,  there  will  be  found  a  distance, 
about  30  cm.,  when  the  circle  will  be  invisible.  This  occurs  when 
the  image  falls  on  the  optic  nerve  at  its  entrance.  The  experiment  of 
Purkinje  as  described  in  the  following  paragraph  demonstrates  also 


Fig.  301. — Diagram  for  Observing  the  Situation  of  the  Blind  Spot. — 

{Helmhollz.) 

the  fact  that  the  sensitive  portion  of  the  retina  is  to  be  found  only  in 
the  layer  of  rods  and  cones. 

It  is  well  known  that  the  blood-vessels  of  the  retina  are  situated 
in  its  innermost  layers  a  short  distance  behind  the  optic-nerve  fibers. 
Owing  to  this  anatomic  arrangement,  a  portion  of  the  hght  coming 
through  the  pupil  will  be  intercepted  by  the  vessels  and  a  shadow 
projected  on  the  layer  of  rods  and  cones.  Ordinarily,  these  shadows 
are  not  perceived,  for  the  reason  that  the  shaded  parts  are  more 
sensitive,  so  that  the  small  amount  of  hght  passing  through  the  vessels 
produces  as  strong  an  impression  on  this  part  as  does  the  full  amount 
of  hght  on  the  unshaded  parts  of  the  retina,  and  perhaps  because  the 
mind  has  learned  to  disregard  them.  But  if  hght  be  made  to  enter 
the  eye  obliquely,  the  position  of  the  shadows  will  be  changed,  when  at 
once  they  become  apparent.  This  can  be  shown  in  the  following  way : 
If  in  a  darkened  room  a  lighted  candle  be  held  several  inches  to 
the  side  and  to  the  front  of  the  eye,  and  then  moved  up  and  down, 
there  will  be  perceived,  apparently  in  the  field  of  vision,  an  arbores- 
cent figure  corresponding  to  the  retinal  blood-vessels.     This  is  due 


THE   SENSE   OF   SIGHT.  643 

to  the  falling  of  the  shadows  on  unusual  portions  of  the  layer  of  rods 
and  cones. 

Excitability  0}  the  Retina. — The  retina  is  not  equally  excitable  in 
all  parts  of  its  extent.  The  maximum  degree  of  sensibility  is  found 
in  the  macula  lutea,  and  especially  in  its  central  portion,  the  fovea. 
In  this  region  the  layers  of  the  retina  almost  entirely  disappear,  the 
layer  of  rods  and  cones  alone  remaining,  and  in  the  fovea  only  the 
latter  are  present.  That  this  area  is  the  point  0}  most  distinct  vision 
is  shown  by  the  observation  that  when  the  eye  is  directed  to  any  given 
point  of  light,  its  image  always  falls  in  the  fovea.  Any  pathologic 
change  in  the  fovea  is  attended  by  marked  indistinctness  of  vision. 
The  sensibihty  of  the  retina  gradually  but  irregularly  diminishes  from 
the  macula  toward  the  periphery.  This  diminution  in  sensibihty 
holds  true  for  monochromatic  as  well  as  white  light. 

As  stated  above,  the  nature  of  the  molecular  processes  which  take 
place  in  the  retinal  tissue,  caused  on  one  hand  by  the  light  vibrations, 
and  on  the  other  hand  developing  nerve  impulses,  is  entirely  un- 
known. The  discovery  of  the  visual  purple  in  the  outer  segment  of 
the  rods  gave  promise  of  some  explanation  of  the  process,  especially 
when  it  was  shown  to  undergo  changes  when  exposed  to  the  action 
of  light.  But  as  the  pigment  is  wanting  in  the  cones,  and  especially 
in  the  fovea,  it  cannot  be  considered  essential  to  distinct  vision, 
although  that  it  plays  some  important  role  in  the  visual  process  is 
highly  probable.  It  was  observed  by  Van  Genderen  Stort,  that 
when  an  animal  is  kept  in  darkness  some  time  before  death,  the 
cones  are  long  and  filiform;  but  if  the  animal  has  been  exposed  to 
light,  they  are  short  and  swollen.  It  was  discovered  by  Boll  that 
if  an  animal  is  kept  in  darkness  an  hour  or  two  before  death  the 
pigment  is  massed  at  the  ends  of  the  rods  and  cones,  but  after  ex- 
posure to  Hght  it  becomes  displaced  and  extends  over  and  between 
the  rods  almost  to  the  external  hmiting  membrane.  These  condi- 
tions are  represented  in  Fig.  302. 

The  Eye  a  Living  Camera. — In  its  construction,  in  the  arrange- 
ment of  its  various  parts,  and  in  their  mode  of  action  the  eye  may  be 
compared  to  a  camera  ohscura.  Though  the  comparison  may  not  be 
absolutely  exact,  yet  in  a  general  way  it  is  true  that  there  are  many 
striking  points  of  similarity  between  them ;  e.  g.,  the  sclera  and  chorioid 
may  be  compared  to  the  walls  of  the  camera ;  the  combined  refracting 
media  to  the  single  lens,  the  action  of  which  results  in  the  focusing 
of  the  light  rays;  the  retina  to  the  sensitive  plate  receiving  the  image 
formed  at  the  focal  point ;  the  iris  to  the  diaphragm  for  the  regulation 
of  the  amount  of  light  to  be  admitted,  and  for  the  partial  exclusion 
of  those  marginal  rays  which  give  rise  to  spheric  aberration;  the  ciliary 
muscle  to  the  adjusting  screw,  by  means  of  which  the  image  is  brought 
to  a  focus  on  the  sensitive  plate,  notwithstanding  the  var}dng  distances 


644 


TEXT-BOOK  OF  PHYSIOLOGY. 


of  the  object  from  the  lens.  The  presence  of  the  visual  purple  in  the 
rods  of  the  retina  capable  of  being  altered  by  light  makes  the  com- 
parison still  more  striking. 

Kiihne  even  succeeded  in  obtaining  a  fixed  image  or  an  optogram 
of  an  external  object  in  a  manner  similar  to  that  by  which  an  image 
is  fixed  on  the  sensitive  plate  of  a  camera.  An  animal  is  kept  in 
the  dark  for  about  ten  minutes  in  order  to  permit  the  retinal  pigment 
to  be  completely  regenerated.  The  animal,  with  the  eyes  covered, 
is  then  brought  into  a  room  with  a  single  window.  While  the  head 
is  steadily  directed  to  the  window,  the  eye  is  exposed  for  several 
minutes.  The  eyes  are  again  covered,  the  animal  killed,  and  the 
eyes  removed  by  the  light  of  a  sodium  flame.     The  retina  is  then 


W&^WJ^, 


ilililtii 


Fig.  302. — Section  of  the  Retina  of  a  Frog.     A.   In  darkness.     B.  In  light. — 
{After  Van  Genderen  Start,  from  Tscherning's  "Physiologic  Optics.") 

placed  in  a  4  per  cent,  solution  of  alum.  In  a  short  time  the  image 
of  the  window,  the  optogram,  will  be  fixed  (Fig.  303).  That  portion 
of  the  retina  corresponding  to  the  image  is  quite  bleached  in  appear- 
ance from  the  action  of  the  light  on  the  pigment.  During  life  the 
regeneration  of  the  visual  purple  must  take  place  with  extreme 
rapidity.  It  is  believed  to  be  derived  from  a  pigment  secreted  by 
the  layer  of  pigment  cells. 

Binocular  Vision. — Though  two  images  are  formed,  one  on  each 
retina,  when  the  eyes  are  directed  to  a  given  object,  there  results  but 
one  sensation.  If  the  direction  of  either  visual  axis  be  changed  by 
pressure  on  the  eyeball,  there  arise  two  sensations,  and  the  object 
appears  to  be  doubled.     The  reason  assigned  for  this,  in  the  first 


THE  SENSE  OF  SIGHT. 


645 


Fig.  303. — Retina  of 
A  Rabbit.  Opto- 
gram OF  A  Win- 
dow Four  Meters 
Distant,  a.  Yellow 
spot,  b,  b.  White 
streak  of  nerve- 
fibers. —  (Kuhne.) 


instance,  is  that  the  two  images  fall  into  the  foveae,  two  corresponding 
points;  while  in  the  second  instance  they  fall  on  non-corresponding 
points.  It  would  appear,  therefore,  that  for  the  purpose  of  seeing 
an  object  singly  when  the  eyes  are  directed  toward  it,  the  rays  eman- 
ating from  it  must  fall  on  corresponding  parts  of  the  retina.  As  all 
portions  of  the  retina  are  sensitive  to  light,  though  in  varying  degrees, 
it  is  not  essential  that  the  images  always  fall  in  the  foveae.  The  parts 
of  the  retinae  which  correspond  physiologicly 
are  shown  in  Fig.  304.  In  this  figure  the 
retinal  area  is  divided  into  quadrants  by 
vertical  and  horizontal  lines  of  separation,  as 
they  are  termed.  If  one  retina  is  placed  in 
front  of  or  over  the  other,  it  will  be  found  that 
the  quadrants  bearing  similar  letters  cover 
each  other.  So  long  as  the  rays  of  light, 
entering  the  eye,  fall  on  corresponding  areas 
the  sensation  of  but  one  object  arises.  If, 
however,  they  fall  on  non- corresponding 
areas,  two  sensations  arise.  Normal  binoc- 
ular vision  enlarges  very  considerably  the 
area  of  the  visual  field,  permits  of  a  better 
estimation  of  the  size  and  distance  of  objects,  enables  the  mind 
to  form  more  readily  a  perception  of  depth,  increases  the 
intensity  of  sensations  and  makes  sensation  more  uniform  by  off- 
setting retinal  rivalry. 

The  Horopter. — When  the  eyes  are  in  the  so-called  secondary 
position, — that  is,  in  a  position  in  which  the  visual  axes  are  con- 
verged and  directed  to  a  point  in  front  of  and  in  the  middle  plane  of 

the  body, — it  will  be  found 
on  examination  that  rays  of 
light  from  a  number  of  other 
objects  enter  the  eye,  pass 
through  the  nodal  point,  and 
fall  on  corresponding  parts  of 
the  two  retinae  and  give  rise 
to  but  single  images.  All 
such  points  lie,  for  the  hori- 
zontal line  of  separation,  on 
a  line  termed  the  horopter.  The  form  of  this  line  is  that  of  a  circle 
which  passes  through  the  fixation  point  and  the  two  nodal  points. 
Any  object  on  the  horopter  will  give  rise  to  but  a  single  image. 
This  is  shown  in  Fig.  305,  in  which  the  objects  I,  II,  III  project 
their  rays  into  both  eyes  which  fall  on  corresponding  areas. 

In  addition  to  the  horopter  for  the  horizontal  line  of  separation, 
there  is  also  an  horopter  for  the  vertical  line  of  separation.     At  a 


Fig.  304. 


-Corresponding  Areas  of  the 
Retina. 


646 


TEXT-BOOK  OF  PHYSIOLOGY. 


distance  of  two  meters  the  vertical  horopter  is  a  plane.  Within  this 
distance  it  is  concave  to  the  face;  beyond  this  distance  it  is  convex. 

An  object  which  lies  either  in  front  of  or  behind  the  fixation  point 
will  project  its  rays  on  parts  of  the  retinae  which  do  not  correspond, 
and  hence  give  rise  to  double  images.  This  is  evident  from  examina- 
tion of  Fig.  306.  While  the  eyes  are  directed  to  figure  2,  of  which 
there  is  but  a  single  image,  the  objects  B  and  A  give  rise  to  double 
images,  for  reasons  already  given.  If  the  eyes  are  now  directed  to 
B,  double  images  will  be  formed  of  2  and  A. 

At  all  times,  therefore,  double  images  are  formed  on  the  retinae 
the  existence  of  which  is  scarcely  noticed  unless  the  attention  is 


B 


Fig.  305. — Horopter  for  the 
Secondary  Position,  with 
Convergence  of  the  Vis- 
ual Axes. — {Landois.) 


Fig.  306. — Scheme  of  Identical  and  Non- 
identical  Points  of  the  Retina. — 
(Landois.) 


directed  to  them.  This  is  due  to  the  fact  that  many  of  the  images 
fall  on  the  peripheral,  less  sensitive  parts  of  the  retinae.  At  the 
same  time,  from  a  want  of  accommodation  and  the  formation  of 
diffusion- circles,  they  are  indistinct.  For  these  reasons  they  are 
readily  neglected. 

In  the  primary  position  of  the  eyes — that  is,  a  position  in  which 
the  visual  axes  are  parallel — the  horopter  is  a  plane  in  infinity.  In 
the  tertiary  position  the  horopter  is  a  curve  of  complex  form. 

Movements  of  the  Eyeball. — The  almost  spheric  eyeball  lies 
in  the  correspondingly  shaped  cavity  of  the  orbit,  like  a  ball  placed 
in  a  socket,  and  is  capable  of  being  moved  to  a  considerable  extent 


THE  SENSE  OF  SIGHT. 


647 


by  the  six  muscles  which  are  attached  to  it.  These  muscles  are 
the  superior  and  inferior  recti,  the  external  and  internal  recti,  and 
the  superior  and  inferior  obliqui  (Fig.  307).  The  four  recti  muscles 
arise  from  the  apex  of  the  orbit  cavity,  from  which  point  they  pass 
forward  to  be  inserted  into  the  sclera  about  7  to  8  mm.  from  the 
corneal  border.  The  superior  oblique  muscle  having  a  similar  origin 
passes  forward  to  the  upper  and  inner  angle  of  the  orbit  cavity,  at 
which  point  its  tendon  passes  through  a  cartilaginous  pulley,  after 
which  it  is  reflected  backward  to  be  inserted  into  the  superior  sur- 
face of  the  sclera  about  16  mm.  behind  the  corneal  border.  The 
inferior  oblique  muscle  arises  from  the  inner  and  inferior  angle  of  the 
orbit  cavity.  It  then  passes  outward,  upward,  and  backward, 
to    be    inserted    into 

the    upper,    posterior  ;  '       10 

and  temporal  portion 
of  the  sclera  about  4 
or  5  mm.  from  the 
optic  nerve  entrance. 

The  movements  of 
each  eye  are  referred 
to  three  fixed  lines 
or  axes,  which 
have  their  origin  at 
the  point  of  rota- 
tion of  the  eyeball, 
this  point  lying  about 
1.7  mm.  behind  the 
center  of  the  globe. 
If  the  eye  looks 
straight  forward  in  the 
horizontal  plane  (the 
head  being  erect),  the 
line  joining  the  center 

of  rotation  with  the  object  looked  at  is  the  line  of  fixation  or  line  of 
regard.  Around  this  antero-posterior  axis  the  eye  may  be  regarded  as 
performing  its  circular  rotation  or  torsion.  At  right  angles  to  this  line, 
and  joining  the  center  of  rotation  of  both  eyes,  is  the  horizontal  or 
transverse  axis,  around  which  the  movements  of  elevation  (up  to 
34  degrees)  and  depression  (down  to  57  degrees)  take  place.  At 
right  angles  to  both  of  these  lines  there  is  the  vertical  axis,  around 
which  the  movements  of  adduction  (toward  the  nose  up  to  45  degrees) 
and  abduction  (toward  the  temple  up  to  42  degrees)  occur.  The  six 
muscles  may  be  divided  into  three  pairs,  each  of  which  has  a  common 
axis  around  which  it  tends  to  move  the  eyeball.  But  only  the  common 
axis  of  the  internal  and  external  recti  coincides  with  one  of  three  axes 


Fig.  307. — Muscles  of  the  Eye.  Tendon  or 
Ligament  of  Zinn.  i.  Tendon  of  Zinn.  2.  Ex- 
ternal rectus  divdded.  3.  Internal  rectus.  4.  Inferior 
rectus.  5.  Superior  rectus.  6.  Superior  oblique. 
7.  Pulley  for  superior  oblique.  8.  Inferior  oblique, 
g.  Levator  palpebrse  superioris.  10,10.  Its  anterior 
expansion.     11.  Optic  nerve. — {Sappey.) 


648  TEXT-BOOK  OF  PHYSIOLOGY. 

before  mentioned — namely,  with  the  vertical  axis — thus  moving  the 
ball  only  inwardly  or  outwardly — respectively.  The  other  two  pairs, 
however,  have  their  own  axes  of  action,  and  their  movements  of  the 
ball  must  be  therefore  analyzed  with  regard  to  all  the  three  axes, 
each  of  these  four  muscles  producing  rotation,  elevation,  and  depres- 
sion, and  abduction  or  adduction.  The  superior  and  inferior  recti 
muscles,  forming  one  pair,  move  the  eye  around  a  horizontal  axis 
which  intersects  the  median  plane  of  the  body  in  front  of  the  eyes  at 
an  angle  of  63  degrees,  and  the  superior  and  inferior  oblique  muscles 
forming  the  third  pair  rotate  the  globe  around  a  horizontal  axis  which 
cuts  the  median  plane  of  the  body  behind  the  eyes  at  an  angle  of 
39  degrees.  Thus  it  is  that  each  muscle  moves  the  eye  as  follows,  the 
movement  for  practical  purposes  being  referred  to  the  cornea:  The 
rectus  externus  draws  the  cornea  simply  to  the  temporal  side,  the 
rectus  internus  simply  to  the  nose;  the  superior  rectus  displaces  the 
cornea  upward,  sHghtly  inward,  and  turns  the  upper  part  toward  the 
nose  (medial  torsion) ;  the  inferior  rectus  moves  the  cornea  downward, 
shghtly  inward,  and  twists  the  upper  part  away  from  the  nose  (lateral 
torsion) ;  the  superior  obhque  displaces  the  cornea  downward,  slightly 
outward,  and  produces  medial  torsion;  while  the  inferior  oblique 
moves  the  cornea  upward,  shghtly  outward,  and  produces  lateral  tor- 
sion. These  facts  show  that  for  certain  movements  of  the  eye  at 
least  three  muscles  are  necessary  (see  following  table) : 


Inward  and  (  Rectus  internus. 

downward, -!  Rectus  inferior. 

(.  Obliquus  superior. 
Outward  and         (  Rectus  externus. 


Inward, Rectus  internus. 

Outward, Rectus  externus. 

Upward       .      ^  I^ectus  superior. 
'^  '  '       '  \  Obliquus  inferior. 

Downward    _    /  Rectus  inferior.  ,  upward, j  Rectus  superior. 

'"   '  \  Obliquus  superior.       '  (  Obliquus  inferior. 

Inward  attd       ( Rectus  internus.  |      Outward  and         I  Rectus  externus. 

upward, -j  Rectus  superior.  '  downward, <  Rectus  inferior. 

[  Obliquus  inferior.  I  Obliquus  superior. 

If  both  eyes  have  their  line  of  vision  in  the  horizontal  plane  parallel 
with  each  other  and  with  the  median  plane  of  the  body,  they  are 
said  to  be  in  the  primary  position.  All  other  positions  are  called 
secondary.  Both  eyes  always  move  simultaneously,  which  is  called 
the  associated  movement  of  the  eyes.  There  are  three  forms  of  asso- 
ciated movements:  (i)  movement  of  both  eyes  in  the  same  direction; 
(2)  movements  of  convergence  by  which  the  visual  lines  are  con- 
verged on  a  point  in  the  middle  line  of  the  body;  (3)  movements  of 
divergence,  by  which  the  eyes  are  brought  back  from  convergence  to 
parallelism,  or  even  to  divergence,  as  in  certain  stereoscopic  exercises. 
A  combination  of  (i)  and  (2)  or  of  (i)  and  (3)  takes  place  for  certain 
positions  of  the  object  looked  at. 

Color-perception. — A  beam  of  sunlight  passed  through  a  glass 
prism  is  decomposed  into  a  series  of  colors — red,  orange,  yellow, 


THE  SENSE  OF  SIGHT. 


649 


green,  blue,  and  violet — the  so-called  spectral  colors,  so  well  exem- 
plified in  the  rainbow.  The  spectral  colors  are  termed  simple 
colors,  because  they  can  not  be  any  further  decomposed  by  a  prism. 
Objectively,  the  spectral  colors  consist  of  very  rapid  transverse  vibra- 
tions of  the  ether,  from  about  400  millions  of  millions  per  second  for 
red  to  about  760  millions  of  milHons  for  violet,  but  subjectively  they 
are  sensations  caused  by  the  impact  of  the  ether-waves  on  the  per- 
cipient layer  of  the  retina. 

It  is  possible  to  mix  or  blend  these  spectral  color-sensations  in 
the  eye  by  stimulating  the  same  area  of  the  retina  by  different  spectral 
colors,  either  at  the  same  time  or  in  rapid  succession.  The  following 
table  shows  the  results  of  such  experiments  as  performed  by  v.  Helm- 
holtz  (Dk.  =  dark;  Wh.  =  whitish): 


Violet. 


Indigo.      Cyan-blue  J 


Bluish- 
green. 


Green. 


Greenish 

YELLOW. 


Yellow. 


Red. 

Orange. 

YeUow. 

Gr.-yellow. 

Green. 

Bluish-green. 

Cyan-blue. 


Purple.         i  Dk.-rose.  Wh.-rose. 

Dk.-rose.  Wh.-rose.  White. 

Wh.-rose.  White.  Wh.-green. 

White.  Wh.-green.  Wh.-green. 

White-blue.  Water-blue.  Bl.-green. 

Water-blue.  Water-blue.         

Indigo.  . .  ' 


White. 
Wh.-yellow. 
Wh. -yellow. 
Green. 


WTi.-yellow.    Gold-yellow   Orange. 

Yellow.  Yellow.  

Gr.-yellow.  


These  are  the  mixed  colors.  But  it  is  to  be  observed  that  only  two 
new  color-sensations  can  be  produced,  white  and  purple,  the  remain- 
ing mixed  colors  already  finding  their  equivalent  in  the  spectrum. 
White  and  purple,  therefore,  are  color-sensations  which  have  no 
objective  equivalent  in  a  simple  number  of  ether-vibrations  hke  the 
spectral  colors. 

Two  spectral  colors  which  by  their  mixture  produce  the  sensation 
of  white  are  called  complementary  colors.  Such  are  red  and  green- 
blue,  golden  yellow  and  blue,  green  and  violet.  The  mixture  of  all 
the  spectral  colors  produces  white  again.  This  is  the  result  of  adding 
two  or  more  color-sensations.  Different  results  are  obtained,  however, 
by  adding  color  pigments.  Yellow  and  blue,  for  example,  produce 
in  the  eye  white,  but  on  the  painter's  palette  green.  The  colors  of 
nature  are  usually  mixtures  of  simple  colors,  as  can  be  shown  by 
spectroscopic  analysis  or  by  a  synthesis  of  spectral  colors. 

In  all  color-sensations  we  must  distinguish  three  primary  qualities : 
(i)  hue;  (2)  purity  or  tint;  (3)  brightness  or  luminosity.  The  first 
quality  gives  the  main  name  to  the  color — e.  g.,  red  or  blue — this  de- 
pending on  the  spectral  color  or  the  mixture  of  two  spectral  colors 
with  which  it  can  be  matched.  The  second  quality,  the  tint,  depends 
on  the  admixture  of  white  with  the  ground  color;  and  the  third  quality, 
brightness,  depends  on  the  objective  intensity  of  the  light  and  the 
subjective  sensitiveness  of  the  retina.     Color-perception  thus  far  refers 


650  TEXT-BOOK  OF  PHYSIOLOGY. 

only  to  the  most  sensitive  part  of  the  retina.  At  the  more  peripheral 
parts  of  the  retina  the  colors  are  seen  somewhat  differently,  as  is 
shown  by  the  following  table  giving  the  limits  up  to  which  the  colors 
are  recognized : 

White.  Blue.  Red.  Green. 

Externally qo°  80°  65°  50° 

Internally 60°  55°  50°  40° 

Superiorly 45°  40°  35°  30° 

Inferiorly 70°  60°  45°  35° 

Theories  of  Color-perception. — The  theory  of  v.  Helmholtz, 
originated  by  Thomas  Young  (1807),  assumes  in  its  latest  form  the 
existence  in  the  human  retina  of  three  different  kinds  of  end-organs, 
each  of  which  is  loaded  with  its  own  photo-chemical  substance 
capable  of  being  decomposed  by  a  certain  color,  and  thus  exciting 
the  fiber  of  the  optic  nerve. 

In  the  first  group  these  end-organs  are  loaded  \\\\h  a  red-sensitive 
substance,  which  is  affected  mainly  by  the  red  part  of  the  spectrum; 
the  second  group  has  its  end-organs  provided  with  a  green-sensitive 
substance,  which  is  mainly  excited  by  the  green  color ;  while  the  third 
group  is  provided  with  a  blue-sensitive  substance,  this  latter  being 
mainly  affected  and  decomposed  by  the  blue-violet  portion  of  the 
spectrum.  All  these  three  different  end-organs  are  present  in  every 
part  of  the  most  sensitive  area  of  the  retina,  and  are  connected  by 
separate  nerve-fibers  with  special  parts  of  the  brain,  in  the  cells  of 
which  each  calls  up  its  separate  sensation  of  red  or  green  or  blue. 

Out  of  these  three  primary  color-sensations  all  other  color-sensa- 
tions arise.  If  a  light  mainly  excites  the  red-  or  green-  or  blue-sensi- 
tive substance  of  a  retinal  area,  we  term  it  red,  green,  or  blue,  re- 
spectively. But  if  two  of  these  photo-chemical  substances  are  stimu- 
lated simultaneously,  quite  different  sensations  arise.  Thus  simul- 
taneous stimulation  of  the  red  and  green  substances  gives  rise  to  the 
sensation  of  yellow,  that  of  red  and  blue  to  the  sensation  of  purple, 
and  that  of  blue  and  green  to  the  sensation  of  blue-green.  Simul- 
taneous stimulation  of  all  three  substances  of  a  certain  area  produces 
the  sensation  of  white.  According  to  this  theory,  complementary 
colors  are  all  those  which  together  excite  all  the  three  substances. 
Color-blindness  is  explained  by  this  theory,  on  the  assumption  that 
two  of  the  photo-chemical  substances  have  become  similar  or  equal 
in  composition  to  each  other. 

The  theory  of  Hering,  brought  forward  in  1874,  has  the  under- 
lying assumption  that  the  process  of  restitution  in  a  nerve-element 
is  capable  of  exciting  a  sensation.  This  theory  asserts  that  there  are 
three  visual  substances  in  the  retina — a  white-black,  a  red-green,  and 
a  yellow-blue  visual  substance.  A  destructive  process  in  the  white- 
black  substance,  such  as  is  induced  not  only  by  white  light,  but  also 


THE  SENSE  OF  SIGHT. 


6;i 


by  any  other  simple  or  mixed  color,  produces  the  sensation  of  white, 
while  the  process  of  restitution  or  assimilation  in  this  substance  pro- 
duces the  sensation  of  black.  Similarly,  red  Hght  produces  dis- 
assimilation  or  decomposition  in  the  red-green  substance,  and  this, 
again,  the  sensation  of  red.  Green  light,  however,  favors  the  process 
of  restitution  or  assimilation  in  the  red-green  substances,  and  thus 
gives  rise  to  the  sensation  of  green.  In  the  same  way  the  sensation 
of  yellow  has  its  cause  in  the  decomposition  of  yellow-blue  substance 
induced  by  yellow  light,  while  the  sensation  of  blue  is  produced 
by  an  assimilative  process  in  the  same  substance.  Simultaneous 
processes  of  disassimilation  and  assimilation  in  the  same  visual  sub- 
stance antagonize  each  other,  and  consequently  produce  no  color-sen- 


FiG.  308. — The  Lacrimal  and  Meibomian  Glands,  and  Adjacent  Organs  of 
THE  Eye.  I,  I.  Inner  wall  of  orbit.  2,  2.  Inner  portion  of  orbicularis  palpe- 
brarum. 3,  3.  Attachment  to  circumference  of  base  of  orbit.  4.  Orifice  for 
transmission  of  nasal  arterj'.  5.  Muscle  of  Horner  (tensor  tarsi).  6,  6.  Mei- 
bomian glands.  7,  7.  Orbital  portion  of  lacrimal  gland.  8,  9,  10.  Palpebral 
portion.    11,  11.  Mouths  of  excretory  ducts.  12,  13.  Lacrimal  puncta. —  (Sappey.) 


sation  by  means  of  this  substance,  but  only  the  sensation  of  white,  by 
reason  of  decomposition,  by  both  colors,  in  the  white-black  substance. 
Thus,  yellow  and  blue,  impinging  on  the  same  retinal  area,  have  no 
effect  on  the  yellow-blue  substance,  because  they  are  antagonistic  in 
their  action  on  this  substance,  but  only  produce  the  sensation  of  white, 
as  both  yellow  and  blue  decompose  the  white-black  material.  Color- 
blindness is  explained  by  the  assumption  of  the  absence  of  either  the 
red-green  or  the  yellow-blue  visual  substance  in  the  retina. 

Accessory  Structures. — The  eyeball  is  protected  anteriorly  by 
the  eyelids  and  their  associated  structures,  the  Meibomian  glands, 
the  lacrimal  glands,  and  tears. 


652  TEXT-BOOK  OF  PHYSIOLOGY. 

The  eyelids  consist  of  a  central  framework  of  connective  tissue 
supporting  muscle  tissue  (the  orbicularis  palpebrarum  muscle)  and 
glands,  and  covered  externally  by  skin  and  internally  by  a  modified 
skin,  the  conjunctiva.  The  free  border  of  each  Hd  is  strengthened 
by  a  semilunar  plate  of  dense  fibrous  tissue,  the  tarsus.  The  cuta- 
neous edge  of  the  lid  is  bordered  vv^ith  short  stiff  hairs.  At  the  inner 
extremity  each  eyelid  presents  a  small  opening,  the  punctum  lacri- 
male,  the  beginning  of  the  lacrimal  duct.  The  two  ducts  after 
uniting  open  into  the  nasal  duct. 

The  Meibomian  glands  are  modified  sebaceous  glands  imbedded 
in  the  posterior  portion  of  the  lids  (Fig.  308).  Their  ducts  open  on 
the  free  border  of  the  lid.  These  glands  secrete  an  oleaginous  ma- 
terial resembling  sebaceous  matter  which  accumulates  along  the 
margin  of  the  lid  and  prevents  the  tears  from  flowing  down  the 
cheek. 

The  lacrimal  gland  is  situated  at  the  upper  and  outer  part  of  the 
orbit  cavity.  It  consists  of  a  series  of  compound  tubules  lined  by 
epithelium.  The  secretion  (the  tears)  is  conducted  from  the  gland 
to  the  outer  part  of  the  conjunctiva  by  seven  or  eight  ducts.  The 
lacrimal  secretion  consists  of  water  and  inorganic  salts.  It  is  dis- 
tributed over  the  corneal  surface  during  the  act  of  winking,  thus 
keeping  it  moist  and  free  from  foreign  particles.  It  eventually  passes 
into  the  lacrimal  ducts  and  then  into  the  nose.  The  lacrimal  glands 
receive  secretory  fibers  by  way  of  the  fifth  nerve  and  the  cervical 
sympathetic.  The  secretion  can  be  excited  reflexly  from  stimulation 
of  sensor  nerves  as  well  as  by  emotional  states. 


CHAPTER  XXVI. 
THE  SENSE  OF  HEARING. 

The  physiologic  mechanism  involved  in  the  sense  of  hearing  in- 
cludes the  ear,  the  auditory  nerve,  its  cortical  connections,  and  nerve- 
cells  in  the  cortex  of  the  temporal  lobe. 

Peripheral  excitation  of  this  mechanism  develops  nerve  impulses 
which,  transmitted  to  the  cortex,  evoke  the  sensation  of  sound  and 
its  varying  qualities — intensity,  pitch,  and  timbre. 

The  specific  physiologic  stimulus  to  the  terminal  organ,  the  organ 
of  Corti,  is  the  impact  of  atmospheric  undulations  of  varying  energy 
and  rapidity. 

THE  PHYSIOLOGIC  ANATOMY  OF  THE  EAR. 

The  ear,  the  organ  of  hearing,  is  lodged  vi^ithin  the  petrous  portion 
of  the  temporal  bone.  It  may,  for  convenience  of  description,  be 
divided  into  three  portions:  viz.,  the  external,  the  middle,  and  the 
internal  portions  (Fig.  309). 

The  external  ear  consists  of  the  pinna  or  auricle  and  the  external 
auditory  canal.  The  pinna  is  composed  of  a  thin  layer  of  cartilage 
which  presents  a  series  of  elevations  and  depressions.  It  is  attached 
by  fibrous  tissue  to  the  outer  edge  of  the  auditory  canal  and  covered 
by  a  layer  of  skin  continuous  with  that  covering  adjacent  structures. 
The  general  shape  of  the  pinna  is  concave.  Its  anterior  surface  pre- 
sents, a  httle  below  the  center,  a  deep  depression — the  concha. 

The  external  auditory  canal  extends  from  the  concha  inward  for 
a  distance  of  from  25  to  30  mm.  It  is  directed  at  first  upward,  for- 
ward, inward,  and  then  downward  to  its  termination.  It  is  composed 
partly  of  bone  and  partly  of  cartilage  and  lined  by  a  reflection  of 
the  skin  covering  the  pinna.  At  the  external  portion  of  the  canal 
the  skin  contains  a  number  of  tubular  glands,  the  ceruminous  glands, 
which  resemble  in  their  conformation  the  perspiratory  glands.  They 
secrete  cerumen  or  ear-wax. 

The  middle  ear,  or  tympanum,  is  an  irregularly  shaped  cavity 
hollowed  out  of  the  temporal  bone  and  situated  between  the  external 
auditory  canal  and  the  internal  ear.  It  is  narrow  from  side  to  side, 
though  wider  above  than  below.  It  is  relatively  long  in  its  antero- 
posterior and  vertical  diameters.  The  upper  portion  is  known  as  the 
attic.     The   middle  ear  is   in   communication   posteriorly  with   the 

653 


654 


TEXT-BOOK  OF  PHYSIOLOGY. 


mastoid  cells,  anteriorly  with  the  pharynx  through  the  Eustachian 
tube. 

The  Eustachian  Tube. — The  passageway  between  the  tympanic 
cavity  and  the  naso-pharynx  is  known  from  its  discoverer  as  the 
Eustachian  tube.  It  is  composed  internally  of  bone,  externally  of 
cartilage,  and  is  lined  by  mucous  membrane  covered  with  ciliated 
epithelium.  Near  the  middle  of  its  course  the  tube  is  contracted, 
though  expanded  at  either  extremity  (Fig.  312).  It  measures  about 
40  mm.  in  length.  Its  general  direction  from  the  pharyngeal  orifice 
is  outward,  backward,  and  upward  at  an  angle  of  about  45  degrees. 


Fig.  309. — The  Ear. — i.  Pinna,  or  auricle.  2.  Concha.  3.  External  auditory  canal 
4.  Membrana  tympani.  5.  Incus.  6.  Malleus.  7.  Manubrium  mallei.  8.  Tensor 
tympani.  9.  Tympanic  cavity.  10.  Eustachian  tube.  11.  Superior  semicircular 
canal.  12.  Posterior  semicircular  canal.  13.  External  semicircular  canal.  14. 
Cochlea.  15.  Internal  auditory  canal.  16.  Facial  nerve.  17.  Large  petrosal  nerve. 
18.  Vestibular  branch  of  auditory  nerve.     19.  Cochlear  branch. — (Sappey.) 


The  middle  ear  cavity  is  separated  from  the  external  ear  by 
a  membrane — the  membrana  tympani— and  from  the  internal  ear  by 
an  osseo-membranous  partition  which  forms  a  common  wall  for  both 
cavities.  The  interior  of  the  cavity  is  crossed  from  side  to  side  by  a 
chain  of  bones  and  lined  by  a  mucous  membrane  continuous  with 
that  lining  the  pharynx. 

The  membrana  tympani  is  a  thin,  translucent,  nearly  circular 
membrane,  measuring  about  10  mm.  in  diameter,  placed  at  the  inner 
termination  of  the  external  auditory  canal.  It  is  inclosed  in  a  ring 
of  bone  which  in  the  fetal  condition  can  be  easily  removed,  but  in 


THE  SENSE  OF  HEARING. 


555 


the  adult  condition  can  not  be  removed,  owing  to  its  consolidation 
with  the  surrounding  bone.  This  membrane  consists  primarily  of 
a  layer  of  fibrous  tissue  which  is  covered  externally  by  a  thin  layer 
of  skin  continuous  with  that  lining  the  auditory  canal,  and  internally 
by  a  thin  mucous  membrane.  The  tympanic  membrane  is  placed 
obliquely  at  the  bottom  of  the  auditory  canal,  inchning  from  above 
and  behind  downward  and  forward  at  an  angle  of  about  forty-five 
degrees.  The  external  surface  of  this  membrane  presents  a  funnel- 
shaped  depression,  the  sides  of  which  are  slightly  convex. 

The  Ear-hones. — Running  across  the  tympanic  cavity  and  form- 
ing an  irregular  line  of 
joined  levers  is  a  chain 
of  bones,  which  articu- 
late one  with  another  at 
their  extremities.  These 
bones  are  known  as 
the  malleus,  incus,  and 
stapes.  The  form  and 
arrangement  of  these 
bones  are  shown  in 
Figs.  310,  311. 

The  malleus,  or  ham- 
mer bone,  consists  of  a 
head,  neck,  and  handle, 
of  which  the  latter  is 
attached  to  the  inner 
surface  of  the  membrana 
tympani.  The  incus  or 
anvil  bone  presents  a 
concave  articular  sur- 
face which  receives  the 
head  of  the  malleus. 
The  stapes,  or  stirrup- 
bone,   articulates  exter- 


FiG.  310. — Tympanic  Membrane  and  the  Audi- 
tory Ossicles  (Left)  seen  from  within, 
i.  e.,  FROM  THE  Tympanic  Cavity'.  M.  Manu- 
brium or  handle  of  the  malleus.  T.  Inser- 
tion of  the  tensor  tympani.  h.  Head.  IF. 
Long  process  of  the  malleus,  a.  Incus,  with 
the  short  {K)  and  the  long  (/)  process.  5. 
Plate  of  the  stapes.  Ax,  Ax,  is  the  common 
axis  of  rotation  of  the  auditory  ossicles.  5'. 
The  pinion-wheel  arrangement  between  the 
malleus  and  incus. — (Laudois.) 


nally  with  the  long  pro- 
cess of  the  incus,  and  internally,  by  its  oval  base,  with  the  edges  of 
an  oval  opening,  the  foramen  ovale.     The  entire  chain  is  partially 
supported  by  a  ligament  attached  to  the  short  process  of  the  incus 
and  to  the  walls  of  the  tympanic  cavity. 

The  Tensor  Tympani  Muscle. — This  is  a  delicate  muscle, 
about  15  mm.  in  length,  situated  in  a  narrow  groove  just  above  the 
Eustachian  tube  (Fig.  312).  It  arises  from  the  cartilaginous  portion 
of  the  Eustachian  tube  and  the  adjacent  portion  of  the  sphenoid  bone. 
From  this  origin  it  passes  nearly  horizontally  backward  to  the  tym- 
panic cavity;  just  opposite  to  the  foramen  ovale,  its  tendon  bends  at 


656 


TEXT-BOOK  OF  PHYSIOLOGY. 


Fig.  311.  —  Audi- 
tory Ossicles. 
—  I.  Head  of 
malleus.  2.  Pro- 
cessus brevis.  3. 
Processus  graci- 
lis. 4.  Manubri- 
um. 5.  Long  pro- 
cess of  incus.  6. 
Articulation  be- 
tween incus  and 
stapes.  7.  Stapes. 
• — (Sappey.) 


a  right  angle  over  the  processus  cochleariformis  and  then  passes 
outward  across  the  tympanic  cavity  to  be  inserted  into  the  handle 
of  the  malleus  near  the  neck. 

The  stapedius  muscle  emerges  from  the  cavity 
of  a  pyramid  of  bone  v^hich  projects  from  the 
posterior  wall  of  the  tympanum.  Its  tendon 
passes  forward  to  be  inserted  into  the  neck  of 
the  stapes  bone  near  its  point  of  articulation  with 
the  incus. 

The  internal  ear,  or  labyrinth,  is  located 
within  the  petrous  portion  of  the  temporal  bone. 
It  consists  of  an  osseous  and  a  membranous  por- 
tion, the  latter  contained  within  the  former. 

The  osseous  labyrinth  is  subdivided  into 
vestibule,  semicircular  canals,  and  cochlea. 

The  vestibule  is  a  small,  triangular-shaped 
cavity  between  the  semicircular  canals  and  the 
cochlea.  It  is  separated  from  the  cavity  of  the 
middle  ear  by  an  osseous  partition  which  pre- 
sents near  its  center  an  oval  opening,  the  foramen 
ovale.  In  the  living  condition  this  opening  is 
closed  by  the  base  of  the  stapes  bone,  which  is 
held  in  position  by  an  annular  ligament.  The  inner  wall  presents 
a  number  of  openings  for  the  passage  of  nerve-fibers  (Fig.  313). 

The  semicircular  canals  are  three  in  number,  a  superior  vertical, 
an  inferior  vertical,  and  a  horizontal, 
each  of  which  opens  by  two  orifices  into 
the  cavity  of  the  vestibule,  with  the  ex- 
ception of  the  two  vertical,  which  unite 
at  one  extremity  and  then  open  by  a 
single  orifice.  Each  canal  near  its  vesti- 
bular orifice  is  enlarged  to  almost  twice 
the  size  of  the  rest  of  the  canal,  forming 
what  is  known  as  the  ampulla. 

The  cochlea,  the  anterior  portion  of 
the  labyrinth,  is  a  gradually  tapering 
canal,  about  35  mm.  in  length,  wound 
spirally  two  and  a  half  times  around  a 
central  bony  axis,  the  modiolus.  The 
cavity  of  the  cochlea  is  partially  subdi- 
vided into  two  cavities  by  a  thin  spiral 
plate  of  bone  which  projects  from  the  inner  wall 
lamina  osseous  spiralis.  In  the  natural  condition  this  partition  is 
completed  by  a  connective-tissue  membrane,  so  that  the  two  passages 
are  completely  separated  from  each  other.     The  upper  passage  or 


Fig.  312. —  M,  The  Tensor 
Tympani  Muscle  —  the 
Eustachian  Tube  (Left). 
— {Landois .) 


known  as  the 


THE  SENSE  OF  HEARING. 


657 


Fig. 


313. — Bony  Cochlea,  i. 
Ampulla  of  superior  semi- 
circular canal.  2.  Horizontal 
canal.  3.  Junction  of  supe- 
rior and  posterior  semicircu- 
lar canals.  4.  The  posterior 
semicircular  canal.  5.  Fora- 
men rotundum.  6.  Foramen 
ovale.     7.  Cochlea. 


scala  is  in  free  communication  with  the  vestibule,  and  is  known  as 
the  scala  vestihuli;  the  lower  passage  or  scala  in  the  dead  condition 
communicates  with  the  tympanum  by  means  of  a  round  opening, 
the  foramen  rotundum,  and  is  therefore  known  as  the  scala  tympani. 
In  the  living  condition  this  opening  is  completely  closed  by  a  mem- 
brane, a  second  membrana  tympani. 
Both  the  scalae  vestibuli  and  tympani 
communicate  at  the  apex  of  the 
cochlea  by  a  small  opening,  the  heli- 
cotrema.  The  modiolus,  the  central 
bony  axis,  is  perforated  from  base  to 
apex  by  a  canal  for  the  passage  of  the 
auditory  nerve-fibers;  lateral  canals, 
diverging  from  the  central  canal,  pass 
through  the  osseous  lamina  spiralis 
and  transmit  fibers  of  the  auditory 
nerve.  The  interior  of  the  bony  laby- 
rinth is  lined  by  periosteum  covered  by 
epithelium  and  in  communication  with 
lymph-spaces  at  the  base  of  the  skull  by 
means  of  the  aqueduct  of  the  vestibule. 
The  membranous  labyrinth, 
lying  within  the  osseous  labyrinth,  corresponds  with  it  in  form,  though 
it  is  smaller  in  size.  It  may  be  subdivided  into  vestibule,  semi- 
circular canals,  and  cochlea  (Fig.  314). 

The  vestibular  portion  consists  of  two  small  sacs,  the  utricle  and 
the  saccule,  which  communicate  with  each  other  by  means  of  the 

two  branches  of  a  duct  passing  through 
the  aqueduct  of  the  vestibule — the  ductus 
endolymphaticus. 

The  semicircular  canals  communicate 
with  the  utricle  in  the  same  manner  as 
the  bony  canals  communicate  with  the 
vestibule.  The  saccule  communicates 
with  the  membranous  cochlea  by  a  short 
canal,  the  canalis  reuniens.  The  walls  of 
the  utricle,  saccule,  and  semicircular 
canals  are  composed  of  connective  tissue 
hned  by  epithehum.  At  the  points  of 
entrance  of  the  auditory  nerve,  the 
maculcB  acusticce,  in  all  three  structures,  the  epithehum  undergoes  a 
marked  change  in  appearance  and  structure.  It  becomes  columnar 
in  shape  and  provided  with  stiff  hair-like  processes  or  threads, 
which  project  into  the  cavity.  In  the  saccule  and  utricle  the  hair- 
like processes  are  covered  by  a  layer  of  small  crystals  of  calcium 
42 


Fig.  .314. — I.  Utricle.  2.  Sac- 
cule. 3.  Vestibular  end  of 
cochlea.  4.  Canalis  reuniens. 

5.  Membranous      cochlea. 

6.  Membranous  semicir- 
cular canal.  —  {Potter's 
"Anatomy.") 


658 


TEXT-BOOK  OF  PHYSIOLOGY. 


Fig 


carbonate  held  together  by  a  gelatinous  material.     The  crystals  are 
known  as  otoliths. 

The  libers  of  the  vestibular  nerve,  arising  from  the  cells  of  the 

ganglion  of  Scarpa  in  the  internal 
auditory  meatus,  send  their  peri- 
pherally directed  branches  through  the 
foramina  in  the  inner  wall  of  the  vesti- 
bule, through  the  walls  of  the  utricle 
and  semicircular  canals  near  the  am- 
pulla. As  the  fibers  approach  the 
maculas  acusticae  they  subdivide  into 
delicate  fibrillae,  which  ultimately 
become  histologically  and  physiologi- 
cally related  to  the  neuro-epithelium. 
From  the  relation  of  the  nerve-fibers 
to  the  epithelium,  the  latter  must  be 
regarded  as  the  highly  specialized 
terminal  organ  of  the  vestibular  portion 
of  the  auditory  nerve. 

The  cochlea  is  a  closed  mem- 
branous tube  situated  between  the 
osseous  lamina  spiralis  and  the  outer 
bony  wall.  A  transection  of  the  entire 
cochlea  shows  the  relation  of  the  os- 
seous and  membranous  portions  (Fig. 
316).  The  cochlear  tube  is  triangular  in  shape.  The  base  is  attached 
to  the  bony  wall,  the  apex  to  the  edge  of  osseous  lamina  spirahs. 
One  side  of  the  tube  forms 
in  part  the  membrane  of 
Reissner,  the  other  side  forms 
in  part  the  basilar  membrane. 
The  sides  of  the  cochlea  to- 
ward the  scala  vestibuli  and 
scala  tympani  are  covered 
with  epithelium.  The  tri- 
angular cavity  of  the  cochlear 
tube  is  known  as  the  scala 
media.  The  inner  surface 
of  the  cochlear  tube  is  lined 
by  epithelium,  which  be- 
comes extraordinarily  modi- 
fied and  specialized  along  the 
surface  of  the  basilar  membrane,  to  constitute  what  is  known  as — 
The  Organ  of  Corti. — In  Fig.  316  this  organ  is  represented  as 
it  appears  on  cross-section  of  the  cochlea.     It  consists  primarily  of 


315. — Section  of  Wall  of 
Utricle  of  the  Internal 
Ear,  through  macular  region, 
from  rabbit,  showdng  otoliths 
(o),  embedded  within  granu- 
lar substance  {g).  h.  Cili- 
ated cells  with  processes  {p), 
extending  between  sustentacu- 
lar  elements  {s).  m.  Base- 
ment membrane,  n.  Nerve- 
fibers  within  fibrous  tissue  (/) 
passing  toward  hair-cells  and 
becoming  non-medullated  at 
basement-membrane.  — {After 
Pier  sot.) 


Scala  lympani 


Fig. 


Org.Corli 
mb.basilaris 


316. — A  Transverse  Section  of 
Turn  of  the  Cochlea. 


THE  SENSE  OF  HEARING.  659 

an  arch  composed  of  two  modified  epithelial  cells  known  as  the  rods 
or  pillars  of  Corti,  which  rest  below  on  the  basilar  membrane,  but 
meet  and  interlock  above ;  it  consists  secondarily  of  a  series  of  colum- 
nar epithelial  cells  provided  with  hair-like  processes  which  rest  upon 
and  are  supported  by  the  rods  both  on  the  inner  and  outer  aspects 
of  the  arch.  The  space  beneath  the  arch  is  known  as  the  tunnel. 
The  inner  hair  cells  are  not  nearly  so  numerous  as  the  outer  hair 
cells.  The  epithelial  cells  external  to  the  outer  and  inner  hair  cells 
are  supporting  or  sustentacular  in  character. 

The  organ  of  Corti  extends  the  entire  length  of  the  cochlea.  The 
number  of  rods  which,  standing  side  by  side,  form  the  inner  limb 
of  the  arch  is  estimated  at  5600;  the  number  which  form  the  outer 
hmb  is  estimated  at  3850.  The  outer, rods  are  broader  than  the 
inner  and  at  some  places  articulate  with  two  or  three  inner  rods. 
The  upper  edges  of  the  rods  are  flattened,  elongated,  and  project 
outward,  forming  a  reticulated  membrane  through  the  meshes  of 
which  the  hair-like  processes  of  the  cells  project. 

From  the  connective-tissue  thickening  on  the  upper  surface  of 
the  osseous  lamina  spirahs  there  extends  outward  over  the  organ  of 
Corti  a  thin  membrane,  the  memhrana  tectoria.  The  common  cavity 
between  the  walls  of  the  osseous  and  membranous  labyrinth  in  the 
vestibule,  the  semicircular  canals,  in  the  scala  vestibuli  and  scala 
tympani  of  the  cochlea,  is  filled  with  a  clear  fluid — the  perilymph;  the 
common  cavity  within  the  walls  of  the  entire  membranous  labyrinth 
is  also  filled  with  a  similar  fluid — the  endolymph.  The  hair-like  pro- 
cesses of  the  epithehal  cells  covering  the  maculae  acusticae  and  the 
rods  of  Corti  are  consequently  bathed  by  endolymph.  Both  fluids 
are  in  relation  with  the  subarachnoid  lymph-spaces  at  the  base  of  the 
brain,  the  perilymph  through  the  aqueduct  of  the  vestibule,  the  endo- 
lymph through  the  endolymphatic  duct. 

The  fibers  of  the  cochlear  nerve,  arising  from  the  ganglion  cells 
of  the  spiral  gangHofi  situated  in  the  osseous  lamina  spiralis  near 
the  modiolus,  send  their  peripheral  branches  to  the  saccule  and  to 
the  organ  of  Corti.  As  they  approach  this  structure  they  lose  their 
medullary  sheath  and  become  naked  axis-cylinders.  The  fibers  then 
divide  into  two  parts,  of  which  one  passes  to  the  inner  hair  cells;  the 
other  passes  between  the  inner  rods  and  crosses  the  tunnel  between 
the  outer  rods  to  the  outer  hair  cells.  The  exact  method  of  termina- 
tion of  these  fibers  in  the  hair  cells  is  unknown,  but  doubtless  it  is 
both  histologic  and  physiologic. 

From  the  relation  of  the  nerve-fibers  to  the  organ  of  Corti  the 
latter  must  be  regarded  as  the  highly  specialized  terminal  organ  of 
the  cochlear  division  of  the  auditorv  nerve. 


66o  TEXT-BOOK  OF  PHYSIOLOGY. 


THE  PHYSIOLOGY  OF  AUDITION. 

The  general  function  of  the  ear  is  the  reception  and  transmission 
of  atmospheric  vibrations  from  the  concha  to  the  percipient  elements 
— the  hair  cells — of  the  organ  of  Corti.  The  vibratory  excitation  of 
these  end-organs  thus  caused,  is  the  basis  of  auditory  perceptions. 
The  atmospheric  vibrations  are  collected  by  the  pinna  and  concha, 
conveyed  by  the  auditory  canal  to  the  tympanic  membrane,  trans- 
mitted by  the  chain  of  bones  to  the  labyrinth  to  pass  successively 
through  the  perilymph,  the  membranous  walls,  the  endolymph,  to 
the  hair  cells.  The  nerve  impulses  generated  by  these  vibrations  are 
then  transmitted  by  the  cochlear  nerve  to  the  auditory  centers  of  the 
cerebrum,  where  the  sensations  of  sound  are  evoked.  In  order  to 
appreciate  the  function  of  the  individual  structures  concerned  in  this 
general  function  there  must  be  kept  in  mind  a  few  of  the  character- 
istics of  atmospheric  vibrations. 

Atmospheric  Vibrations. — The  vibrations  of  the  atmosphere 
which  are  the  objective  causes  of  the  sensations  of  sound  are  com- 
municated to  it  by  the  vibrations  of  elastic  bodies  such  as  tuning- 
forks,  rods,  strings,  membranes,  etc.  These  produce  in  the  air  a 
to-and-fro  movement  of  its  particles,  resulting  in  a  succession  of 
alternate  condensations  and  rarefactions  which  are  propagated  in 
all  directions.  The  impact  of  a  rhythmic  succession  of  such  con- 
densations on  the  ear  gives  rise  to  musical  sounds;  the  impact  of  an 
arrhythmic  or  irregular  succession  gives  rise  to  noises. 

If  a  writing  point  attached  to  a  tuning-fork  in  vibration  be  placed 
in  contact  with  a  travehng  recording  surface,  each  vibration  will  be 
recorded  in  the  form  of  a  wave.  For  this  reason  atmospheric  vibra- 
tions are  generally  spoken  of  as  sound-waves.  A  line  drawn  hori- 
zontally through  such  a  curve  indicates  the  position  of  rest  of  the 
fork;  the  extent  of  the  curve  on  each  side  of  this  line  indicates  the 
excursion  of  the  fork  or  the  amplitude  of  its  movement. 

The  sounds  which  physiologically  result  from  the  impact  and 
transmission  of  the  effects  of  sound-waves,  possess  intensity,  pitch,  and 
quality  or  tone. 

The  intensity  or  loudness  of  a  sound  depends  on  the  amplitude 
of  the  vibration  which  causes  it.  The  greater  the  amplitude  or 
swing  of  the  vibrating  body,  the  greater  is  the  energy  with  which  it 
strikes  the  ear. 

The  pitch  of  a  sound  depends  on  the  number  of  vibrations  which 
strike  the  ear  in  a  unit  of  time — a  second.  The  greater  the  number, 
the  higher  the  pitch.  Thus  while  the  pitch  of  the  sound  caused  by 
the  note  C,  on  the  first  leger  line  below,  of  the  music  scale,  corre- 
sponds to  256  vibrations,  the  pitch  of  the  sound  caused  by  the  note  C 
an  octave  above,  corresponds  to  512  vibrations.     The  lowest  rate  of 


THE  SENSE  OF  HEARING.  66i 

vibration  which  can  produce  a  distinct  sound  varies  in  different 
individuals  from  14  to  18;  the  liighest  rate  varies  from  35,000  to 
40,000  per  second.  Between  these  two  extremes  lies  the  range  of 
audibihty,  which  embraces  about  11  octaves.  Vibrations  less  than 
14  per  second  can  not  be  perceived  as  a  continuous  sound;  vibrations 
beyond  40,000  also  fail  to  be  so  perceived.  In  the  ascent  of  the 
music  scale  from  the  lowest  to  the  highest  regions  there  is  a  gradual 
increase  in  the  vibration  frequency. 

The  quality  of  a  sound  depends  on  the  jorm  of  the  vibration.  It 
is  this  feature  which  gives  rise  to  those  differences  in  sensations  which 
permit  one  to  distinguish  one  instrument  from  another  when  both 
are  emitting  the  same  note.  The  form  of  the  sound-wave  in  any 
given  instance  is  the  resultant  of  a  combination  of  a  fundamental 
vibration  and  certain  secondary  vibrations  of  subdivisions  of  the 
vibrating  body.  These  secondary  vibrations  give  rise  to  what  is 
known  as  overtones.  By  their  union  with  and  modification  of  the 
fundamental  vibration  there  is  produced  a  special  form  of  vibration 
which  gives  rise  not  to  a  simple  but  a  composite  sensation.  It  is 
for  this  reason  that  the  same  note  of  the  piano,  the  violin,  and  the 
human  voice  varies  in  quality. 

The  Function  of  the  Pinna  and  External  Auditory  Canal. — 
In  those  animals  possessing  movable  ears  the  pinna  plays  an  im- 
portant part  in  the  collection  of  sound-waves.  In  man  it  is  doubtful 
if  it  plays  a  part  at  all  necessar}^  for  hearing.  Nevertheless  an  indi- 
vidual with  defective  hearing  may  have  the  perception  of  sound 
increased  by  placing  the  pinna  at  an  angle  of  90  degrees  to  the  side 
of  the  head  or  by  placing  the  hand  behind  it.  The  external  auditory 
canal  transmits  the  sonorous  vibrations  to  the  tympanic  membrane. 
From  the  obliquity  of  this  canal  it  has  been  supposed  that  the  vibra- 
tions, after  passing  the  concha,  undergo  a  series  of  reflections  on  their 
way  to  the  tympanic  membrane,  which,  owing  to  its  inclination,  would 
be  struck  by  them  in  a  much  more  effective  manner. 

The  Function  of  the  Tympanic  Membrane. — The  function  of 
the  tympanic  membrane  is  the  reception  of  the  atmospheric  vibrations 
which  are  transmitted  to  it.  This  it  does  by  vibrating  in  unison  with 
them.  The  vibrations  which  the  membrane  exhibits  correspond  in 
amphtude,  in  frequency,  and  in  form  to  those  of  the  atmosphere. 
That  this  membrane  actually  reproduces  all  vibrations  within  the 
range  of  audibility  has  been  experimentally  demonstrated.  The 
membrane  not  being  fixed,  as  far  as  its  tension  is  concerned,  does 
not  possess  a  fixed  fundamental  note,  like  a  stationary  fixed  mem- 
brane, and  is  therefore  just  as  well  adapted  for  the  reception  of  one 
set  of  vibrations  as  another.  This  is  made  possible  by  variations  in 
its  tension  in  accordance  with  the  pitch  or  frequency  of  the  atmos- 
pheric vibrations.     In  the  absence  of  vibration  the  membrane  is  in 


662  TEXT-BOOK  OF  PHYSIOLOGY. 

a  condition  of  relaxation;  with  tlic  advent  of  sound-waves  possessing 
a  gradual  increase  of  pitch,  as  in  the  ascent  of  the  music  scale,  the 
tension  of  the  membrane  increases  until  its  maximum  is  reached  at 
the  upper  limit  of  the  range  of  audibility.  By  this  change  in  tension 
certain  tones  become  perceptible  and  distinct,  while  others  become 
imperceptible  and  indistinct. 

The  Function  of  the  Tensor  Tympani  Muscle. — The  function 
of  this  muscle  is,  as  its  name  indicates,  to  change  and  to  fix  the 
tension  of  the  tympanic  membrane,  so  that  it  can  most  readily  vibrate 
in  unison  with  vibrations  of  varying  degrees  of  rapidity.  The  tendon 
of  this  muscle  playing  around  the  processus  cochleariformis  is  attached 
almost  at  a  right  angle  to  the  handle  of  the  malleus.  Hence  as  the 
muscle  contracts  it  exerts  its  traction  from  the  process  and  draws  the 
handle  of  the  malleus  inward,  thus  increasing  the  convexity  of  the 
tympanic  membrane  and  at  the  same  time  its  tension.  With  the 
relaxation  of  the  muscle  the  handle  of  the  malleus  passes  outward, 
and  the  convexity  and  tension  diminish. 

In  the  ascent  of  the  music  scale,  each  note  corresponding  to  an 
increase  in  vibration  frequency,  requires  for  its  perception  an  increase 
in  tension  and  an  increase  in  the  force  of  the  contraction  of  the  tensor 
muscle.  In  the  descent  of  the  music  scale  the  reverse  conditions 
obtain.  The  contraction  of  the  muscle  is  of  the  nature  of  a 
single  twitch,  and  of  just  sufhcient  force  and  duration  to  tense  the 
membrane  for  a  given  rate  of  vibration. 

The  contraction  of  the  muscle  is  excited  reflexly.  The  afferent 
path  is  through  fibers  of  the  trigeminal  nerve  distributed  to  the  tym- 
panic membrane ;  the  efferent  path  is  through  fibers  in  the  small  root 
of  the  trigeminal.  The  stimulus  is  sudden  pressure  on  the  tympanic 
membrane.  The  more  frequently  and  forcibly  the  stimulus  is  applied, 
the  greater  is  the  muscle  response.  The  tensor  tympani  muscle  may 
therefore  be  regarded  as  an  accommodative  apparatus  by  which  the 
tympanic  membrane  is  adjusted  for  the  reception  of  vibrations  of 
varying  degrees  of  frequency. 

The  Function  of  the  Chain  of  Bones. — The  function  of  the 
chain  of  bones  is  to  transmit  the  effects  of  the  atmospheric  vibrations 
to  the  fluid  of  the  labyrinth.  The  manner  in  which  this  is  accom- 
plished becomes  evident  from  the  relation  which  the  bones  of  this 
chain  bear  to  one  another  and  to  the  tympanic  membrane  on  the 
one  hand  and  to  the  fluid  of  the  labyrinth  on  the  other. 

When  pressure  is  made  on  the  outer  surface  of  the  tympanic  mem- 
brane it  is  at  once  pushed  inward,  carrying  with  it  the  handle  of 
the  malleus,  the  head  at  the  same  time  rotating  outward  around  an 
axis  corresponding  to  its  ligamentous  attachments.  As  the  handle 
moves  inward  a  small  ledge  of  bone  just  below  the  malleo-incudal 
joint  locks  with,  and  hence  pushes  inward,  the  long  process  of  the 


THE  SENSE  OF  HEARING.  663 

incus.  Since  this  process  is  united  at  almost  a  right  angle  to  the 
stapes  bone,  the  latter  is  forced  toward  and  into  the  foramen  ovale, 
thus  producing  a  pressure  on  the  perilymph.  With  the  cessation  of 
the  pressure  the  elastic  forces  of  the  membrane  and  of  the  hgaments 
return  the  handle  of  the  malleus  to  its  former  position;  by  the  un- 
locking of  the  malleo-incudal  joint  the  entire  chain  also  returns  to 
its  former  position  without  exerting  undue  traction  on  the  basal 
attachment  of  the  stapes. 

As  the  long  process  of  the  incus  is  shorter  than  the  handle  of  the 
malleus,  and  as  the  movement  between  them  takes  place  around  an 
axis  from  before  backward,  it  follows  that  the  excursion  of  the  incus 
and  stapes  will  be  less  than  that  of  the  malleus,  while  the  force  will 
be  greater.  Hence  as  the  vibrations  are  transferred  from  the  tym- 
panic membrane  of  large  area  to  the  base  of  the  stapes  of  small  area 
(20  to  1.5),  they  lose  in  amphtude  but  increase  in  force.  Their  pres- 
sure on  the  perilymph  is  therefore  thirty  times  greater  than  on  the 
membrana  tympani.  In  addition  to  its  function  as  a  transmitter  of 
vibrations,  the  chain  of  bones  serves  as  a  point  of  attachment  for 
muscles  which  regulate  the  tension  of  the  tympanic  membrane  and 
the  pressure  on  the  labyrinth. 

The  Function  of  the  Stapedius  Muscle. — The  function  of  the 
stapedius  muscle  is  a  subject  of  much  discussion.  According  to 
Henle,  its  function  is  to  so  adjust  the  stapes  bone  that  it  will  be 
prevented  from  exerting  an  undue  pressure  on  the  perilymph  during 
the  inward  excursions  of  the  incus  process.  According  to  Toynbee, 
its  function  is  to  press  the  posterior  part  of  the  stapes  inward,  make 
it  a  fixed  point,  and  place  the  anterior  part  in  such  a  position  that 
it  will  vibrate  freely  and  accurately. 

The  Function  of  the  Eustachian  Tube.^ — In  order  that  the  tym- 
panic membrane  may  vibrate  freely  it  is  essential  that  the  air  pressure 
on  both  sides  shall  be  equal  at  all  times.  This  is  made  possible  by 
the  Eustachian  tube.  Were  it  not  for  this  passageway,  with  each 
inward  swing  of  the  membrane  the  air  in  the  tympanic  cavity  would 
be  condensed  and  its  pressure  raised,  in  consequence  of  which  the 
movement  of  the  membrane  would  be  retarded;  with  each  outw^ard 
swing,  the  air  would  be  rarified  and  its  pressure  lowered  below  that 
of  the  atmosphere,  and  in  consequence  the  movement  outward  would 
be  retarded;  the  maximum  response,  therefore,  of  the  membrane  to 
a  given  vibration  could  not  be  attained  and  the  resulting  sound 
would  be  muffied  and  indistinct.  But  as  with  each  vibration  of  the 
membrane  the  air  can  pass  into  and  out  of  the  tympanum  through 
this  tube,  inequalities  of  pressure  are  prevented  and  a  free  vibration 
peiTuitted. 

The  impairment  in  the  acuteness  of  hearing  which  is  caused  by 
either  a  rise  or  fall  of  pressure  in  the  middle  ear  can  be  shown — 


664  TEXT-BOOK  OF  PHYSIOLOGY. 

1.  By  closing  the  mouth  and  nose  and  then  forcing  air  from  the 

lungs  through  the  Eustachian  tube  into  the  tympanum,  thus  in- 
creasing the  pressure. 

2.  By  closing  the  mouth  and  nose  and  then  making  an  effort  of 

deglutition.     As  this  act  is  attended  by  an  opening  of  the  phar- 
yngeal end  of  the  Eustachian  tube,  the  air  in  the  tympanum  is 
partly  withdrawn  and  the  pressure  lowered.     In  each  instance 
hearing  is  impaired.     After  either  experiment  the  normal  con- 
dition is  restored  by  swallowing  with  the  nasal  passages  open. 
The   Functions   of   the   Internal   Ear. — From    the   anatomic 
arrangement  of  the  structures  of  the  internal  ear  it  is  evident  that  if 
the  vibrations  of  the  stapes  bone  are  to  reach  the  peripheral  organs — 
the  hair  cells — of  both  the  vestibular  and  cochlear  nerves,  they  must 
traverse  successively  the  perilymph,  the  membranous  walls,  and  the 
endolymph.     As  the  perilymph  is  incompressible,  the  inward  move- 
ment of  the  stapes  would  be  prevented  were  it  not  for  the  elastic 
character  of  the  membrane  closing  the  foramen  rotundum.     The 
pressure  wave  occasioned  by  each  inward  movement  of  the  stapes 
is  transmitted  through  the  scala  vestibuli,  the  helicotrema,  the  scala 
tympani,  to  this  membrane,  which  by  virtue  of  its  elasticity  is  pressed 
into  the  tympanic  cavity.     With  the  outward  movement  of  the  stapes, 
equilibrium  is  at  once  restored. 

The  Functions  of  the  Cochlea. — The  cochlea  is  the  portion 
of  the  internal  ear  which  is  concerned  in  the  perception  of  tones. 
The  arrangement  of  the  histologic  elements  of  the  organ  of  Corti 
indicates  that  they  in  some  way  respond  to  the  vibrations  of  varying 
frequency  and  form,  and  through  the  development  of  nerve  im- 
pulses, evoke  the  sensations  of  pitch  and  quahty.  The  manner  in 
which  this  is  accomphshed  is  largely  a  matter  of  speculation.  While 
many  theories  have  been  offered  in  explanation  of  the  power  to 
distinguish  the  pitch  and  the  quahty  or  timbre  of  a  tone,  most  physiol- 
ogists prefer  that  of  Helmholtz,  who  regarded  the  transverse  fibers 
of  the  basilar  membrane  as  the  elements  immediately  concerned, 
and  compared  them,  both  in  their  arrangement  and  power  of  sympa- 
thetic vibration,  with  the  strings  of  a  piano.  He  said:  "If  we  could 
so  connect  every  string  of  a  piano  with  a  nerve-fiber  that  the  nerve- 
fiber  would  be  excited  as  often  as  the  string  vibrated,  then,  as  is 
actually  the  case  in  the  ear,  every  musical  note  which  affected  the 
instrument  would  excite  a  series  of  sensations  exactly  corresponding 
to  the  pendulum-like  vibrations  into  which  the  original  movements 
of  the  air  can  be  resolved;  and  thus  the  existence  of  each  individual 
overtone  would  be  exactly  perceived,  as  is  actually  the  case  with 
the  ear.  The  perception  of  tones  of  different  pitch  would,  under 
these  circumstances,  depend  upon  different  nerve-fibers,  and  hence 
would  occur  quite  independently  of  each  other.     Microscopic  in- 


THE  SENSE  OF  HEARING.  665 

vestigation  shows  that  there  are  somewhat  similar  structures  in 
the  ear.  The  free  ends  of  all  the  nerve-fibers  are  connected  with 
small  elastic  particles  which  we  must  assume  are  set  into  sympathetic 
vibration  by  sound-waves."     (Stirhng.) 

The  mechanism  might  be  regarded,  therefore,  somewhat  as 
follows:  The  sound-waves  received  by  the  membrana  tympani  and 
transmitted  by  the  chain  of  bones  to  the  fenestra  ovahs  produce 
variable  pressures  in  the  fluids  of  the  internal  ear;  these  pressures 
vsiry  in  intensity,  in  number,  and  in  quahty,  and  correspond  with 
the  intensity,  pitch,  and  quahty  of  the  tones.  If,  therefore,  a  com- 
pound wave  of  pressure  be  communicated  by  the  base  of  the  stapes, 
it  will  be  resolved  into  its  constituents  by  the  different  transverse 
fibers  of  the  basilar  membrane,  each  picking  out  its  peculiar  portion  of 
the  wave  and  thus  stimulating  corresponding  nerve  filaments.  Thus 
different  nerve  impulses  are  transmitted  to  the  brain,  where  they  are 
fused  in  such  a  manner  as  to  give  rise  to  a  sensation  of  a  particular 
quahty,  but  still  so  imperfectly  fused  that  each  constituent,  by  a 
strong  efi'ort  of  attention,  may  be  still  recognized.  The  transverse 
fibers  of  the  basilar  membrane  vary  in  length  from  0.04125  mm.  at 
the  base  of  the  cochlea  to  0.495  ^^-  ^.t  the  apex,  and,  according  to 
Retzius,  are  about  24,000  in  number.  As  the  human  ear  usually 
cannot  distinguish  more  than  11,064  tones,  it  is  evident  that  there 
is  a  sufiiicient  anatomic  basis  for  this  theory. 

The  functions  of  the  semicircular  canals  have  already  been 
stated  in  connection  with  the  chapter  relating  to  the  functions  of  the 
cerebellum. 


CHAPTER  XXVII. 
REPRODUCTION. 

Reproduction  is  the  process  by  which  a  new  individual  is  initiated 
and  developed  and  the  species  to  which  it  belongs  is  preserved. 
Reproduction  is  the  result  of  the  union  and  subsequent  development 
of  germ-  and  sperm-cells.  These  cells  are  produced  and  their  union 
accomplished  by  the  cooperation  of  the  reproductive  organs  charac- 
teristic of  the  two  sexes. 

Embryology  is  a  department  of  anatomic  science  which  has  for 
its  object  the  investigation  of  the  successive  stages  that  the  new 
being  passes  through  during  its  gradual  development  prior  to  birth. 

THE  REPRODUCTIVE  ORGANS  OF  THE  FEMALE. 

The  reproductive  organs  of  the  female  comprise  the  ovaries, 
Fallopian  tubes,  uterus,  and  vagina  (Fig.  317). 

The  Ovaries. — The  ovaries  are  two  small,  flattened  bodies, 
measuring  about  40  mm.  in  length  and  20  in  breadth.  They  are 
situated  in  the  cavity  of  the  pelvis,  one  on  either  side,  and  embedded 
in  a  fold  of  the  peritoneum,  known  as  the  broad  ligament.  A  section 
of  the  ovary  shows  that  it  consists  externally  of  a  thin,  firm,  connective- 
tissue  membrane  and  internally  of  a  fine  connective-tissue  stroma, 
supporting  blood-vessels,  non-striated  muscle-fibers  and  nerves,  and 
containing  in  its  meshes  a  very  large  number  of  spheric  sacs  named 
after  their  discoverer,  de  Graaf,  the  Graafian  sacs  or  follicles.  These 
follicles  are  very  numerous  and  are  present  in  all  portions  of  the 
ovary,  though  they  are  most  abundant  toward  its  peripheral  portions. 
It  is  estimated  that  the  human  ovary  contains  from  20,000  to  40,000 
follicles.  The  follicles  vary  considerably  in  size ;  while  many  are  visible 
to  the  unaided  eye,  others  require  for  their  detection  high  powers 
of  the  microscope.  Although  the  folHcles  are  present  in  the  ovary 
at  the  time  of  birth,  it  is  not  until  the  period  of  puberty  that  they 
assume  functional  activity. 

From  this  time  on  to  the  catamenial  period  there  is  a  constant 
growth  and  development  of  these  follicles.  Each  follicle  consists  of 
an  external  investment  of  fibrous  tissue  and  blood-vessels,  and  an 
internal  investment  of  cells,  the  memhrana  granulosa.  At  the  lower 
portion  of  this  membrane  there  is  an  accumulation  of  cells,  the  pro- 

666 


REPRODUCTION. 


66: 


ligerous  disc  (Fig.  318).  The  cavity  of  the  foUicle  contains  a  shghtly 
yellowish,  alkaline,  albuminous  fluid,  a  transudate  in  all  probability 
from  the  blood-vessels.  The  Graafian  follicle  is  of  especial  interest, 
for  it  is  in  this  structure,  and  more  especially  in  the  proligerous 
disc,  that  the  true  germ- cell  or  ovum  is  developed. 

The  ovum  is  a  spheric  body  measuring  about  0.3  mm.  in  diameter. 
It  consists  of  a  mass  of  living,  protoplasmic  material,  cytoplasm,  a 
nucleus  or  germinal  vesicle,  and  a  nucleolus  or  germinal  spot.  The 
cytoplasm  presents  toward  its  central  portion  a  quantity  of  granular 
material,  partly  fatty  in  character,  the  deutoplasm  or  vitellus.  The 
peripheral  portion  of  the  cytoplasm  is  surrounded  by  a  delicate 
radially  striated  border,  the  zona  pellucida  or  radiata  (Fig.  319). 


^  ■ijH 


Fig.  317. — Uterus,  Fallopian  Tubes  amj  U\aries;  Posterior  View.  1,  i. 
Ovaries.  2,2.  Fallopian  tubes.  t„  t,-  Fimbriated  extremity  of  the  left  Fallopian 
tube  seen  from  its  concavity.  4.  Opening  of  the  left  tube.  5.  Fimbriated 
extremity  of  the  right  tube,  posterior  view.  6,  6.  Fimbriae  which  attach  the 
extremity  of  each  tube  to  the  ovary.  7,  7.  Ligaments  of  the  ovar>'.  8,  8,  9,  9. 
Broad  hgament.  10.  Uterus.  11.  Cervix  uteri.  12.  Os  externum.  13,  13. 
Vagina. 


The  nucleus  consists  of  a  nuclear  membrane  enclosing  contents. 
The  latter  consist  of  an  amorphous  material  in  which  is  embedded 
a  network,  some  of  the  threads  of  which  have  a  strong  affinity  for 
certain  staining  materials,  and  hence  are  known  as  chromatin,  while 
others  stain  less  deeply  and  are  known  as  achromatin. 

The  Fallopian  Tubes. — The  Fallopian  tubes  are  about  12  centi- 
meters in  length  and  extend  from  the  upper  angles  of  the  uterus 
to  the  ovaries.  Each  tube  is  somewhat  trumpet-shaped,  the  narrow 
portion  being  close  to  the  uterus,  the  wide  portion  close  to  the  ovary. 
The  outer  extremity  of  the  tube  is  expanded  and  subdivided,  and 
presents  a  series  of  processes  termed  fimbriae,  one  of  which  is  attached 


668 


TEXT-BOOK  OF  PHYSIOLOGY, 


to  the  ovary.  The  tube  consists  of  three  coats — an  external  or 
serous;  a  middle  or  muscular,  the  fibers  of  which  are  arranged 
longitudinally  and  transversely;  and  an  internal  or  mucous.  The 
surface  of  the  mucous  coat  is  covered  with  a  layer  of  ciliated  epithelial 
cells,  the  motion  of  which  is  toward  the  uterus. 

The  Uterus. — The  uterus  is  pyriform  in  shape  and  divided  into 
It  measures,  before  the  first  pregnancy,  about 

7  cm.  in  length,  5  cm. 
b 
^K>  -^  ..  f  \'^ 


a  body  and  neck. 


■^. 


^  V 


t-^fjT 


I 


/' 


>-' 
U 


■/ 


in  breadth  and  2\ 
cm.  in  thickness.  A 
frontal  section  of  the 
uterus  shows  a  central 
cavity  which  in  the 
body  is  triangular  in 
shape,  in  the  neck 
oval  or  fusiform  (Fig. 
320).  At  the  upper 
angles  of  the  uterus 
the  cavity  is  contin- 
uous with  the  cavity  of 
each  Fallopian  tube. 
At  the  junction  of  the 
body  and  the  neck, 
the  cavity  presents  a 
constriction,  the  inter- 
nal OS.  The  constric- 
tion at  the  end  of  the 
neck  is  known  as  the 
external  os.  The 
walls  of  the  uterus 
are  extremely  thick 
and  composed  of  non- 
striated  muscle-fibers 
arranged  in  a  very 
complicated  manner. 
The  interior  of  the 
uterus  is  Hned  by 
mucous  membrane 
covered  with  cylindric 
ciliated  epithelial  cells,  the  motion  of  which  is  toward  the  external 
OS.  Tubular  glands  are  found  in  large  numbers  in  the  mucous 
membrane  lining  the  cavity,  while  racemose  glands  are  found  in 
the  mucous  membrane  lining  the  neck.  Owing  to  the  flattening  of 
the  uterus  from  before  backward  the  walls  are  almost  in  contact 
and  the  cavitv  almost  obliterated. 


t 


Fig.  31S. — Section  of  Cortex  of  Cat's  Ovary, 
Exhibiting  Large  Graafian  Follicles. — a. 
Peripheral  zone  of  condensed  stroma,  b.  Groups 
of  immature  follicles,  c.  Theca  of  follicle,  d. 
Membrana  granulosa,  e.  Discus  proligerus.  /. 
Zona  pellucida.  g.  Vitellus.  h.  Germinal  vesi- 
cle, i.  Germinal  spot.  k.  Cavity  of  licjuor  fol- 
liculi. — {After  Piersol.) 


REPRODUCTION. 


669 


The  Vagina. — The  vagina  is  a  musculo-membranous  canal,  from 
12  to  18  cm.  in  length,  situated  between  the  rectum  and  bladder. 
It  extends  from  the  surface  of  the  body  to  the  brim  of  the  pelvis, 
and  embraces  at  its  upper  extremity  the  neck  of  the  uterus. 

Ovulation. — After  the  estabhshment  of  puberty  a  Graafian  follicle 
develops  and  ripens  or  matures  periodically,  usually  every  twenty- 
eight  days.  During  the  time  of  maturation  the  follicle  increases  in 
size,  from  an  augmentation  of  its  fluid  contents,  and  approaches  the 
surface  of  the  ovary,  where  it  forms  a  projection  varying  from  6 
to  12  mm.  in  size.     When  maturation  is  complete  the  vesicle  rup- 


^-dBr 


''C^^'    r'-^-^^:'^^^^^'^^^^^ 


Fig.  319.— Ovum  of  a  Cow. — i.  Zona  pel- 
lucida.  2.  Cytoplasm,  vitellus.  3.  Nu- 
cleus, germinal  vesicle.  4.  Nucleolus, 
germinal  spot.  5.  Corona  radiata.  The 
radial  striation  of  the  zona  pellucida 
can  not  be  seen. — (Siohr.) 


Fig.  320. — Frontal  Section 
OF  THE  Uterus,  i.  Cav- 
ity of  the  body.  2,  3. 
Lateral  walls.  4,  4.  Cor- 
nua.  5.  Os  internum.  6. 
Cavity  of  the  cervix.  7. 
Arbor  vitae  of  the  cervix. 
8.  Os  externum.  9.  Va- 
gina.— {Sappey.) 


tures,  and  the  ovum  and  liquid  contents  are  discharged.  The  ovum, 
by  a  mechanism  not  fully  understood,  is  received  by  the  fimbriated 
e-xtremity  of  the  Fallopian  tube  and  enters  its  cavity.  The  ovum  is 
then  transferred  through  the  tube  by  the  peristaltic  contraction  of 
its  muscle- fibers  and  by  the  action  of  the  cilia  of  its  lining  epithelium. 
The  time  occupied  in  the  transference  of  the  ovum  from  the  ovary 
to  the  interior  of  the  uterus  has  been  estimated  to  be  from  four  to 
ten  days. 

Either  at  the  time,  or  very  shortly  after,  its  discharge  from  the 
follicle,  the  ovum,  and  more  especially  the  nucleus,  undergoes  a  series 


670  TEXT-BOOK  OF  PHYSIOLOGY. 

of  histologic  changes  which  eventuates  in  an  extrusion  of  a  portion 
of  the  chromatin  material.  The  extruded  portions  are  known  as  the 
polar  bodies.  The  non-extruded  portion  of  the  chromatin  material 
is  known  as  the  female  pronucleus.  The  succession  of  changes  which 
the  nucleus  undergoes  is  termed  maturation.  As  the  nucleus  is 
regarded  as  the  part  of  the  ovum  which  transmits  parental  character- 
istics it  is  assumed  that  the  extrusion  of  a  portion  of  the  nuclear 
material  is  a  means  by  which  an  excess  of  inherited  substance  is 
prevented. 

Menstruation. — Menstruation  is  a  periodic  discharge  of  blood 
and  mucus  from  the  surface  of  the  mucous  membrane  of  the  uterus, 
and  occurs  about  every  twenty-eight  days.  The  duration  of  the 
menstrual  period  extends  over  four  or  five  days  and  the  amount  of 
blood  discharged  varies  from  180  c.c.  to  200  c.c.  Menstruation  is 
usually  an  accompaniment  of  ovulation,  though  the  latter  process  may 
take  place  independently  of  the  former.  It  is  characterized  by  both 
local  and  systemic  changes.  The  local  changes  are  most  marked 
in  the  uterus,  the  mucous  membrane  of  which  increases  in  thickness 
from  a  proliferation  of  the  connective  tissue  and  a  hyperemic  condi- 
tion of  the  blood-vessels.  Subsequently  to  these  changes  the  epithe- 
lial surface,  as  well  as  the  more  superficial  portions  of  the  connective 
tissue,  undergo  degeneration  and  exfoliation,  after  which  the  finer 
blood-vessels  rupture  and  permit  of  an  escape  of  blood  into  the 
uterine  cavity.  At  the  end  of  the  menstrual  period  regenerative 
changes  set  in  which  continue  until  the  normal  condition  of  the 
mucous  membrane  is  reestablished. 

The  Corpus  Luteum. — With  the  rupture  of  the  Graafian  follicle 
there  is  an  effusion  of  blood  into  the  follicular  cavity  which  soon 
coagulates,  loses  its  color  and  assumes  the  characteristics  of  fibrin. 
The  walls  of  the  follicle,  which  have  become  thickened  from  the 
deposition  of  a  reddish-yellow  glutinous  substance,  now  become  con- 
voluted and  undergo  a  still  further  hypertrophy,  until  they  encroach 
upon  and  almost  obliterate  the  follicular  cavity.  In  a  few  weeks  the 
mass  loses  its  red  color  and  becomes  decidedly  yellow,  when  it  is 
known  as  the  corpus  luteum.  With  the  continuance  of  reparative 
changes  this  body  gradually  disappears  until  at  the  end  of  two  months 
nothing  remains  but  a  small  cicatrix  on  the  surface  of  the  ovary. 
Such  are  the  changes  in  the  follicle  if  the  ovum  has  not  been  im- 
pregnated. 

The  corpus  luteum,  after  impregnation  has  taken  place,  undergoes 
a  much  slower  development,  becomes  larger,  and  continues  during 
the  entire  period  of  gestation.  The  diiJerence  between  the  corpus 
luteum  of  the  unimpregnated  and  pregnant  condition  is  expressed  in 
the  following  table  by  Dalton: 


REPRODUCTION. 


671 


Corpus  Luteum  of  Menstruation. 


At  the  end  of 

weeks. 
One  month. 


Two  months. 
Four  months. 
Six  months. 
Nine  months. 


Corpus  Luteum  of  Pregnancy. 


central  clot  reddish 


three        Three-quarters  of  an  inch  in  diameter 
convoluted  wall  pale. 

I      Smaller;     convoluted  ,      Larger  ;  convoluted  wall  bright 
wall    bright    yellow  ;     clot    yellow  ;  clot  still  reddish, 
still  reddish. 

Seven-eighths  of  an  inch  in 
diameter  ;  convoluted  wall  bright 
yellow  ;  clot  perfectly  decolorized. 
Seven-eighths  of  an  inch  in 
diameter  ;  clot  pale  and  fibrinous; 
convoluted  wall  dull  yellow. 

Still  as  large  as  at  the  end  of 
second  month  ;  clot  fibrinous ; 
convoluted  wall  paler. 

Half  an  inch  in  diameter  ;  cen- 
tral clot  converted  into  a  radiating 
cicatrix ;  external  wall  tolerably 
thick  and  convoluted,  but  without 
any  bright  yellow  color. 


Reduced  to  the  condition 
of  an  insignificant  cicatrix. 

Absent  or  unnoticeable. 


Absent. 


Absent. 


THE  REPRODUCTIVE  ORGANS  OF  THE  MALE. 

The  reproductive  organs  of  the  male  comprise  the  testicles,  vasa 
deferentia,  vesiculae  seminales,  and  penis. 

The  Testicles. — The  testicles  are  oblong  glands,  about  40  mm.  in 
length,  30  mm.  in  breadth  and  20  mm.  in 
thickness,  and  contained  within  the  cavity 
of  the  scrotum.  A  section  of  the  testicle 
(Fig.  321)  reveals  the  presence  externally 
of  a  dense  fibrous  membrane,  the 
tunica  albuginea,  and  internally  a  con- 
nective -  tissue  framework  consisting 
mainly  of  septa,  which  enter  the  organ 
on  its  posterior  aspect  at  the  mediastinum 
testis,  passing  inward  in  a  diverging 
manner.  The  spaces  between  the  septa 
are  occupied  by  the  true  gland  substance, 
the  seminiferous  tubules. 

The  seminiferous  tubules  are  very 
numerous,  the  estimate  as  to  their 
number  varying  from  800  to  1000. 
When  unraveled  they  measure  from  30 
to  40  cm.  in  length  and  0.3  mm.  in 
diameter.  At  their  peripheral  extrem- 
ities the  tubules  are  very  much  con- 
voluted, but  as  they  pass  toward  the 
mediastinum  testis,  the  convolutions 
disappear,  and  after  uniting  with  one  another  terminate  in  from 
twenty  to  thirty  straight  tubes,  the  vasa  recta,  which  pass  through 


321. — Diagram  of  a  Ver- 
tical Section  through  a 
Testicle,  i.  Mediastinum 
testis.  2,  2.  Trabeculae. 
3.  One  of  the  lobules.  4,  4. 
Vasa  recta.  5.  Globus  ma- 
jor of  the  epididymus.  6. 
Globus  minor.  7.  Vas  def- 
erens.— {H  olden.) 


6/2 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  mediastinum  and  form  the  rele  testis.  At  the  upper  part  of 
the  mediastinum  the  tubules  unite  to  form  from  nine  to  thirty  small 
ducts,  the  vasa  efferentia,  which  soon  become  very  much  convoluted. 
After  a  short  course  they  unite  to  form  a  single  tortuous  tube,  about 
7  meters  in  length  and  0.4  mm.  in  diameter,  which  descends  behind 
the  testicle  to  its  lower  border.  This  tube  is  known  as  the  epididy- 
mis. The  seminal  tubule  consists  of  a  basement  membrane  lined 
by  granular  nucleated  epithelium. 

The  vas  dejerens,  the  excretory  duct  of  the  testicle,  is  about  60  cm. 
in  length  and  from  2  to  3  mm.  in  diameter,  and  extends  upward 

from  the  epididymis  to  the 
inguinal  canal,  through  which 
it  passes  into  the  abdominal 
cavity  and  then  to  the  under 
surface  of  the  base  of  the 
bladder,  where  it  unites  with  the 
duct  of  the  vesicula  seminalis  to 
form  the  ejaculatory  duct. 

The  vesiculae  seminales 
are  two  lobulated  pyriform 
bodies,  about  40  mm.  in  length, 
situated  on  the  under  surface 
of  the  bladder.  Each  vesicula 
seminalis  consists  of  an  external 
fibrous  coat,  a  middle,  muscular 
coat,  and  an  external  mucous 
coat.  The  mucous  coat  con- 
tains a  number  of  small  tubu- 
lar albumin-producing  glands 
which  secrete  a  characteristic 
fluid. 

The  ejaculatory  duct,  formed 
by  the  union  of  the  vas  deferens 
and  the  duct  of  the  vesicula  semi- 
nalis, opens  into  the  prostatic 
portion  of  the  urethra  (Fig.  322). 
The  prostate  gland  is  a  musculo-glandular  mass  situated  at  the 
posterior  extremity  of  the  urethra.  It  contains  a  large  number  of 
tubules,  more  or  less  branched  and  convoluted,  and  hned  by  columnar 
epithelium.  They  secrete  a  fluid  which  is  poured  into  the  urethra 
at  the  time  of  the  ejaculation  of  semen. 

The  penis  consists  of  three  parts:  the  corpus  spongiosum  below, 
through  which  passes  the  urethra,  and  the  two  corpora  cavernosa, 
one  on  either  side  and  above.  The  corpus  spongiosum  terminates 
anteriorly  in  a  conic-shaped  structure,  the  glans  penis;   the  corpora 


Fig 


322. — Vas  Deferens,  Vesicula 
Seminales,  and  Ejaculatory 
Ducts. — a.  Vas  deferens,  b.  Semi- 
nal vesicle,  c.  Ejaculatory  duct.  d. 
Termination  of  the  ejaculatory  duct. 
e.  Opening  of  the  prostatic  utricle. 
/,  g.  Veru  montanum.  /;,  /.  Pros- 
tate.— {Liegeois.) 


REPRODUCTION.  673 

cavernosa  consist  externally  of  a  fibrous  investment  and  internally 
of  erectile  tissue.  These  bodies  are  abundantly  supplied  with  blood, 
which  after  entering  their  substance  by  the  arteries,  passes  into  sinuses 
or  reservoirs,  from  which  it  is  carried  away  by  veins.  These  vessels 
pass  to  the  dorsum  of  the  penis  and  unite  to  form  a  large  vein  by 
which  the  blood  is  returned  to  the  general  circulation.  By  virtue 
of  the  erectile  tissue  in  the  corpora  cavernosa  the  penis  becomes 
erect  and  rigid  when  the  blood  supply  is  increased.  This  takes  place 
in  response  to  peripheral  stimulation  or  emotional  states,  or  both 
combined.  When  these  conditions  are  established  nerve-impulses 
pass  outward  through  nerves,  the  nervi  erigentes,  which  have  their 
origin  in  the  lumbar  region  of  the  spinal  cord,  and  bring  about  an 
active  dilatation  of  the  arteries  and  a  relaxation  of  the  non-striated 
muscle-fibers  in  the  corpora  cavernosa.  With  these  events  there  is  a 
rapid  influx  of  blood  and  a  distention  and  an  erection  of  the  organ. 
This  condition  is  furthered  and  maintained  by  a  partial  compression 
of  the  dorsal  vein  by  the  fibrous  capsule. 

Semen.- — The  semen  is  a  complex  fluid  composed  of  the  secretions 
of  the  testicles,  the  vesiculae  seminales,  the  prostatic  tubules,  and 
urethral  glands.  It  is  grayish-white  in  color,  mucilaginous  in  con- 
sistence, characteristic  in  odor,  and  somewhat  heavier  than  water. 
In  response  to  appropriate  stimulation  the  muscle-fibers  in  the  walls 
of  the  vasa  deferentia,  vesiculae  seminales,  and  prostatic  tubules 
contract  and  discharge  their  contents  into  the  urethra,  from  which 
they  are  forcibly  ejected  by  the  rhythmic  contraction  of  the  ejaculatory 
muscles,  the  ischio  and  hidbo  cavernosi.  The  amount  of  semen  dis- 
charged at  each  ejaculation  varies  from  i  to  5  c.c. 

Spermatozoa. — The  spermatozoa  are  peculiar  morphologic  ele- 
ments which  arise  within  the  seminiferous  tubules  as  a  result  of 
complex  histologic  changes  in  the  lining  epithelium.  An  adult  sper- 
matozoon consists  of  a  conoid  slightly  flattened  head,  from  the  pos- 
terior part  of  which  there  projects  a  short  straight  rod,  provided  with 
a  long  filamentous  tail  or  cilium  and  an  end-piece  (Fig.  2>^'^).  The 
head  contains  a  nucleus  of  chromatin  material.  The  total  length  of 
a  spermatozoon  varies  from  50  to  80  micromillimeters.  The  char- 
acteristic physiologic  feature  of  spermatozoa  is  incessant  locomotion 
when  in  a  suitable  medium.  So  long  as  they  are  confined  to 
the  vas  deferens  they  are  quiescent,  but  with  their  advent  into  the 
vesicula  seminalis  and  dissemination  in  its  contained  fluid,  they  be- 
come extremely  active  and  move  around  with  considerable  rapidity. 
The  power  of  locomotion  depends  on  the  possession  of  the  tail, 
which,  by  lashing  the  surrounding  fluid  now  in  this  and  now  in  that 
direction,  propels  the  head  from  place  to  place.  The  vitality  of 
spermatozoa  is  such  as  to  enable  them  to  retain  their  physiologic 
activities  in  the  uterus  for  more  than  eight  days. 
43 


674 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  development  of  spermatozoa  from  testicular  cells  as  observed 
in  lower  animals  indicates  that  each  cell  gives  rise  to  four  embryonic 
forms — spermatids — which  subsequently  develop  into  adult  sperma- 
tozoa. In  this  process  the  primary  nuclear  chromatin  undergoes  a 
division,  so  that  each  spermatozoon  receives  but  a  fractional  amount. 
The  changes  by  which  this  condition  is  brought  about  are  comparable 
to  the  changes  exhibited  by  the  ovum,  and  have  for  their  result  a 

reduction  in  the  quantity  of  hereditary  substance 

to  be  transmitted. 

Fecundation. — Fecundation  is  the  union  of 
the  spermatozoon  (the  sperm- cell)  with  the  ovum 
(the  germ-cell)  and  takes  place  in  the  great 
majority  of  instances  in  the  Fallopian  tube. 
After  the  introduction  of  the  spermatozoa  into 
the  vagina  during  the  act  of  copulation,  they 
soon  begin  to  pass  upward,  into  and  through, 
the  uterine  cavity  and  out  into  the  Fallopian  tube, 
where  they  accumulate  in  large  numbers  and 
retain  their  vitality  for  some  days.  The  migration 
is  effected  by  the  propelling  power  of  the 
filamentous  tail. 

From  observations  made  on  the  behavior  of 
the  spermatozoa  toward  the  ovum  in  lower 
animals,  and  on  the  manner  by  which  their 
union  is  effected,  the  inference  may  be  drawn 
that  a  similar  procedure  takes  place  in  mammals. 
In  lower  animals  the  spermatozoa  on  approaching 
an  ovum  take  on  increased  activity,  swimming 
around  it  in  all  directions  and  apparently  seeking 
a  point  of  entrance.  In  fish  and  molluscs  the 
zona  pellucida  presents  a  distinct  opening,  the 
micropyle,  through  which  the  spermatozoon 
passes.  Inasmuch  as  the  mammalian  ovum  is 
devoid  of  such  an  opening,  the  mechanism  of 
entrance  of  the  spermatozoon  is  not  clearly 
understood.  Notwithstanding  their  enormous  numbers  it  is  gener- 
ally believed  that  but  a  single  spermatozoon  effects  an  entrance 
into  the  ovum.  With  the  accomplishment  of  this,  however,  the 
spermatozoon  loses  its  vitality,  after  which  the  body  and  tail  dis- 
appear. The  head,  which  in  this  instance  also  is  the  transmitter  of 
the  inherited  material,  advances  to  meet  and  unite  with  the  nucleus 
of  the  ovum.  A  series  of  histologic  changes  now  arise,  which 
eventuate  in  the  production  of  a  new  cell,  a  parent  cell,  possessing 
all  the  features  of  cell  structure  and  the  physiologic  activities  and 


Fig.    32 


-Human 


Spermatozoon. 

1 .  Front    view, 

2,  side  view,  of 
the  head.  k. 
Head.  m.  mid- 
dle piece.  /. 
Tail.  e.  Termi- 
nal filament. — 
{After  Retzius.) 


REPRODUCTION. 


675 


characteristics  of  both  ancestral  cells.  From  this  parent  cell  the 
new  being  develops  through  successive  division,  multiphcation,  and 
differentiation  of  cells. 

The  Fixation  of  the  Ovum. — If  the  ovum  is  to  develop  into  a 
new  being  it  is  essential  that  it  be  retained  within  the  cavity  of  the 
uterus.  This  is  accomplished  by  the  development  of  specialized 
structures  on  the  surface  of  the  uterine  mucosa  and  on  the  surface 
of  the  ovTam.  With  the  fertilization  of  the  ovum,  the  mucous  mem- 
brane of  the  uterus  takes  on  an  increased  growth ;  it  becomes  hyper- 
trophied  and  vascular,  and  develops  small  elevations  known  as  villi. 
Inasmuch  as  this  membrane  is  detached  and  discharged  at  the  birth 
of  the  fetus,  it  is  known  as  the  decidua  vera.     With  the  fertilization 


Fig.  324. — Impregnated  Uterus,  with 
Folds  of  Decidua  Growing  up 
Around  the  Egg.  The  narrow 
opening,  where  the  folds  approach 
each  other,  is  seen  over  the  most 
prominent  portion  of  the  egg. — 
(Dalion.) 


Fig.  325.  —  Impregnated  Uterus; 
showing  the  connection  between  the 
villosities  of  the  chorion  and  the 
decidual  membranes. — (Dalton.)    ^ 


of  the  ovum,  the  zona  pellucida  or  radiata  also  develops  villosities, 
and  as  it  passes  from  the  Fallopian  tube  into  the  uterus  the  villi 
interdigitate,  and  its  further  progress  is  retarded.  (Figs.  324  and 
325.)  In  a  short  time  a  portion  of  the  decidua  vera  grows  up 
on  all  sides  and  encloses  the  ovum.  Its  retention  is  thus  secured. 
That  portion  of  the  decidua  which  grows  around  the  ovum 
is  termed  the  decidua  reflexa;  while  the  portion  to  which  the 
ovum  attaches  itself  is  termed  the  decidua  serotina,  and  is  of  interest 
for  the  reason  that  it  becomes  the  seat  of  development  of  the  placenta, 
the  organ  by  w^hich  the  fetus  is  nourished.  As  development  advances 
the  decidua  reflexa  also  increases  in  size  and  extent,  and  about  the 
end  of  the  fourth  month  comes  into  contact  with  the  decidua  vera, 
with  which  it  ultimately  fuses. 


676 


TEXT-BOOK  OF  PHYSIOLOGY. 


DEVELOPMENT  OF  FETAL  ACCESSORY  STRUCTURES. 

Segmentation  of  the  Ovum. — Shortly  after  the  formation  of  the 
parent  cell,  segmentation  of  the  nucleus  and  cytoplasm  takes  place 
in  accordance  with  karyokinetic  methods.  The  two  new  cells  thus 
formed  undergo  a  similar  division  into  four,  the  four  into  eight,  the 
eight  into  sixteen,  and  so  on  until  the  space  within  the  zona  pellucida 
is  completely  filled  with  a  large  number  of  small  cells,  each  possessing 
the  characteristic  cell  structures.  The  peripheral  cells  then  arrange 
themselves  in  the  form  of  a  membrane,  and  as  they  are,  at  the  same 
time,  subjected  to  mutual  pressure  they  assume  a  polyhedral  shape, 
and  give  to  the  membrane  a  mosaic  appearance  (Fig.  326).     The 

central  cells  then 
undergo  hquefac- 
tion.  At  some 
point  on  the  inner 
surface  of  the 
membrane,  cells  ac- 
cumulate which  by 
their  division  and 
multiplication  form 
a  second  mem- 
brane. The  two 
together  are  known 
as  the  external  and 
internal  blasto- 
dermic membranes. 
Germinal  Area. 
— At  about  this 
period  there  is  an 
accumulation  of 
cells  at  a  certain 
spot  in  the  sub- 
stance of  the  blastodermic  membranes  which  marks  the  position  of 
the  future  embryo.  This  spot,  at  first  circular,  soon  becomes 
elongated.  A  slight  indentation  now  develops  into  what  is  known  as 
the  primitive  trace.  Around  this  area  there  is  a  clear  space,  the 
area  pellucida,  which  is  in  turn  surrounded  by  a  darker  region,  the 
area  opaca.  The  primitive  trace  soon  disappears  and  the  area 
pellucida  becomes  guitar-shaped.  A  second  groove,  the  medullary 
groove,  is  now  formed,  which  develops  from  before  backward  and 
becomes  the  neural  medullary  canal. 

Blastodermic  Membranes. — The  embryo,  at  this  period,  con- 
sists of  three  layers — viz.,  the  external  and  the  internal  blastodermic 
membranes  and  a  middle  membrane  formed  by  a  genesis  of  cells 


Fig.  326. — Primitive  Trace  OF  THE  Embryo,  a.  Primi- 
tive trace,  b.  Area  pellucida.  c.  Area  opaca.  d. 
Blastodermic  cells,  e.  Villi  beginning  to  appear  on 
the  surface  of  the  zona  pellucida. — (Lugois.) 


REPRODUCTION. 


677 


from  their  internal  surfaces.     These  layers  are  known  from  without 
inward  as  the  epiblast,  mesoblast,  and  hypoblast. 

The  epiblast  gives  rise  to  the  central  nerve  system,  the  epidermis 
and  its  appendages,  and  the  primitive  kidneys. 

The  mesoblast  gives  rise  to  the  dermis,  muscles,  bones,  nerves, 
blood-vessels,  sympathetic  nerve  system,  connective  tissue,  the 
urinary  and  reproductive  apparatus,  and  the  w^alls  of  the  alimentary 
canal. 

The  hypoblast  gives  rise  to  the  epithelial  hning  of  the  alimentary 
canal  and  its  glandular  appendages, 
the  hver  and  pancreas,  and  the  epithe- 
lium of  the  respiratory  tract. 

Dorsal  Laminae. — As  develop- 
ment advances,  the  true  medullary 
groove  deepens,  and  there  arise  two 
longitudinal  elevations  of  the  epiblast 
— the  dorsal  lamincB,  one  on  either  side 
of  [the  groove, — which  grow  up,  arch 
over,  and  unite  so  as  to  form  a  closed 
tube,  the  primitive  central  nerve 
system. 

The  Chorda  Dorsalis. — Just  be- 
neath the  neural  canal  there  arises  a 
group  of  hypoblastic  cells  which  ar- 
range themselves  in  the  form  of  a 
cylindric  rod,  which  marks  out  the 
position  of  the  future  bony  axis  of  the 
body.  This  rod  is  known  as  the 
chorda  dorsalis  or  notochord. 

Primitive  Vertebrae. — On  either 
side  of  the  neural  canal  the  cells  of 
the  mesoblast  undergo  a  longitudinal 
thickening,  which  develops  and  extends 
around  the  neural  canal  and  the 
chorda  dorsalis,  and  forms  the  arches 

and   bodies  of    the  vertebras.      They  become  divided  transversely 
into  segments. 

The  mesoblast  now  separates  into  two  layers :  the  external,  joining 
with  the  epiblast,  forms  the  somatopleura;  the  internal,  joining  with 
the  hypoblast,  forms  the  splanchno pleura;  the  space  betw^een  them 
constitutes  the  pleuro- peritoneal  cavity  (Fig.  327). 

Visceral  Laminae. — The  walls  of  the  pleuro-peritoneal  cavity 
are  formed  by  a  downward  prolongation  of  the  somatopleura  (the 
visceral  lamince),  which,  as  they  extend  around  in  front,  pinch  off 
a  portion  of  the  yolk-sac  (formed  by  the  splanchnopleura),  which 


-  pp 


Fig.  327. — Diagram  Represent- 
ing THE  Relation  of  Prim- 
ary Structures  in  a  Devel- 
oping Chicken;  Vertical 
Transverse  Section.  The 
medullary  groove  and  chorda 
dorsalis  are  seen  in  section; 
the  ahmentary  canal  pinched 
off  from  the  yolk-sac  is  com- 
pletely closed,  a.  Amnion. 
a,  c.  Amniotic  cavity  filled 
with  amniotic  fluid,  pp. 
Space  between  amnion  and 
chorion  continuous  with  the 
pleuro-peritoneal  cavity  in- 
side the  body.  vt.  VitelHne 
membrane,  or  zona  pellucida. 
ys.  Yolk-sac,  or  umbilical 
vesicle. — {Foster  and  Balfour.) 


678 


TEXT-BOOK  OF  PHYSIOLOGY. 


becomes  the  primitive  alimentary  canal;  the  lower  portion,  remaining 
outside  of  the  body  cavity,  forms  the  umbilical  vesicle. 

The  Fetal  or  Embryonic  Membranes. — With  the  appearance 
of  the  visceral  laminie  two  membranes  develop  in  succession,  both 
of  which  play  an  important  part  in  the  subsequent  life  of  the  embryo. 
These  are  known  as  the  amnion  and  the  allantois. 

The  amnion  is  formed  by  folds  of  the  epiblast  and  the  external 
layer  of  the  mesoblast  rising  up  in  front,  behind,  and  at  the  sides. 
These  folds  gradually  extend  over  the  back  of  the  embryo  to  a  certain 
point  where  they  meet,  coalesce,  and  enclose  a  cavity  known  as  the 
amniotic  cavity.  The  membranous  partition  between  the  folds  is 
absorbed,  after  which  the  outer  layer  recedes  and  becomes  blended 
with   the  primitive  enveloping  membrane  of  the  ovum  and  thus 


Fig. 


328 

Egg.  a 
Amniotic 
{Dalton.) 


Diagram   of    Fecundated 

a.    Umbilical     vesicle.      b. 

cavitv.      c.  Allantois. — 


Fig.  329. — Fecundated  Egg  with 
Allantois  nearly  Complete,  a. 
Inner  layer  of  amniotic  fold.  b. 
Outer  layer  of  ditto,  c.  Point  where, 
the  amniotic  folds  come  in  contact. 
The  allantois  is  seen  penetrating 
between  the  outer  and  inner  layers 
of  the  amniotic  folds. — {Dalton.) 


assists  in  the  formation  of  the  chorion — the  external  covering  of  the 
embryo. 

The  cavity  enclosed  by  the  amnion  is  at  firat  quite  small,  but 
soon  enlarges  from  the  accumulation  of  a  clear,  transparent  fluid,  the 
amniotic  fluid.  It  gradually  increases  in  amount  up  to  the  latter 
period  of  gestation,  when  its  volume  reaches  about  one  liter.  This 
fluid  is  derived  mainly  from  the  blood,  as  it  contains  albumin,  sugar, 
fatty  matter,  and  inorganic  salts.  Traces  of  urea  indicate  that  some 
of  its  constituents  are  derived  from  the  embryo  itself. 

The  allantois  is  primarily  a  pouch  or  diverticulum  which  develops 
from  the  posterior  portion  of  the  alimentary  canal.  As  it  develops 
it  enlarges,  and  in  its  growth  inserts  itself  between  the  two  layers 
of  the  amnion,  coming  into  contact  more  especially  with  the  external 
layer.  It  finally  completely  surrounds  the  embryo,  after  which  its 
edges  become  fused  together  (Figs.  328  and  329). 


REPRODUCTION. 


679 


The  allantois  is  of  especial  interest  and  importance,  as  it  is  the 
means  by  which  the  blood  of  the  embryo  is  brought  into  relation 
with  the  blood  of  the  mother.  As  it  develops,  two  arteries,  the 
hypogastrics,  one  from  each  internal  iliac,  pass  out  of  the  abdominal 
cavity  within  the  walls  of  the  allantois,  and  follow  it  in  its  course 
around  the  embryo.  The  ultimate  branches  of  these  arteries  pene- 
trate the  villous  processes  which  develop  on  the  surface  of  the  chorion 
and  which  take  part  in  the  formation  of  the  placenta.  A  single 
large  vein  emerges  from  the  placenta  and  returns  the  blood  to  the 
embryo.  In  its  course  it  winds  around  the  arteries  in  a  spiral 
manner  a  number  of  times.  These  vessels — the  umbilical  arteries 
and  vein — are  enclosed  by  the  walls  of  the  allantois  and  amnion, 
and  together  constitute  the  umbilical  cord  which  at  the  end  of  gesta- 
tion is  about  60  cm.  in  length. 

The  Chorion.— The  cho- 
rion, the  external  investment 
of  the  embryo,  is  formed  by 
the  fusion  of  the  primitive  egg 
membrane — the  zona  pellu- 
cida — the  external  layer  of  the 
amnion,  and  the  allantois. 
Very  early  in  development  its 
external  surface  becomes 
covered  with  homogeneous, 
granular,  club-shaped  proces- 
ses, which  by  continued  bud- 
ding and  growth,  give  to  the 
membrane  a  shaggy  appear- 
ance. At  about  the  end  of 
the  second  month  these  pro- 
cesses  begin  to   atrophy  and 

disappear  from  the  surface  of  the  chorion,  with  the  exception  of  that 
portion  which  is  in  contact  with  the  decidua  serotina.  At  this  point 
the  processes  or  villi  continue  to  grow  and  develop,  and  insert  them- 
selves more  deeply  into  the  mucous  membrane.  Corresponding 
processes  from  the  mucous  membrane  insert  themselves  between 
the  vilh  of  the  chorion,  which  by  their  growth  and  fusion  secure, 
among  other  things,  the  retention  of  the  embryo. 

The  Nutrition  of  the  Embryo. — Coincidently  with  the  develop- 
ment of  the  amnion,  allantois,  and  chorion,  there  arises  within  the 
body  of  the  embryo  the  early  forms  of  many,  if  not  all,  of  the  future 
viscera.  The  nutritive  material  required  for  their  growth  is  partly 
contained  within  the  umbilical  vesicle  lying  without  the  body  cavity. 
That  this  material  may  be  utihzed,  blood-vessels  emerge  from  the 
body  and  ramify  within  the  walls  of  the  vesicle.     The  capillaries  to 


Fig.  330. — Human  Embryo  and  its  En- 
velopes AT  THE  End  of  the  Third 
Month. 


68o 


TEXT-BOOK  OF  PHYSIOLOGY. 


which  these  vessels  gi\'e  rise  come  into  close  relation  with  and  absorb 
the  food  material,  after  which  it  is  carried  by  veins  to  the  heart, 
by  which  it  is  distributed  to  all  parts  of  the  embryo.  These  vessels 
are  collectively  known  as  the  omphalo-mesenteric  arteries  and  veins. 
This  primitive  or  vitelline  circulation  is  of  short  duration  in  mam- 
mals, as  the  nutritive  material  in  the  vesicle  is  small  in  amount  and 
is  soon  exhausted.  In  birds,  however,  it  is  of  primary  importance. 
The  main  supply  of  nutritive  material,  however,  is  derived  from 
the  mother  by  means  of  a  highly  developed  and  specialized  organ — - 
The  Placenta. — Of  all  the  embryonic  structures  the  placenta 
is  the  most  important.     It  is  formed  by  the  end  of  the  third  month, 

after  which  it  gradually 
increases  in  size  u  p  to  the 
end  of  the  eighth  imonth, 
by  which  time  it  is  fully 
developed.  It  then  mea- 
sures from  1 8  to  24  cm. 
in  diameter  and  weighs 
from  400  to  600  grams. 
It  is  most  frequently  situ- 
ated at  the  upper  and 
back  part  of  the  uterine 
cavity.  Though  exceed- 
ingly complex  in  structure 
it  consists  essentially  of 
two  portions,  a  fetal  and  a 
maternal. 

The  fetal  portion  con- 
sists primarily  of  those  villi 
on  the  chorion  in  relation 
with  the  decidua  serotina. 
These  structures  gradu- 
ally increase  in  size  and 
number,  and  receive  the 
ultimate  branches  of  the  umbilical  arteries.  The  maternal  portion 
consists  primarily  of  the  decidua  serotina.  As  gestation  advances  the 
chorionic  villi  rapidly  increase  in  size  and  number,  and  receive  the 
branches  of  the  umbilical  arteries.  At  the  same  time  the  decidua 
serotina  becomes  hypertrophied  and  vascular.  With  the  continued 
growth  and  development  of  these  two  structures  they  gradually  fuse 
together  and  finally  become  inseparable.  In  accordance  with  the 
needs  of  the  embryo,  the  decidua  serotina  and  its  contained  blood- 
vessels undergo  certain  histologic  changes  which  result  in  the  forma- 
tion of  large  cavities,  sinuses,  or  lakes,  into  which  the  blood  of  the 
uterine  vessels  is  emptied.     Coincidently  the  villi  of  the  chorion  grow 


7  /^<5v,- 


Fig.  331. — Human  Embryo,  with  Amnion  and 
Allantois,  in  the  Third  Week.  There 
are  as  yet  no  limbs;  the  embryo  and  its 
appendages  are  surrounded  by  the  tufted 
chorion. — (Haeckel.) 


REPRODUCTION. 


68i 


and  give  off  numerous  branches,  which  project  themselves  in  all  direc- 
tions into  the  blood  of  uterine  sinuses  (Figs.  332,  333).  As  the 
placenta  develops,  the  structures  separating  the  blood  of  the  mother 
from  that  of  the  child  gradually  become  modified  until  they  are  repre- 
sented by  a  thin  cellular  or  homogeneous  membrane.  The  conditions 
now  are  such  as  to  permit  of  a  free  exchange  of  material  between  the 
mother  and  child.  Whether  by  osmosis  or  by  an  act  of  secretion,  the 
nutritive  materials  of  the  maternal  blood  pass  through  the  intervening 
membrane  into  the  fetal  blood  on  the  one  hand,  while  waste  products 


Fig.  332. — Diagram  showing  the  Relations  of  the  Fetal  Membranes.  A^n. 
Amnion.  Ch.  Chorion.  M.  Muscle  wall  of  uterus.  R.  Decidua  reflexa.  5. 
Serotina.     V.  Decidua  vera.     Y.  Yolk  stalk. — {McMurrich.) 


pass  in  the  reverse  direction  into  the  maternal  blood  on  the  other 
hand.  Inasmuch  as  oxygen  is  absorbed  and  carbon  dioxid  exhaled 
by  the  same  structures,  the  placenta  is  to  be  regarded  as  both  a 
digestive  and  a  respiratory  organ.  So  long  as  these  exchanges  are 
permitted  to  take  place  in  a  normal  manner  the  nutrition  of  the 
embryo  is  secured. 

The  Fetal  Circulation. — The  composition  of  the  blood  as  well 
as  the  course  it  pursues  through  the  heart  and  vascular  apparatus 
presents  peculiarities  which  have  arisen  in  consequence  of  the  neces- 
sity of  obtaining  nutritive  material  through  the  placenta  and  the 


682 


TEXT-BOOK  OF  PHYSIOLOGY. 


almost  impervious  condition  of  the  pulmonary  capillaries.  On  re- 
turning from  the  placenta,  the  blood  in  the  umbilical  vein  is  relatively 
rich  in  nutritive  material  and  scarlet  red  in  color  from  the  presence 
of  oxygen.  As  it  passes  into  the  abdominal  cavity  a  portion  of  the 
blood  is  directed  by  the  ductus  venosus  into  the  vena  cava,  while 
another  portion  is  emptied  into  the  branches  of  the  portal  vein,  by 
which  it  is  distributed  to  the  liver  and  from  which  it  emerges  by 
the  hepatic  veins  and  poured  into  the  vena  cava.  The  blood  in  the 
vena  cava  is  thus  a  mixture  of  venous  blood  from  the  lower  extremi- 
ties and  liver,  and  oxygenated  blood  from  the  placenta.  After  its 
discharge  into  the  right  auricle  the  blood  is  directed  bv  a  fold  of 


Amnion 
Chorion 


S^^o^ 


cj 

'Compact 

C 

layer. 

c 

t 

«• 

3 

r2 

o 

Cavernous  J 

o 

I      layer.     ', 

Muscularis.  'i 


Fig.  2)?,2)- — Diagram  op  Human  Placenta  at  the  Close  of  Pregnancy. — 

(^Schdper.) 


Chorionic  villi. 

Internllous  spaces. 
Floating  villus. 


|-  Attached  ^^lIi. 


the  lining  membrane,  the  Eustachian  valve,  through  an  opening  in 
the  interauricular  septum,  the  foramen  ovale,  into  the  left  auricle. 
It  then  flows  through  the  auriculo-ventricular  opening  into  the  left 
ventricle,  thence  into  the  aorta,  and  by  its  branches  is  distributed 
to  all  parts  of  the  body. 

^The  blood  from  the  head  and  upper  extremities  is  emptied  by 
the  superior  vena  cava  into  the  right  auricle,  but  as  it  passes  in 
front  of  the  Eustachian  valve,  it  flows  directly  into  the  right  ventricle 
and  then  into  the  pulmonary  artery.  On  account  of  the  unexpanded 
condition  of  the  lungs  and  the  almost  impervious  condition  of  the 
pulmonary  capillaries,  but  a  small  portion  of  the  blood  passes 
through  them,  while  the  larger  portion  by  far  passes  into  the  aorta 


PLATE  III. 


DIAGRAM  OF  FCETAL  CIRCULATION. 


W  Preyer  del- 


REPRODUCTION.  683 

directly  through  a  duct,  the  ductus  arteriosus,  which  enters  at  a  point 
below  the  origin  of  the  left  carotid  and  subclavian  arteries.  A  com- 
parison of  the  blood  distributed  to  the  head  and  upper  extremities,  with 
that  distributed  to  the  lower  extremities,  will  show  a  larger  percentage 
of  nutritive  material  and  oxygen  in  the  former  than  in  the  latter, 
a  fact  which  has  been  offered  as  an  explanation  of  the  more  rapid 
growth  of  the  upper  half  of  the  body.  As  the  blood  passes  through 
the  aorta,  a  portion  is  directed  from  the  main  current  by  the  hypogas- 
tric and  umbilical  arteries  to  the  placenta,  where  it  loses  carbon 
dioxid  and  gains  oxygen,  and  changes  in  color  from  a  bluish  red  to  a 
scarlet  red. 

Parturition. — At  the  end  of  gestation — approximately  280  days 
from  the  time  of  conception — a  series  of  changes  occur  in  the  uterine 
structures  which  lead  to  an  expulsion  of  the  child,  the  placenta,  and 
decidua  vera.  To  this  process  in  its  entirety  the  term  parturition 
is  given.  At  this  time,  from  causes  not  clearly  defined,  the  uterine 
walls  begin  to  exhibit  throughout  their  extent  a  series  of  slight  con- 
tractions which  are  somewhat  peristaltic  in  character;  these  con- 
tractions, which  gradually  increase  in  frequency  and  vigor,  bring 
about  a  dilatation  of  the  internal  os  and  a  descent  of  the  membranes 
into  the  cervical  canal.  The  pressure  exerted  by  these  membranes 
during  the  time  of  the  contraction  materially  assists  in  the  relaxation 
of  the  circular  fibers  and  a  dilatation  of  the  external  os.  When  the 
dilatation  has  so  far  advanced  that  the  diameter  of  the  external  os 
attains  a  measure  of  7  or  8  cm.,  the  tension  of  the  membranes  becomes 
sufficiently  great  to  lead  to  their  rupture  and  to  a  partial  escape  of 
the  amniotic  fluid.  With  this  event,  the  presenting  part  of  the 
child,  usually  the  head,  descends  into,  the  cervical  canal.  After  a 
short  period  of  rest  the  uterine  contractions  return  and  rapidly 
increase  in  vigor  and  duration.  As  a  result  of  the  pressure  thus 
exerted  from  all  sides  on  the  body  of  the  child,  the  head  gradually 
descends  into  the  vagina  and  finally  emerges  through  the  vulva  to 
be  followed  in  a  short  time  by  the  expulsion  of  the  trunk  and  limbs, 
and  a  discharge  of  the  remaining  amniotic  fluid.  With  the  expulsion 
of  the  child  the  uterine  contractions  cease  for  a  period  of  ten  or 
fifteen  minutes,  when  they  again  recur,  with  the  result  of  detaching 
the  placenta  and  expelling  it  into  the  vagina.  It  is  then  removed 
by  the  co-operative  action  of  the  abdominal  and  perineal  muscles. 
The  hemorrhage  which  would  naturally  occur  with  the  detachment  of 
the  placenta  and  the  laceration  of  the  maternal  vessels  is  prevented 
by  the  firm  continuous  contraction  of  the  uterine  walls,  by  which 
the  vessels  are  compressed  and  permanently  closed. 

The  Establishment  of  Inspiration  and  the  Adult  Circulation. 
— After  the  birth  of  the  child  and  the  detachment  of  the  placenta, 
there  speedily  occurs  a  decrease  in  the  quantity  of  oxygen  and  an 


684  TEXT-BOOK  OF  PHYSIOLOGY. 

increase  in  the  quantity  of  carbon  dioxid  in  the  blood,  a  condition 
which  causes  a  discharge  of  nerve  energy  from  the  inspiratory  center, 
a  contraction  of  the  inspiratory  muscles,  an  expansion  of  the  thorax, 
and  an  inflow  of  air  into  the  lungs. 

In  the  later  months  of  intrauterine  life  the  vascular  apparatus 
undergoes  certain  anatomic  changes  which  favor  the  transition  from 
the  placental  to  the  adult  circulation.  Thus  the  ductus  venosus  con- 
tracts, and  shunts  a  larger  volume  of  blood  into  and  through  the 
liver;  the  Eustachian  valve  diminishes  in  size  and  at  the  time  of 
birth  has  almost  disappeared;  a  membranous  fold  grows  upward 
and  backward  from  the  edge  of  the  foramen  ovale  on  the  left  side; 
the  ductus  arteriosus  also  contracts.  With  the  first  inspiration  and 
the  expansion  of  the  lungs,  the  blood  which  enters  the  pulmonary 
artery  passes  through  the  pulmonary  capillaries  in  large  volume  and 
is  returned  by  the  pulmonary  veins  to  the  left  auricle.  The  en- 
trance of  the  blood  into  this  cavity  presses  the  membranous  fold 
against  the  margins  of  the  foramen  ovale  and  thus  prevents  the 
further  flow  of  blood  from  the  right  auricle.  The  blood  entering  the 
right  auricle  by  the  inferior  vena  cava  now  flows  into  the  right  ven- 
tricle, which  is  favored  by  the  small  size  of  the  Eustachian  valve. 
The  foramen  ovale  is  permanently  closed  at  the  end  of  a  week  or 
ten  days;  the  ductus  arteriosus  at  the  end  of  four  days.  The  um- 
bilical vein  and  ductus  venosus,  at  the  end  of  four  or  five  days, 
have  also  become  almost  impervious  from  the  contraction  of  their 
walls.  The  hypogastric  arteries  remain  open  and  carry  blood  to 
the  walls  of  the  bladder. 

Lactation. — As  pregnancy  advances  the  mammary  glands  in- 
crease in  size,  partly  from  a  deposition  of  fat  and  connective  tissue 
and  partly  from  a  multiplication  of  the  secreting  acini.  The  lining 
epithelial  cells  at  the  same  time  increase  in  size,  and  tow^ard  the 
end  of  pregnancy  begin  to  exliibit  functional  activity.  At  the  time 
of  birth,  or  within  a  day  or  so  after  birth,  the  acini  are  filled  with 
a  fluid  which  in  its  qualitative  composition  resembles  milk  and  is 
known  as  colostrum.  It  is  distinguished  from  milk  more  especially 
in  the  fact  that  it  contains  in  large  quantity  a  proteid  which  coagulates 
on  boiling,  and  certain  inorganic  salts  which  have  a  laxative  effect 
on  the  new-born  child.  Normal  lactation  and  the  phenomena  which 
accompany  it  are  fufly  established  by  the  end  of  the  second  or  third 
day. 

The  composition  of  milk  and  the  mechanism  of  its  production 
have  been  stated  in  the  chapter  on  Secretion,  page  401. 

Physiologic  Activities  of  the  Embryo. — ^During  intrauterine 
life  the  evolution  of  structure  is  accompanied  by  an  evolution  of 
function.  The  relatively  simple  and  uniform  metabolism  of  the 
undifferentiated    blastodermic    membranes    gradually    increases    in 


REPRODUCTION.  685 

complexity  and  variety,  as  the  individual  tissues  and  organs  make 
their  appearance  and  assume  even  a  slight  degree  of  functional 
activity.  As  to  the  periods  at  which  different  organs  begin  to  func- 
tionate, but  Httle  is  positively  known. 

The  primitive  heart,  in  all  probability,  begins  to  pulsate  very 
early,  as  in  an  embryo  from  fifteen  to  eighteen  days  old  and  measuring 
but  2.2  mm.  in  length,  Coste  found  the  amnion,  the  allantois,  the 
omphalo-mesenteric  vessels,  and  the  two  primitive  aortas  developed. 
In  the  earlier  weeks,  all  products  of  metabolism  are  doubtless  elimi- 
nated by  the  placental  structures;  but  as  metabolism  increases  in 
complexity  the  liver  and  kidney  assume  excretory  activity.  Thus, 
at  the  end  of  the  third  month  the  intestine  contains  a  dark,  greenish, 
viscid  material — meconium — composed  of  bile  pigments,  bile  salts, 
and  desquamated  epithelium ;  the  amniotic  fluid,  as  well  as  the  fluid 
within  the  bladder,  contains  urea  at  the  end  of  the  sixth  month, 
indicating  the  estabhshment  of  both  hepatic  and  renal  activity.  Con- 
tractions of  the  skeletal  muscles  of  the  limbs  begin  about  the  fifth 
month,  from  which  it  may  be  inferred  that  the  mechanism  for  muscle 
activity,  viz.,  muscles,  efferent  nerves,  and  spinal  centers,  has 
become  anatomically  developed  and  associated,  and  capable  of 
coordinate  activity.  These  contractions  are,  in  all  probabihty,  auto- 
matic or  autochthonic  in  character  due  to  stimuli  arising  within  the 
spinal  centers.     The  remaining  organs  remain  more  or  less  inactive. 

After  birth,  with  the  first  inspiration  and  the  introduction  of  food 
into  the  ahmentary  canal,  the  physiologic  mechanisms  which  sub- 
serve general  metabolism  begin  to  functionate  and  in  the  course 
of  a  week  are  fully  estabhshed.  At  this  time  the  cardiac  pulsation 
averages  about  135  a  minute;  the  respiratory  movements  vary  from 
30  to  35  a  minute,  and  are  diaphragmatic  in  type;  the  urine,  which 
was  at  first  scanty,  is  now  abundant  and  proportional  to  the  food 
consumed;  the  digestive  glands  are  elaborating  their  respective 
enz}'mes,  digestion  proceeding  as  in  the  adult.  The  hepatic  secre- 
tion is  active  and  the  lower  bowel  is  emptied  of  its  contents;  the 
coordinate  activities  of  the  nerve-,  muscle-,  and  gland-mechanisms 
are  entirely  reflex  in  character.  Psychic  activities  are  in  abeyance  by 
reason  of  the  incomplete  development  of  the  cerebral  mechanisms. 


APPENDIX. 


PHYSIOLOGIC  APPARATUS. 

The  study  of  the  physical  and  physiologic  properties  of  muscles  and 
nerves  necessitates  the  employment  of  some  stimulus  which,  when  applied 
to  either  tissue,  will  call  forth  a  contraction  of  the  muscle,  or  the  develop- 
ment of  a  nerve  impulse  in  the  nerve.  The  most  convenient  stimulus 
is  electricity,  for  the  reason  that,  with  appropriate  apparatus,  its  intensity 
and  duration  can  be  graduated  with  the  utmost  nicety.  Moreover,  it 
does  not  destroy  the  tissues,  as  do  many  chemic,  physical,  and  mechanic 
stimuli. 

It  is  therefore  necessary  that  the  student  should  have  a  practical  acquaint- 
ance with   those   appliances  by  means  of  which  elec- 
tricity is  generated,  appUed  and  controlled. 

The  electric  cell  is  an  apparatus  composed  of 
different  elements,  which,  by  virtue  of  chemic  actions 
taking  place  among  them,  generate  and  conduct  elec- 
tricity. In  its  simplest  form  an  electric  cell  consists  of 
two  metals — zinc  and  copper,  or  carbon,  or  platinum, 
etc.,  immersed  in  an  exciting  fluid,  usually  dilute  sul- 
phuric acid  (Fig.  334). 

The  zinc  element  is  the  one  acted  on  chemically  by 
the  sulphuric  acid,  and  at  the  expense  of  which  the 
electricity  is  maintained.  It  is  kno^vn  as  the  generating 
element.  The  copper  is  the  collecting  and  conducting 
element. 

With  the  immersion  of  these  elements  in  a  solution 
of  H2SO4  a  chemic  action  at  once  takes  place  between 
the  zinc  and  the  acid,  with  the  formation  of  zinc  sulphate  and  the  libera- 
tion of  hydrogen,  as  expressed  in  the  following  formula: 

Zn  -f  H2SO1  =  ZnSO,  +  H,. 

The  zinc  sulphate  passes  into  the  solution,  while  the  hydrogen  accumulates 
on  the  surface  of  the  copper  element. 

As  all  chemic  action  is  accompanied  by  the  development  of  electricity, 
it  can  be  sho\ATi  by  appropriate  means  that  this  is  the  case  at  the  surface 
of  the  zinc.  Such  a  combination  is  the  means  of  establishing  a  difference 
of  potential  between  two  points;  the  point  of  highest  potential  bemg  the 
surface  of  the  zinc  or  the  positive  element,  the  point  of  lowest  potential 

687 


Fig.      334.  —  An 
Electric  Cell. 


688 


TEXT-BOOK  OF  PHYSIOLOGY. 


being  the  copper  or  the  negative  element.     So  long  as  the  elements  remain 
unconnected  there  is  no  movement  of  electricity,  no  current. 

If  the  ends  of  the  elements  projecting  beyond  the  fluid  are  connected 
by  a  copper  wire,  a  pathway  or  circuit  is  estabUshed,  and  a  movement  of 
the  electricity  takes  place.  As  electricity  flows  from  the  pomt  of  high  to 
the  point  of  low  potential,  it  follows  that  inside  the  cell  the  current  flows 
from  the  zinc  to  the  copper,  and  outside  the  cell  from  the  copper  to  the 
zinc.  Such  a  current  is  termed  a  continuous,  a  galvanic  or  a  voltaic  cur- 
rent. Inasmuch  as  there  is  a  progressive  fall  in  potential  between  the 
highest  and  lowest  points,  it  follows  that  any  two  points  in  the  circuit  will 
exhibit  a  similar  difference  of  potential.  For  this  reason  the  projecting 
end  of  the  copper  element  is  at  a  higher  potential  than  the  projecting  end 
of  the  zinc  element.  The  end  of  the  copper  is,  therefore,  termed  the  posi- 
tive, +  pole  or  anode,  the  end  of  the  zinc  the  negative,  • —  pole  or  kathode. 
Electric  Units. — Owing  to  the  difference  of  the  electric  potential  in 
the  cell,  the  electricity  leaves  the  cell  under  a  certain  degree  of  pressure, 
termed  the  "electro-motive  force."  As  it  passes  through  the  circuit  it 
meets  with  resistance,  the  amount  of  which  will  depend  on  the  nature  of 

the  circuit  material,  its 
length,  and  the  area'  of  its 
cross-section.     In   accord- 
ance  with    the    resistance 
will  depend  the  quantity  of 
electricity  that  a  given  elec- 
tro-motive force  will  press 
through  in  a  unit  of  time. 
The  strength  of  the  current 
will  therefore  not  depend 
entirely    on     the    electro- 
motive force,  but,   rather, 
on  the   ratio   between   the 
electro-motive     force    and 
the  resistance. 
For  the  measurement  of  electric  quantities,  a  system  of  imits  has  been 
devised.     The  unit  of  electro-motive  force  is  the  volt;  the  unit  of  resistance 
is  the  ohm,  i.  e.,  the  resistance  offered  by  a  column  of  mercury  106.3  cm. 
long  and  i  sq.  mm.  in  section  at  0°  C;  the  unit  of  quantity  is  the  coulomb; 
the  unit  of  time  is  one  second.     One  volt  is  the  electro-motive  force  which, 
when  steadily  appUed,  will  press  through  a  resistance  of  one  ohm,  one 
coulomb  of  electricity  in  one  second  of  time  yielding  a  current  strength 
of  one  ampere. 

This  relation  may  be  expressed  in  the  following  formula.  Ohm's  law: 


T  -f- 


FlG. 


335. — Two  Simple  Electric  Cells  Joined 
IN  Series.    C.  Copper.      Z.  Zinc. 


C  (current  strength) 


Electro-motive  force  (E.  M. 
Resistance  (R) 


F.)        .  Volt 

— ^  or  Ampere  = 

^  Ohm 


In  practical  work  it  is  often  necessary  to  increase  the  strength  of  the 
current.  This  is  done  by  uniting  two  or  more  cells  in  series,  i.  e.,  uniting 
the  copper  of  one  cell  to  the  zinc  of  a  second,  and  so  on  (Fig.  335).  If  the 
resistance  remains  the  same  the  total  voltage  is  that  of  one  cell  multi- 
plied by  the  number  of  cells  united. 


PHYSIOLOGIC  APPARATUS. 


The  cell  as  above  described  cannot  maintain  a  current  of  constant 
strength  for  any  length  of  time,  for  the  following  reasons: 

1.  The  sulphuric  acid  solution,  in  consequence  of  its  chemic  action, 
soon  becomes  nothing  more  than  a  saturated  solution  of  zinc  sulphate, 
after  which  its  chemic  activity  ceases.  The  current,  therefore,  soon 
diminishes  in  strength. 

2.  The  accumulation  of  hydrogen  bubbles  on  the  surface  of  the  copper 
hinders  the  passage  of  the  electricity.  In  a  short  time  they  develop  a 
current  in  the  opposite  direction,  which  also  tends  to  weaken  the  original 
current.     This  action  is  termed  polarization  of  the  elements. 

Cells  of  this  character  are  not  suited  for  physiologic  work,  in  which 
constancy  in  the  strength  of  the  current  is  absolutely  necessary.  To 
overcome  these  disadvantages,  cells  have  been  devised  which  are  less 
violent  in  action,  which  prevent  polarization,  and  which  main  tarn  a  cur- 
rent of  constant  strength  for 
a  long  period  of  time.  One 
of  the  most  generally  used  for 
physiologic  purposes  is — 

The  Daniell  cell.  This 
consists  of  a  porous  cup  con- 
taining a  saturated  solution  of 
CUSO4,  copper  sulphate,  in 
which  is  immersed  a  copper 
plate  or  rod.  This  combina- 
tion is  placed  in  a  glass  vessel 
containing  a  solution  of  H2SO4 
(i  :  15).  In  this  solution  is 
immersed  a  roll  of  sheet  zinc 
(Fig.  336).  Each  of  the  plates 
is  provided  with  a  binding 
screw.  When  the  cell  is  in 
action  the  sulphuric  acid  at- 
tacks the  zinc,  forming  zinc 
sulphate,  and  liberates  hydro- 
gen; the  cup  being  porous,  the  hydrogen  passes  into  the  copper  sulphate 
solution,  where  it  combines  with  the  sulphuric  acid  radicle,  and  liberates 
metalhc  copper.  Polarization  of  the  copper  is  thus  prevented.  The 
metallic  copper  is  deposited  on  the  copper  plate,  which  is  thus  kept 
bright.  The  copper  sulphate  solution  is  kept  at  the  point  of  saturation 
by  packing  around  the  copper  cylinder  a  quantity  of  the  crystals  of  the 
salt.  The  sulphuric  acid  passes  back  into  the  porous  cup,  to  take  the 
place  of  that  used.  This  cell  is  remarkably  constant  for  these  reasons, 
and  well  adapted  for  physiologic  as  well  as  other  purposes  where  a  current 
of  uniform  strength  is  necessary. 

The  projecting  ends  of  the  copper  and  zinc  plates  are  termed  respec- 
tively the  positive  pole  or  anode,  and  the  negative  pole  or  kathode.  The 
electro-motive  force  of  a  Daniell  cell  is  practically  i  volt;  but  when  the 
two  poles  are  connected  by  a  wire  of  i  ohm  resistance,  the  current  strength 
will  be  less  than  i  ampere,  possibly  only  0.7,  owing  to  the  resistance  offered 

44 


i-ir 


-JJAXIELL    L  KLL. 


690 


TEXT-BOOK  OF  PHYSIOLOGY. 


to  the  flow  of  electricity  by  the  fluids  between  the  zinc  and  the  copper. 
In  all  measurements,  the  internal  resistance  of  the  cell  must  be  taken  into 
consideration. 

The  Dry  Cell. — The  commercial  dry  cell  is  a  convenient  source  of 
electricity  for  general  laboratory  work.  It  consists  of  a  cup  of  zinc,  the 
inner  surface  of  which  is  covered  over  with  a  thick  layer  of  a  paste  of  plaster 
of  Paris,  saturated  with  ammonium  chlorid.  In  the  center  of  the  cup 
there  is  a  rod  of  carbon.  Surrounding  this  rod  and  occupying  the  space 
between  it  and  the  plaster  of  Paris  paste,  is  a  mixture  of  manganese  dioxid 
and  charcoal.  The  upper  surface  of  the  cell  is  sealed  to  prevent  evapora- 
tion.    The  electricity  is  generated  at  the  surface  of  the  zinc  cup  by  the 

chemic  action  of  the  chlorin  which 
arises  from  the  dissociation  of  the  am- 
monium chlorid.  When  the  plates 
are  united  by  a  conjunctive  wire  the 
current  within  the  cell  flows  from  the 
zinc  (the  positive  element)  to  the 
carbon  (the  negative  element),  and 
without  the  cell  from  the  carbon  (the 
positive  pole)  to  the  zinc  (the  negative 
pole) . 

Leads. — By  means  of  insulated 
wires  attached  to  the  poles  of  a  cell, 
the  electricity  may  be  conducted  from 
the  cell  and  used  for  exciting  or 
stimulating  purpose.  As  the  wires 
thus  become  practically  prolongations 
of  the  plates  their  ends  become  the 
corresponding  poles.  In  experimental 
work  the  ends  of  the  wires  are  provided 
with  special  devices,  termed — 

Non-polarizable  electrodes. 
The  necessity  for  the  employment  of 
such  electrodes  arises  from  the  fact 
that  when  the  ends  of  the  wires  from 
a  cell  are  placed  in  direct  contact  with 
the  tissues  chemic  changes  are  pro- 
duced in  a  short  time,  which  lead  to  their  polarization.  As  a  result,  a 
current  opposite  in  direction  to  that  of  the  cell  is  developed,  which  tends 
to  weaken  or  neutralize  it.  This  polarization  current  vitiates  the  result 
of  many  experiments  made  with  highly  irritable  tissue  such  as  nerve  tissue. 
Whether  for  stimulating  purposes  or  for  the  purpose  of  detecting  the  exist- 
ence of  electric  currents  in  living  tissues,  it  is  essential  that  the  electrodes 
used  shall  be  non-polarizable.  The  earliest  electrodes  of  this  character 
were  made  by  du  Bois-Reymond  and  were  based  on  the  fact  discovered 
by  Regnault  that  a  strip  of  chemically  pure  zinc  or  amalgamated  zinc 
(Matteucci)  immersed  in  a  saturated  solution  of  zinc  sulphate  would  not 
polarize.     One  form  made  by  du  Bois-Reymond  is  shown  in  Fig.  337.     It 


Fig.  337. — Non-polarizable  Elec- 
trodes. I.  Du  Bois-Reymond's. 
2.  Von    Fleischl's.      3.  d'Arson- 

'        val's. 


PHYSIOLOGIC  APPARATUS. 


691 


consists  of  a  flattened  glass  tube  attached  to  a  universal  joint  and  supported 
by  an  insulated  brass  stand.  The  lower  end  of  the  tube  is  closed  with 
kaolin  or  China  clay  made  into  a  paste  with  a  0.6  per  cent,  solution  of 
sodium  chlorid.  It  can  be  molded  into  any  desired  shape.  The  interior 
of  the  tube  is  partially  filled  with  a  saturated  solution  of  sulphate  of  zinc 
in  which  is  immersed  the  strip  of  amalgamated  zinc.  To  the  upper  end 
of  the  zmc  the  conducting  wire  is  attached. 

The  V.  Fleischl  brush  electrode  is  similar  to  the  preceding  except  that 
the  end  of  the  tube  is  closed  by  the  brush  of  a  camel's-hair  pencil. 

The  d'Arsonval  electrode  consists  of  a  glass  tube  containing  a  silver 
rod  coated  with  fused  silver  chlorid.     The  interior  of  the  tube  is  filled 
with  normal  salt  solution  0.6  per  cent,  and 
the   end  closed   with  a   thread   or   plug    of 
asbestos  which  is  made  to  project  beyond 
the  tube  for  a  short  distance. 

Any  one  of  the  these  three  electrodes  is 
suitable  for  physiologic  experimentation,  as 
their  free  ends  neither  corrode  the  tissues  nor 
develop  electric  currents. 

Keys. — Muscle  and  nerve  tissues  are  con- 
ductors of  electricity.  When,  therefore,  the 
termmals  (the  non-polarizable  electrodes)  of 
the  wires  of  a  cell  are  placed  in  contact  with 
either  a  muscle  or  a  nerve  a  circuit  is  made 
through  which  a  current  of  electricity  flows; 
when  one  or  both  are  removed,  the  circuit  is 
broken  and  the  current  ceases.  In  practical 
work  it  is  often  necessary  to  keep  the  elec- 
trodes in  contact  with  the  tissues  for  a  varia- 
ble length  of  time.  The  circuit,  however, 
may  be  alternately  made  and  broken  at  will 
by  interposing  along  the  return  wire  a 
mechanic  contrivance  kno^\Tl  as  a  key,  of 
which  there  are  many  forms. 

The  du  Bois  Reymond  Friction  Key. — 
This  consists  of  a  plate  of  vulcanite  attached 
to  a  screw  clamp  by  which  it  can  be  fastened 
to  the  edge  of  a  table  (Fig.  338).  The 
surface    of   the   vulcanite   plate   carries  two 

rectangular  blocks  of  brass,  each  of  which  has  two  holes  drilled  through 
it,  for  the  insertion  of  wires,  which  are  held  in  position  by  small  screws.  A 
movable  bridge  of  brass,  provided  with  an  ebonite  handle,  serves  to  make 
connection  between  the  blocks.  There  are  two  ways  of  interposing  this 
key  in  the  circuit. 

1.  As  a  Simple  Key. — For  this  purpose  one  of  the  wires,  usually  the  nega- 

tive, is  carried  from  the  cell  to  one  block  and  then  continued  from 
the  second  block.  When  the  bridge  is  dowTi,  the  circuit  is  made  and 
the  current  passes;  when  it  is  up,  the  circuit  is  broken. 

2.  As  a  Short-circuiting  Key. — When  used  for  this  purpose,  the  wires  of 


Fig.  338. — Du  Bois-Rey- 
MOND  Friction  Key. 


692 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  cell  are  carried  to  the  inner  holes  of  each  block  and  then  continued 
from  the  outer  holes  to  the  tissues  or  to  some  form  of  apparatus 
which  it  is  desired  to  actuate.     When  the  key  is  closed,  i.  e.,  when 
the  bridge  is  down,  the  current  on  reaching  the  key,  will  divide,  one 
portion  passing  across  the  bridge  and  so  back  to  the  cell,  the  other 
portion  passing  to  the  tissue  or  apparatus.     The  amount  of  the  cur- 
rent which  is  returned  to  the  cell  through  the  short  circuit  will  be 
proportional  to  the  resistance  of  the  longer  circuit.     As  the  latter  is 
usually  great  in  comparison  with  the  former,  practically  all  the  current 
is  short-circuited.     When  the  bridge  is  lowered,  therefore,  the  current 
is  short-circuited;  when  it  is  raised,  the  current  flows  into  the  longer 
circuit  through  the  tissue  or  apparatus. 
The  Mercury  Key. — In  this  form  the  connection  is  established  by 
means  of  mercury.     It  consists  of  a  circular  block  in  the  center  of  which 
there  is  a  cup  containing  mercury  (Fig.  339).     At  opposite  points  there 
are  bindmg  posts,  one  of  which  is  provided  with  a  rigid  fixed  copper  rod 

passing  into  the  mercury;  the 
other  is  provided  with  a  mova- 
ble bent  rod  which  may  be 
made  to  dip  into  or  be  with- 
drawal from  the  mercury  by 
the  ebonite  handle. 

The  effect  of  a  constant  or 
galvanic  current  on  a  muscle 
or  nerve  will  depend  to  some 
extent  on  its  strength.  This 
may  be  accurately  regulated 
by  means  of  an  apparatus 
known  as— 

The  Rheocord.  With  this 
apparatus  an  electric  current 
may  be  divided,  one  portion  continuing  through  a  conductor  back  to  the 
battery,  the  other  portion  being  sent  off  through  the  nerve.  The  strengths 
of  these  two  currents  are  inversely  proportional  to  the  resistances  of  their 
circuits.  A  simple  form  of  rheocord  (Fig.  340)  consists  of  a  long  wire 
arranged  for  convenience  in  parallel  Imes  on  a  small  wooden  base  and 
connected  at  its  two  ends  with  binding  posts  A  and  B.  The  resistance 
of  this  wire,  1.6  ohms,  can  be  increased  by  the  introduction  of  small  re- 
sistance coils,  between  D  and  B,  varying  from  5  to  20  ohms. 

The  two  binding  posts  A  and  B  are  connected  with  the  positive  and 
negative  poles  of  an  electric  cell  respectively.  A  simple  key  is  placed  in 
the  circuit. 

From  A,  a  wire  passes  to  one  of  the  electrodes  on  which  the  muscle  or 
nerve  rests.  A  second  wire  passes  from  the  second  electrode  to  a  clamp 
S,  by  way  of  the  binding  post  C,  which  can  be  fastened  to  the  long  wire 
at  any  given  point.  The  current,  on  reaching  A,  will  divide  into  two 
branches,  one  of  which  will  pass  along  the  wire  A,  B,  and  thence  back  to 
the  cell;   the  other  will  pass  through  the  nerve  and  back  to  S  and  thence 


Fig.  339. — A  Mercury  Key. 


PHYSIOLOGIC  APPARATUS. 


693 


also  to  the  cell.  The  amount  of  current  passing  through  the  nerve  circuit 
will  be  inversely  proportional  to  the  resistance  of  the  nerve  and  directly 
proportional  to  the  difference  of  potential  between  A  and  S.  If  S  is  close 
to  A,  the  difference  of  potential  is  slight.  If  S  is  removed  from  A  toward 
B,  the  difference  of  potential  is  increased  and  the  current  sent  through 
the  nerve  circuit  is  increased. 

In  many  experiments  it  is  necessary  to  reverse  the  direction  of  the  cur- 
rent, in  other  experiments  to  de-flecl  it,  without  changing  the  position  of 
the  electrodes.     Both  these  results  may  be  accomplished  by  the  use  of — 

Pohl's  commutator.  This  is  a  round  block  of  wood  with  six  cups, 
each  of  which  is  in  connection  with  a  binding  post  (Fig.  341).  In  each  of 
the  two  cups  marked  i  and 
2,  +  and  — ,  is  inserted  one 
end  of  a  copper  wire  bent  at 
right  angles.  The  other  ends 
of  the  wires  are  supported  and 
insulated  by  a  hard -rubber 
handle..  To  the  top  of  each 
wire  is  soldered  a  semicircular 
copper  wire.  This  arrange- 
ment permits  of  a  rocking 
movement,  whereby  the  oppo- 
site ends  of  the  semicircular 
wires  can  be  made  to  dip  into 
cups  3  and  4,  and  into  cups 
5  and  6  alternately.  Two 
wires  crossed  in  the  middle  of 
the  block  serve  to  connect- 
opposite  pairs  of  cups.  When 
in  use,  the  cups  are  filled  with 
clean  mercury.  The  method 
of  using  the  commutator  is  as 
follows : 

I .  Asa  Current  Reverser. — -The 
positive  and  negative  poles 

of  the  electric  cell  are  connected  by  wires  with  binding  posts  i  and  2 
respectively.  A  key  is  interposed  in  the  circuit.  Wires  are  then  carried 
from  binding  posts  3  and  4  to  the  electrodes  in  connection  with  the 
muscle  or  nerve.  The  rocker  of  the  commutator  is  so  turned  that  the 
ends  of  the  semicircular  wires  dip  into  cups  3  and  4.  The  direction  of 
the  current  will  be  on  the  closure  of  the  circuit  from  i  to  3,  then  from 
3  along  a  wire  to  and  through  the  tissue  and  back  to  4,  and  thence 
to  the  cell.  If  the  position  of  the  rocker  be  now  reversed  so  that  the 
opposite  ends  of  the  semicircular  wires  dip  into  cups  5  and  6,  the 
direction  of  the  current  through  the  tissue  will  be  reversed.  The 
positive  current,  after  entering  binding  post  i,  will  flow  to  5;  then 
along  one  of  the  cross  wires  to  4;  then  along  a  wire  to  and  through 
the  tissue  and  back  to  3,  along  the  opposite  cross  wire  to  6,  thence  to  2 
and  so  back  to  the  cell. 


Fig    340.— Rheocord. 


694 


TEXT-BOOK  OF  PHYSIOLOGY. 


2.  As  a  Current  Deflector. — When  it  is  desirable  to  deflect  the  current  to 

two  pairs  of  electrodes  differently  situated,  wires  are   carried  from 

binding  posts  3  and  4  to  one  pair,  and  from  5  and  6  to  the  other  pair. 

The  cross  wires  are  then  removed.     According  to  the  position  of  the 

rocker  the  current  will  be  deflected  to  one  or  the  other. 

The  Inductorium. — This  is  an  apparatus  designed  for  the  purpose 

of  obtaining  single  or  rapidly  succeeding  electric  currents  by  induction. 

Its  construction  is  based  on  facts  discovered  by  Faraday,  some  of  which 

are  the  following: 

If  two  circuits,  a  primary  and  a  secondary,  are  placed  parallel  to  each 
other,  the  former  connected  with  a  galvanic  cell,  the  latter  with  a  galvan- 
ometer, it  is  found  that,  at  the  moment  the  primary  circuit  is  made,  and 
at  the  moment  it  is  broken,  a  current  is  induced  in  the  secondary  circuit, 
as  shown  by  a  momentary  deflection  of  the  galvanometer  needle.  During 
the  continuous  flow  of  the  current  through  the  primary  circuit  there  is  no 


Fig.  341. — Pohl's  CoiiiiuxATOR. 


A.    Arranged  as  a  current  reverser;    B,  as  a  cur- 
rent deflector. 


evidence  of  a  current  in  the  secondary  circuit.  The  induced  current  is 
but  of  momentary  duration.  The  current  flowing  through  the  primary 
circuit  is  termed  the  inducing,  the  current  flowing  through  the  secondary 
circuit  the  induced  current. 

The  induced  current  is  opposite  in  direction  to  that  of  the  inducing 
current  when  the  circuit  is  made  or  closed;  it  is  in  the  same  direction, 
however,  when  the  circuit  is  broken  or  opened. 

If  the  circuits  are  arranged  in  the  form  of  coils,  it  is  found  that,  other 
things  being  equal,  the  strength  of  the  induced  currents  will  be  proportional 
to  the  number  of  turns  in  the  coils. 

If  the  coils  are  placed  at  varying  distances  from  each  other,  the  strength 
of  the  induced  current  varies,  increasing  as  the  coils  are  approximated, 
decreasing  as  they  are  separated. 

Approximation  or  separation  of  the  coils  while  the  current  is  flowing 
through  the  primary  circuit  develops  an  induced  current,  which  disappears, 


PHYSIOLOGIC  APPARATUS. 


69s 


however,  the  moment  the  movement  of  the  coil  ceases.  A  sudden  increase 
or  decrease  in  the  strength  of  the  inducing  current  also  develops  an  induced 
current. 

When  the  coils  are  approximated  or  the  primary  current  increased  in 
strength,  the  induced  current  is  opposite  in  direction  to  that  of  the  inducing 
current;  with  the  reverse  conditions,  the  induced  current  has  the  same 
direction . 

The  induced  currents  have  been  termed,  in  honor  of  their  discoverer, 
Faradic  currents. 

The  du  Bois-Reymond  inductorium,  based  on  the  foregoing  facts, 
consists  essentially  of  two  coils  of  insulated  copper  wire,  termed  primary 
and  secondary  (Fig.  342). 

The     primary 
coil,   R',    consists  S 

of  thick  copper 
wire  wound 
around  a  wooden 
spool  attached  to 
a  vertical  support. 
The  beginning  of 
this  coil  is  at  the 
binding  post  S", 
its  termination 
either  at  binding 
post  P"  or  S"'. 
In  the  course  of 
this  primary  wire 
or  circuit,  there 
are  placed  two 
vertical  bars  of 
soft  iron,  B',  con- 
nected at  their 
bases  to  form  a 
horseshoe  magnet, 

around  the  ends  of  which  the  wire  is  coiled, 
will  be  explained  later. 

Inside  the  primar}^  coil  there  is  placed  a  bimdle  of  soft  iron  wires, 
which,  as  soon  as  the  circuit  is  made,  become  magnetized,  with  the  effect 
of  increasing  the  action  of  the  inducing  current. 

The  secondary  coil,  R'',  consists  of  a  much  greater  number  of  turns 
of  a  finer  copper  wire,  the  ratio  bemg  about  40  to  i,  also  wound  around  a 
spool,  having  a  timnel  sufl&ciently  large  to  enable  it  to  slide  over  the  pri- 
mary. By  these  means  the  strength  of  the  induced  current  is  increased. 
As  a  result  of  the  construction  of  the  inductorium,  the  low  electro-motive 
force  of  the  cell  is  transformed  into  the  high  electro-motive  force  charac- 
teristic of  the  induced  current.  As  the  number  of  turns  of  wire  in  the 
secondary  coil  is  to  the  number  in  the  primary,  so  are  the  electro-motive 
forces  in  the  secondary  coil  to  those  m  the  primary  coil. 

The   secondary  coil   slides   along  a  track,  B,  which   permits   it  to  be 


Fig. 


342. — iNDUCTORiuii  OF  DU  Bois-Reymond.  R',  Pri- 
mary,  R",  secondary  spiral.     B.    Board  on  which  R" 

moves.     I.   Scale.     -1 .  Wires  from  battery.     P',  P". 

Pillars.  H.  Neef's  hammer.  B'.  Electro-magnet.  S'. 
Binding  screw  touching  the  steel  spring  (H).  S"  and 
S'".  Binding  screws  to  which  to  attach  \vires  where  Neef's 
hammer  is  not  required. 


The  object  of  this  device 


696  TEXT-BOOK  OF  PHYSIOLOGY. 

moved  toward  or  away  from  the  primary.  The  distance  between  the  two 
coils  can  be  measured  and  the  strength  of  the  induced  current  again  re- 
produced, other  things  being  equal,  by  means  of  a  centimeter-millimeter 
scale  pasted  on  the  edge  of  B. 

The  ends  of  the  wire  of  the  secondary  coil  are  fastened  to  two  binding 
posts  to  which  conducting  w^ires  provided  with  hand  electrodes  can  be 
attached. 

The  inductorium  may  be  used  for  obtaining  either  a  single  current  or 
a  series  of  rapidly  repeated  induced  currents. 

The  Single  Induced  Current. — On  account  of  its  high  electro-motive 
force,  its  penetrative  power,  and  short  duration,  the  single  induced  current 
is  a  most  convenient  and  suitable  form  of  stimulus  for  many  purposes. 
In  order  to  obtain  such  a  current,  the  positive  wire  of  the  cell  is  carried  to 
bindmg  post  S",  and  the  negative  wire  either  to  S"'  or  P".  A  key  is  placed 
in  the  primary  circuit.  The  course  of  the  current  will  then  be  on  the 
closure  of  the  circuit  from  the  cell  to  S'^,  thence  around  R'  to  S'",  and  so 
back  to  the  cell;  or  if  the  negative  wire  is  connected  with  P",  the  course 
of  the  current  on  leaving  R'  will  be  through  the  coils  surrounding  the  two 
vertical  bars  B',  thence  to  V",  and  so  back  to  the  cell.  If  the  secondary 
coil  be  placed  close  to  the  primary  and  the  wires  of  the  secondary  brought 
into  contact  wdth  a  muscle,  it  will  be  fomid  that  with  both  the  make  and 
the  break  of  the  primary  circuit  a  current  is  induced  in  the  secondary,  as 
shown  by  a  short  quick  pulsation  of  the  muscle;  but  during  the  time  of 
closure  of  the  circuit,  the  induced  current  is  wanting,  as  shown  by  the 
quiescent  condition  of  the  muscle.  It  will  be  apparent,  however,  from 
the  energy  of  the  contraction  that  the  break  induced  current  is  a  more 
efficient  stimulus  than  the  make  induced  current.  That  this  is  the  case 
is  made  evident  by  removing  the  secondary  to  the  end  of  the  slidew'ay  and 
then  gradually  bringing  it  toward  the  primary  half  a  centimeter  at  a  time, 
making  and  breaking  the  circuit  after  each  movement  until  a  pulsation  of 
the  muscle  occurs.  It  will  be  fomid  to  occur  first  on  the  break  of  the 
circuit.  As  the  secondary  approaches  the  primary  a  position  will  be 
reached  when  a  pulsation  occurs  on  the  make  as  well  as  on  the  break  of 
the  circuit,  though  it  will  be  less  pronounced. 

The  explanation  offered  for  this  difference  in  the  strength  of  the  two  induced 
currents  is  as  follows:  With  the  make  of  the  circuit  and  the  passage  of  the  battery 
current  through  the  primary  coil  there  is  induced  in  the  neighboring  and  parallel 
turns  of  the  \vire  an  extra  current  opposite  in  direction  to  the  primary  current.  This 
extra  or  self-induced  current  antagonizes  and  prevents  the  current  from  attaining 
its  maximum  development  as  quickly  as  it  otherwise  would,  and  therefore  its  efficiency 
as  an  inducer  of  a  current  in  the  secondary  is  diminished.  On  the  break  of  the  circuit 
the  primary  current  disappears  quickly,  arid  as  there  is  nothing  to  retard  its  disappear- 
ance its  efhciency  as  an  inducer  of  a  current  in  the  secondary  coil  is  not  diminished. 
It  is  not  infrequently  stated  that  the  disappearance  of  the  primary  current  induces  in 
the  neighboring  coils  a  break  extra  current  corresponding  in  direction  which  assists 
in  the  development  of  the  induced  current.  This  is  not  the  case,  however,  as  no 
break  extra  current  is  developed  in  the  inductorium  as  ordinarily  used  when  actuated 
by  a  battery  current  of  moderate  strength. 

As  it  is  not  so  much  the  intensity  of  the  current  as  it  is  rapid  variations  in  intensity 
that  produce    effects,  it  is  readily  apparent  why  the   induced  current  developed  at 
the  break  of  the  primary  is  more  effective  as  a  stimulus  than  the  induced  current 
developed  at  the  make  of  the  primary  circuit.     The  quantity  of  the  electricity  is 
however,  the  same  in  both  cases. 


PHYSIOLOGIC  APPARATUS.  697 

If  the  secondary  be  pushed  further  along  the  slideway  until  it 
largely  covers  the  primar}^  coil,  a  position  will  be  reached  when  the  make 
induced  current  equals  in  its  efficiency  as  a  stimulus  the  break  mduced 
current;  and  if  the  secondar}^  be  yet  further  advanced,  a  position  is 
reached  when  the  make  induced  current  becomes  more  powerful  and 
efficient  than  the  break  induced  current,  as  sho^Mi  by  the  greater  contrac- 
tion of  the  muscle.  This  result  is  explained  by  the  fact  that  the  make 
extra  current  is  now  able  of  itself  to  induce  a  current  in  the  secondary 
coil,  on  account  of  its  proximity,  which,  added  to  that  induced  by  the 
batten-  current,  produces  a  current,  greater  than  that  induced  on  the 
break  of  the  circuit.* 

Rapidly  Repeated  Induced  Currents. — As  the  single  induced  current 
is  of  extremely  short  duration,  it  is  inefficient  as  a  stimulus  in  the  conduct 
of  many  experiments.  It  is  necessary,  therefore,  to  develop  it  with  a  fre- 
quency that  is  sufficient  to  give  rise  to  a  summation  of  effects.  The  dura- 
tion of  the  stimulation  may  be  thus  considerably  prolonged.  This  is 
accomplished  by  introducing  in  the  primary  circuit  close  to  the  primary 
coil  an  automatic  interrupter,  usually  Neef's  modification  of  Wagner's 
hammer  (Fig.  342).  This  consists  of  a  vertical  post,  P',  to  the  top  of 
which  is  fastened  a  metallic  spring  carrying  at  its  opposite  end  a  steel  or 
iron  hammer,  H,  which  hangs  over,  but  does  not  touch,  the  two  vertical 
bars  of  soft  iron  around  which  the  wire  of  the  primary  coil  is  woimd. 
About  the  middle  of  the  spring  on  its  upper  surface  there  is  a  small 
plate  of  platmum  which  is  in  contact  with  an  adjustable,  platinum-tipped 
screw,  S',  carried  by  a  plate  of  brass  in  connection  with  binding  post  S". 

For  the  purpose  of  interrupting  the  primar}'  circuit  frequently  in  a 
imit  of  time,  and  thus  developing  induced  currents  in  quick  succession,  the 
apparatus  is  arranged  in  the  following  way:  The  positive  and  negative 
poles  of  the  electric  cell  are  connected  by  wires  with  binding  posts  P'  and 
P",  a  key  being  interposed  in  the  circuit.  If  the  screw  S'  is  in  contact 
with  the  spring,  the  current  on  the  closure  of  the  circuit  will  enter  P',  pass 
along  the  spring  to  S',  thence  into  and  through  the  primary  coil  R',  to  the 
coils  surrounding  the  vertical  bars  B',  then  to  P'^,  and  so  back  to  the  cell. 

As  the  current  passes  around  the  vertical  bars,  they  are  magnetized. 
The  magnetization  draws  down  the  hammer,  and,  in  so  doing,  breaks 
the  circuit  at  the  tip  of  the  screw,  S'.  The  vertical  bars  are  at  once 
demagnetized,  and  the  hammer  is  restored  to  its  original  position  by  the 
elasticity  of  the  spring.  The  circuit  is  thus  re-established,  the  current 
flows  through  the  coils,  the  bars  are  again  magnetized,  the  hammer  is 
drawn  down,  to  be  followed  by  a  second  break  of  the  circuit. 

The  number  of  times  the  circuit  is  thus  made  and  broken  per  second  will 
vary  with  the  length  of  the  spring. 

As  each  interruption  of  the  primary  circuit  develops  an  mduced  current, 
it  follows  that  the  latter  must  succeed  each  other  with  a  frequency  corre- 
sponding with  the  frequency  of  the  former.  If  while  the  primary  circuit 
is  thus  being  interrupted  the  wires  of  the  secondary  coil  be  placed  in  con- 

*  "On  certain  peculiarities  of  the  inductorium/'  Prof.  Colin  C.  Stewart,  "Univ. 
Pa.  Medical  Bulletin,"  Feb.,  1904. 


698 


TEXT-BOOK  OF  PHYSIOLOGY. 


tact  with  a  muscle,  the  mduced  current  will  give  rise  to  contractions  which 
will  succeed  each  other  so  rapidly  that  they  fuse  together,  producing  a 
spasm  or  tetanus  of  the  muscle.  For  this  reason  these  currents  are  fre- 
quently spoken  of  as  tetanizing  currents,  and  the  procedure  as  tetanization 
or  Faradization.  These  currents  also  increase  in  strength  as  the  secondary 
approaches  the  primary. 

Helmholtz's  Modification  of  the  Inductorium. — With  a  view  of  equalizing 
the  strengths  of  the  induced  currents,  HelmhoUz  suggested  a  device  the  adoption  of 
which  accomphshes  this  to  a  certain  extent.  It  consists  (Fig.  342)  in  connecting  with  a 
wire  binding  posts  P'  and  S",  and  in  providing  binding  post  P"  with  an  adjustable  screw 
which  can  be  raised  until  the  spring  comes  in  contact   with  it,  when  the  hammer  is 

drawn  down  by  the  electromagnet  B'.  This 
latter  arrangement  is  practically  a  short- 
circuiting  key  by  which  a  portion  of  the  cur- 
rent is  returned  to  the  cell  without  ever  enter- 
ing the  primary  coil.  The  same  arrange- 
ment, though  differently  lettered,  is  shown  in 
Fig.  343.  By  the  use  of  the  entire  device 
the  changes  in  the  primary  coil  are  made 
not  by  making  and  breaking  the  primary 
current,  but  by  alternately  long-  and  short- 
circuiting  the  current.  "When  the  short- 
circuiting  key  is  opened,  the  full  volume  of 
the  primary  current  flows  through  the  pri- 
mary coil.  When  the  short-circuiting  key 
is  closed,  most  of  the  current  fails  to  enter 
the  coil,  taking  the  easier  path  through 
the  key.  Some  of  the  current,  however, 
always  flows  through  the  coil  and  is  never 
diverted.  The  cycle  of  changes  in  the 
electric  condition  of  the  primary  coil  is 
thus  altered  for  two  reasons: 

"First,  we  no  longer  have  an  alternation 
between  a  full  primary  current  and  none 
at  all — rather  an  alternation  between  a  full 
primary  current  and  a  weaker  one.     The 
difference   in   the  phases  is  thus   lessened, 
the  extent  of   the  change   on  making   and 
breaking  is  lessened,  and   correspondingly 
the  efficiency  of  the  make  and  break  cur- 
rents  induced    in   the    secondary    coil    is 
slightly  decreased. 
"Second,  on  making  the  primary  current,  as  in  the  ordinary  coil,  the  sudden 
appearance  of  the  primary  current  is  antagonized  by  the  opposing  make  extra  current, 
with  the  result  that  the  make  induced  current  is  still  further  reduced;   while  on  break- 
ing the  current  the  break  e.xtra  current  can  now  flow  through  the  primary  coil  across 
the  short-circuiting  key.     This   current,   traihng  behind  the   disappearing   primary 
current  in  the  same  direction,  produces  the  same  effect  as  if  the  primary  current  itself 
were  to  disappear  slowly.     As  a  result  the  disappearance  of  the  primary  current  loses 
its  former  efficiency  as  an  inducer  of  secondary  currents,  and  the  break  induction 
current  is  reduced  to  about  the  efficiency  of  the  make. 

"This  so-called  'ecjualizing'  of  the  make  and  break  induced  currents  is  never 
perfect,  if  for  no  other  reason,  because  the  make  extra  current  must  take  the  long 
circuit  through  the  battery,  while  the  break  extra  current  has  an  easier  path  through 
the  short-circuiting  key,  and  is  thus  greater  than  the  make  extra  current."  (C.  C. 
Stewart.) 


Fig.  343. — Helmholtz's  Modifica- 
tion OF  NeEF'S  HAMilER.  As 
long  as  c  is  not  in  contact  with  d, 
g  h  remains  magnetic;  thus  c  is 
attracted  to  d  and  a  secondary 
circuit,  0,  b,  c,  d,  e,  is  formed;  c 
then  springs  back  again,  and 
thus  the  process  goes  on.  A  new 
wire  is  introduced  to  connect  a 
with  /.     K.  Battery 


PHYSIOLOGIC  APPARATUS. 


699 


THE  GRAPHIC  METHOD. 

The  term  graphic  is  applied  to  a  method  by  which  curves  or  tracings 
are  obtained  which  represent  the  extent,  duration,  and  time  relations  of 
the  movements  accompanymg  physiologic  processes.  If  these  movements 
can  be  translated  in  one  direction,  they  may  be  recorded  in  different  ways: 

1.  By  attaching  the  movmg  structure — e.  g.,  heart,  muscle,  etc. — to  a 

delicate  lever  the  free  extremity  of  which  is  provided  with  a  writmg 
point. 

2.  By  transmitting  the  movement  through  a  column  of  air  enclosed  in  a 

rubber  tube  the  two  ends  of  which  are  attached  to  a  metallic  capsule, 
covered  by  a 
rubber  mem- 
brane, termed 
a  drum  or  tam- 
bour. When 
the  membrane 
of  the  first  tam- 
bour is  pressed 
or  driven  in- 
ward, the  air  is 
■  forced  through 
the  rubber  tube 
into  the  second 
tambour  and  its 
membrane  is 
pushed  out- 
ward.   As  soon 

as  the  primary  pressure  is  removed,  the  membranes  return  to  their 
former  condition.  If  the  membrane  of  the  first  tambour  is  drawii 
outward,  the  air  in  the  system  is  rarefied  and  the  membrane  of  the 
second  tambour  is  pressed  inward.  For  the  purpose  of  register- 
ing the  movement 
transmitted  by  the 
column  of  air,  tte 
second  tambour  is 
provided  with  a 
light  lever  support- 
ed by  a  vertical 
bearing  resting  on 
a  small  metallic 
disk.  The  mem- 
brane of  the  first 
tambour  is  frequently  provided  with  a  button,  which  is  placed  over 
the  moving  structure.  The  inward  movement  of  the  membrane  of 
the  first  tambour  produces  an  outward  movement  of  the  membrane  of 
the  second  tambour,  indicated,  though  magnified,  by  the  rise  of  the 
free  end  of  the  lever.  The  reverse  movement  of  the  membrane  is 
attended  by  a  fall  of  the  lever.  The  first  tambour  is  termed  the 
receiving,  the  second  the  recording  tambour  (Figs.  344,  345)- 


Fig.  344. — A  Receiving  Tambour. 


Fig.  345. — A  Recording  T.\iibour. — (Marey.) 


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TEXT-BOOK  OF  PHYSIOLOGY 


iiiiiiiiiir''iiiM 


By  enclosing  an  organ — e.  g.,  kidney,  spleen,  arm,  finger,  etc. — in  a 
rigid  glass  or  metal  vessel  which  at  one  point  is  in  communication 
with  a  recording  apparatus — e.  g.,  (i)  a  piston  provided  with  a  lever 
(page  431);  or  (2)  a  tambour  and  lever  (page  320);  or  (3)  a  mercurial 
manometer  carrying  a  float  and  pen  (page  305).  The  space  between 
the  part  investigated  and  the  vessel  is  filled  with  fluid.  The  varia- 
tions in  volume  of  the  organ  cause  a  displacement  of  the  fluid  and 

give  rise  to  a  to-and-fro 
movement  which  is  taken 
up  and  reproduced  by  the 
recording  apparatus. 
The  writing  point  may  be 
(i)  some  form  of  pen  carry- 
ing   ink   which    records    the 
movement  on  a  white  paper 
surface,    or    (2)    a    piece    of 
metal,  glass,  or  paper  which 
records    the     movement     on 
smoked  paper  or  glass. 

The  Recording  Sur- 
face.— The  surface  which 
receives  and  records  the 
movements  of  a  pen  or  lever 
is  usually  that  of  a  cylinder 
wliich  is  covered  with  glazed 
jjaper  and  coated  with  a  thin 
layer  of  soot,  obtained  by 
passing  the  cylinder  through 
the  flame  of  a  gas  burner. 
The  axis  of  the  cylinder  is 
supported  by  a  metal  frame- 
work. If  the  writing  point  of 
the  lever  be  placed  against 
the  cylinder  and  a  movement 
be  imparted  to  it,  a  portion 
of  the  soot  is  rubbed  off,  leav- 
ing a  white  line  behind.  If 
the  cyhnder  be  stationary,  the 
rise  and  fall  of  the  lever  are 
Such  a  record  shows  only  the  extent  of 


Fig.  346. — Kymograph.     (Boruttau's,  Pet 
zold,  Leipzig.) 


recorded  as 
a  movement 


a  vertical  line. 
If  the  cylinder  is  traveling,  however,  at  a  uniform  rate,  the 
rise  and  fall  of  the  lever  are  recorded  in  the  form  of  a  curve  the  width 
of  the  two  arms  of  which  will  depend  partly  on  the  rapidity  of  the  move- 
ment of  the  lever  and  partly  on  the  rate  of  movement  of  the  cylinder. 
The  cylinder  movement  is  initiated  and  maintained  by  clock-work  or  by 
the  transmission  of  power  by  belting  to  a  system  of  pulleys  m  connection 
with  its  axis.  As  the  tracing  is  wave-like  in  form,  the  cylinder  is  frequently 
spoken  of  as  a  kymograph  or  wave  recorder  (Fig.  346). 

From  the  record  thus  obtained  it  is  possible  to  determine  not  only  the 


PHYSIOLOGIC  APPARATUS. 


701 


-Signal  Magnet. 


extent  but  also  the  duration,  the  form,  and  the  rate  of  recurrence  of  any 
given  movement. 

The  Extent  of  a  Movement. — As  the  lever  not  only  takes  up  and  repro- 
duces a  movement,  but  at  the  same  time  magnifies  it,  it  is  essential  that 
the  degree  of  magnification  be  known,  in  order  to  determine  the  actual 
extent  of  the  movement.  The  magnification  of  the  lever  is  readily  deter- 
mined by  dividing  the  distance  between  the  axis  of  the  lever  and  its  writing 
point  by  the  distance  between  the  axis  and  the  point  of  attachment  of  the 
structure,  and  then  dividing  the  height  of  the  tracing  by  this  quotient. 
The  final  quotient 
represents  the  extent 
of  the  movement. 

The  Time  Rela- 
tions of  a  Movement. 
— When  recorded  in 
the  form  of  a  curve, 

the   duration    of   the  ^^^-  247- 

entire  movement,   or 

of  any  one  portion  of  it,  can  be  determined  by  means  of  a  time  marking 
or  chronographic  apparatus,  consisting  of  (i)  a  small  signal  magnet 
provided  with  a  movable  armature,  to  which  is  attached  a  writing  style; 
(2)  an  automatic  interrupter;  and  (3)  an  electric  cell. 

The  Signal  Magnet. — The  magnet  (Fig.  347)  is  actuated  by  the  electric 
current  made  and  broken  at  regular  and  known  intervals  by  an  auto- 
matically-acting interrupter 
placed  in  the  circuit.  With 
each  make  and  break  of  the 
circuit  the  armature  and  style 
move  alternately  downward 
and  upward.  The  excursion 
of  the  style  can  be  readily 
recorded  on  a  travelmg  sur- 
face. The  character  and  num- 
ber of  the  interruptions  per 
second  will  determine  the 
character  of  the  tracing.  If 
they  occur  in  a  rhythmic 
manner,  the  tracing  will  be 
sinusoidal  or  wave-like  in 
form.  If  the  time  of  interruption  is  of  short  duration  as  compared  with 
the  time  of  closure  of  the  circuit,  the  tracing  will  be  a  horizontal  line 
with  short  vertical  elevations  at  regular  intervals. 

The  Automatic  Interrupter. — The  circuit  may  be  interrupted  by 
virbating  reeds,  tuning-forks,  metronomes,  etc.  A  well-kno\Mi  form  of  vibrat- 
ing reed  is  sho^^^l  in  Fig.  348,  This  consists  of  a  metallic  frame  carrying  a 
coil  of  wire  in  the  center  of  which  there  is  a  core  of  soft  iron.  To  the 
vertical  part  of  the  frame  there  is  fastened  the  reed,  the  distal  end  of  which 
is  bent  to  dip  into  an  adjustable  mercury  cup.  When  in  circuit  the  current 
enters  the  coil,  then  flows  into  and  through  the  frame  and  the  reed  to  the 


Fig.  348. — Page's  Vibrating  Reed. 
ert's  modification.) 


(Reich- 


702 


TEXT-BOOK  OF  PHYSIOLOGY. 


mercury,  and  thence  back,  to  the  cell.  On  the  closure  of  the  circuit  and  the 
magnetization  of  the  iron  core  the  reed  is  withdrawn  from  the  mercury,  the 
circuit  broken,  and  the  core  demagnetized.  The  elasticity  of  the  spring 
returns  it  to  the  mercury,  when  the  circuit  is  again  restored.  The  reed 
may  be  so  constructed  that  it  will  be  raised  and  lowered  50,  100,  or  200 
times  a  second.  The  armature  of  the  signal  magnet  undergoes  a  corre- 
sponding number  of  elevations  and  depressions.  If  the  reed  vibrates 
100  times  in  a  second,  the  distance  from  crest  to  crest  of  the  wave  tracing 
will  represent  yl^  of  a  second.  Interrupters  of  various  kinds  have  been 
devised  which  make  and  break  the  circuit  from  i  to  250  times  a  second. 

Moist  Chamber. — In  many  experiments,  it  is  necessary  to  keep  the 
nerve  or  muscle  preparation  in  a  uniformly  moist  atmosphere.     To  secure 


Fig.  349. — Moist  Chamber. 


this,  a  moist  chamber  is  employed.  This  consists  of  a  hard-rubber  plat- 
form, supported  by  a  piece  of  brass,  which  slides  up  and  down  a  vertical 
rod,  and  which  can  be  clamped  at  any  height.  By  means  of  a  short  lever 
the  vertical  rod  can  be  turned,  carrying  the  platform  from  side  to  side. 
The  rod  is  secured  to  a  firm  iron  base. 

Six  double  bindmg  posts  for  the  attachment  of  wires  pass  through  the 
platform.  Near  the  side  of  the  upper  surface  of  the  platform  there  rises  a 
vertical  rod,  carrying  a  clamp  for  holding  the  femur  of  a  nerve-muscle 
preparation,  as  well  as  a  horizontal  rod  for  supporting  three  pairs  of  non- 
polarizable  electrodes.  A  groove  around  the  outer  edge  of  the  platform 
receives  a  glass  shade,  which  covers  the  whole.  The  air  of  the  chamber 
is  kept  moist  by  placing  in  it  pieces  of  blotting-paper  saturated  with  water. 


PHYSIOLOGIC  APPARATUS.  703 

From  the  under  surface  of  the  platform  there  descends  a  rod,  which, 
by  means  of  a  double  bindmg  screw,  supports  a  horizontal  rod,  modified 
at  one  end  to  carry  the  delicate  axis  of  a  Ught  stiff  recording  lever.  The 
end  of  this  lever  is  pointed,  to  enable  it  to  write  on  a  smoked  glass  or  paper. 
Beneath  the  axis  is  a  strip  of  brass,  carr}-ing  a  screw,  which  gives  support 
to  the  lever  until  the  instant  the  contraction  of  the  muscle  begins.  This 
screw,  the  after-loading  screw,  also  enables  the  lever  to  be  placed  in  a 
horizontal  position.  The  portion  of  the  lever  near  the  axis  is  provided 
with  a  double  hook,  the  lower  portion  of  which  serves  for  the  attach- 
ment of  the  weight  by  which  the  muscle  is  counterpoised. 

In  some  experiments,  as  in  the  registration  of  a  muscle  contraction 
under  varying  conditions,  it  is  necessary  to  give  the  lever  mass  by  attaching 
weights  directly  beneath  the  muscle.  This,  however,  introduces  certain 
errors  in  the  movements  of  the  lever,  which  somewhat  deform  what  would 
otherwise  be  the  normal  curve.  If  the  weight  be  attached,  not  opposite 
to  the  muscle  attachment,  but  close  to  the  axis  of  the  lever,  the  undesirable 
acceleration  of  the  lever  movement,  during  both  contraction  and  relaxa- 
tion, is  largely  prevented.  The  lever  may  be  a  straw,  a  strip  of  celluloid 
or  alummium.  It  should  be  as  light  as  possible.  The  writmg  point  may 
be  made  of  stiff  paper,  a  piece  of  tinsel,  glass  or  aluminium.  It  should 
have  sufficient  elasticity  to  keep  it  m  contact  with  the  cylmder  during  the 
excursion  of  the  lever.  The  writing  point  should  be  placed  as  nearly 
parallel  as  possible  to  the  surface  of  the  cylinder. 

Normal  Saline  Solution. — To  prevent  drying  and  a  loss  of  irrita- 
bihty  the  tissue  vmder  investigation  should  be  kept  moist  with  the  normal 
saline  solution  (NaCl  0.6  per  cent.).  This  solution  very  largely  prevents 
either  absorption  or  extraction  of  water  from  the  tissues  and  thus  retards 
chemic  changes  in  their  composition. 

Ringer's  solution,  largely  used  for  the  same  purpose,  is  made  by 
saturating  0.65  per  cent.  NaCl  solution  with  calcium  phosphate  and  then 
adding  2  c.c.  of  a  i  per  cent,  solution  of  potassium  chlorid  to  each  100  c.c. 

The  Galvanometer  and  Capillary  Electrometer. — In  the  detection 
and  investigation  of  the  electric  currents  of  muscles,  nerves,  and  other 
tissues,  the  physiologist  is  limited  to  the  galvanometer  and  capillary  elec- 
trometer. The  prmciple  of  the  galvanometer  is  based  on  the  fact  that  an 
electric  current  flowing  through  a  wire  parallel  in  direction  with  a  magnetic 
needle  will  tend  to  set  the  needle  at  right  angles  to  the  direction  of  the 
current.  The  essential  requisite  of  any  galvanometer  used  for  physiologic 
purposes  is  that  it  will  respond  quickly  to  the  influence  of  extremely  weak 
currents.  This  is  realized  by  the  use  of  small  Hght  needles,  the  adoption 
of  the  astatic  system,  or  some  similar  device  by  which  the  directive  mflu- 
ence  of  the  earth's  magnetism  is  eliminated,  and  the  multiplication  of 
the  number  of  turns  of  the  wire  in  the  coils  which  surround  the  needle. 

The  tangent  galvanometer,  or  boussole,  as  constructed  by  Wiedemann,  is 
the  form  most  frequently  employed  in  physiologic  investigations  (Fig.  350). 
It  consists  primarily  of  a  thick  copper  cylinder,  through  which  a  tunnel  has 
been  bored.  Within  this  tunnel  is  suspended  a  magnetized  ring,  just 
large  enough  to  swing  clear  of  the  sides  of  the  chamber.     The  object  of 


704 


TEXT-BOOK  OF  PHYSIOLOGY 


making  the  magnet  ring-shaped  is  to  increase  its  strength  in  proportion  to 
its  size,  and  to  get  rid  of  the  central  inactive  part.  Connected  with  and 
passing  upward  from  the  magnetized  ring  through  the  copper  cylinder  is 
an  aluminium  rod,  surmounted  by  a  circular  plane  mirror.  Above  the 
mirror  rises  a  glass  tube,  which  carries  on  top,  on  an  ebonite  support,  a 
little  windlass,  capable  of  being  centered  by  three  small  screws.  On  the 
windlass  is  wound  a  single  filament  of  silk,  which  passes  down  the  tube 
and  is  attached  to  the  mirror.  The  magnet  can,  by  this  contrivance,  be 
raised  or  lowered  and  centered  in  the  copper  chamber.  Deflections  of 
the  mirror  from  currents  of  air  are  prevented  by  inclosing  it  with  a  brass 
cover  provided  with  a  glass  window.  The  coils  are  placed  on  each  side 
of  the  copper  chamber,  and  supported  by  a  rod,  on  which  they  slide.  By 
this  arrangement  they  can  be  approximated  until  they  meet  and  completely 

conceal  the  cy Un- 
der. By  varying 
the  position  of  the 
coils  the  mfluence 
of  the  current 
upon  the  needle 
can  be  increased 
or  diminished. 
An  advantage 
which  this  galvan- 
ometer possesses 
is  the  damping  of 
the  oscillation  of 
the  needle,  so  that 
it  quickly  comes  to 
rest  after  deflec- 
tion. This  is  ac- 
complished by  the 
development  of 
induction  currents 
in  the  copper  cyl- 
mder,  the  direc- 
tion of  which  is 
opposite  to  that  of  the  movement  of  the  needle.  The  instrument, 
therefore,  is  aperiodic — that  'is  to  say,  when  the  needle  is  influenced 
by  a  current  it  moves  comparatively  slowly  until  the  maximum  deflec- 
tion is  reached,  when  it  comes  to  rest  without  oscillations.  When  the 
circuit  is  broken  the  needle  swings  slowly  back  to  zero,  and  again  comes 
to  rest  without  oscillations. 

Inasmuch  as  the  needle  is  not  astatic,  it  is  rendered  so  by  the  use  of  an 
accessory  magnet — the  so-called  Hauy's  bar.  This  magnet,  supported 
by  a  rod  directed  perpendicular  to  the  coils,  is  placed  in  the  magnetic 
meridian,  horizontal  to  the  needle,  with  its  north  pole  pointing  north.  By 
sliding  the  magnet  toward  the  needle  the  directive  influence  of  the  earth's 
magnetism  is  gradually  diminished,  and  when  it  is  reduced  to  a  minimum 
the  needle  acquires  its  highest  degree  of  instabihty.     By  means  of  a  pulley 


Fig.  350. — Wiedemann's  Boussole. 


PHYSIOLOGIC  APPARATUS. 


70s 


an  angular  movement  can  be  imparted  to  the  end  of  the  accessory  magnet 
in  the  direction  of  the  magnetic  meridian,  which  serves  to  keep  the  needle 
on  the  zero  of  the  scale.  The  deflections  of  the  needle  are  observed  by 
means  of  an  astronomic  telescope,  above  which  is  placed  a  scale  divided 
into  centimeters  and  millimeters,  and  distant  from  the  galvanometer  about 
six  or  eight  feet.  As  the  numbers  on  the  scale  are  reversed,  they  will  be 
seen  in  the  mirror  in  their  natural  position,  and  with  the  deflection  of  the 
needle  the  numbers  will  appear  as  if  drawn  across  the  mirror.  The  extent 
of  the  deflection  is  readily  determined  when  the  needle  comes  to  rest. 

The  reflecting  galvanometer  of  Sir  William  Thompson  is  also  used  for 
the  same  purposes. 

The  Capillary  Electrometer. — Notwithstanding  the  extreme  sensi- 
tiveness of  the  modem  galvanometer,  it  has  been  found  desirable,  in  the 
investigation  of  many 
physiologic  processes,  to 
possess  some  means 
which  will  respond  even 
more  promptly  to  slight 
variations  in  electromo- 
tive force.  This  has 
been  realized  in  the  con- 
struction by  Lippmann 
of  the  capillary  electro- 
meter. The  principle 
of  this  apparatus  rests 
upon  the  fact  that  the 
capillary  constant  or  the 
surface-tension  of  mer- 
cury undergoes  a  change 
upon  the  passage  of  an 
electric  current,  in  con- 
sequence of  a  polariza- 
tion by  hydrogen  taking 
place  at  its  surface.  If 
a  capillary  glass  tube  be 
filled  with  mercury  and 
its  lower  end  inserted 
into  a  solution  of  sul- 
phuric   acid,    and    the 

former  connected  with  the  positive  and  the  latter  with  the  negative 
electrode,  it  will  be  observed,  upon  the  passage  of  the  current,  that  a 
definite  movement  of  the  mercury  takes  place,  in  the  direction  of  the  nega- 
tive electrode,  in  consequence  of  the  diminution  of  its  capillary  constant  or 
the  tension  of  its  surface  in  contact  with  the  acid.  As  a  reverse  movement 
follows  a  cessation  of  the  current,  a  series  of  oscillations  will  follow  a  rapid 
making  and  breaking  of  the  current.  If  the  direction  of  the  current  is 
reversed,  the  capillary  constant  is  increased  and  the  mercury  ascends  the 
tube  toward  the  negative  pole.  From  facts  such  as  these  Lippmann  con- 
structed the  capillary  electrometer,  a  convenient  modification  of  which 


Fig.  351. — Von  Frey's  Capillary  Electrometer. 


44 


7o6 


TEXT-BOOK  OF  PHYSIOLOGY. 


devised  by  M.  v.  Frey,  is  shown  in  Fig.  351.  This  consists  of  a  glass 
tube,  A ,  forty  miUimeters  in  length,  three  millimeters  in  diameter,  the  lower 
end  of  which  is  drawn  out  to  a  fine  capillary  point.  The  tube  is  filled  with 
mercury  and  its  capillary  point  immersed  in  a  10  per  cent,  solution  of 
sulphuric  acid.  The  vessel  containing  the  acid 
is  filled  to  the  extent  of  several  millimeters  with 
mercury  also.  The  mercury  in  the  tube  is  put 
in  connection  with  a  platinum  wire  (a),  and  the 
acid  in  the  vessel  with  a  second  wire  (b).  When 
a  constant  current  passes  into  the  apparatus  in 
the  direction  from  b  to  a  the  mercury  is  pushed 
up  the  tube,  and,  upon  the  breaking  of  the  cur- 
rent, it  may  or  may  not  return  to  the  zero-point. 
For  the  purpose  of  measuring  in  millimeters  of 
mercury  the  pressure  necessary  to  compensate  this 
change  in  the  capillar}^  constant  produced  by  the 
electro-motive  force  of  polarization,  the  apparatus 
is  provided  with  a  pressure-vessel,  H,  and  a 
manometer,  B.  This  electrometer  can  be  applied 
to  any  microscope  having  a  reversible  stage.  The 
oscillations  of  the  mercury  can  then  be  observed 
with  the  microscope  provided  with  an  ocular 
micrometer  (Fig.  352).  The  special  advantage  of 
the  electrometer  is,  that  it  will  respond  instantly  to 
any  variation  in  the  electro-motive  force,  and  indi- 
cate a  difference  of  potential,  according  to  Lipp- 
mann's  observation,  as  shght  as  the  roi¥o'  °^  ^ 
Daniell.  These  rapid  oscillations  can  be  recorded 
by  photographic  methods. 

In  using  either  the  galvanometer  or  the  elec- 
trometer for  detecting  the   existence   of   electric 
currents   or   differences  of   potential   in    living   tissues,  it   is    absolutely 
essential  that  non-polarizable  electrodes  be  employed  in  connection  with  it 


Fig.  352. — Capillary 
Electrometer.  R. 
Mercury  in  tube ; 
capillary  tube.  s. 
Sulphuric  acid.  q. 
Hg.  B.  Observer. 
M.  Microscope. 


DISSECTION  OF  THE  HIND-LEG  OF  THE  FROG. 

Much  of  our  knowledge  of  the  physiologic  properties  of  muscles  and 
nerves  has  been  derived  from  the  study  of  the  muscles  and  nerves  of  the 
cold-blooded  animals,  especially  of  the  frog,  for  the  reason  that  in  these 
animals  the  tissues  retain  their  vitahty  under  appropriate  conditions  for  a 
considerable  period  of  time  after  death  or  removal  from  the  body.  The 
muscles  generally  employed  for  experimental  purposes  are  the  gastroc- 
nemius, the  sartorius,  the  semi-membranosus,  the  gracilis,  and  the  hyo- 
glossus.  The  nerve  generally  employed  is  the  sciatic.  Both  muscle  and 
nerve  may  be  studied  independently  of  each  other,  or  they  may  be  studied 
together,  as  when  in  their  usual  physiologic  relation.  For  this  latter  pur- 
pose the  gastrocnemius  muscle  and  sciatic  nerve  are  employed,  constituting 
the  so-called  "nerve-muscle  preparation." 

For  these,  and  many  other  reasons,  the  student  should  familiarize 
himself  with  the  general  anatomy  of  the  frog,  and  especially  with  the 
anatomy  of  the  posterior  extremities. 


PHYSIOLOGIC  APPARATUS. 


707 


Preparation  of  the  Frog. — Destroy  the  frog  by  plunging  a  pin  through 
the  skin  and  soft  tissues  covering  the  space  between  the  occipital  bone  and 
the  first  vertebra  until  the  pomt  is  stopped  by  the  vertebra.  Turn  the 
pin  toward  the  head  and  push  it  into  the  brain  cavity;  move  it  from 
side  to  side  and  destroy  the  brain.  Pass  the  pin  into  the  spinal  canal 
and  destroy  the  spinal  cord.  With  a  stout  pan-  of  scissors  cut  off  the  body 
behind  the  fore-Umbs.  Remove  the  viscera  and  the  abdominal  walls. 
Draw  the  hmd-legs  out  of  the  skin.  Place  the  legs  on  a  glass  plate,  back 
uppermost,  and  moisten  them  freely  with  normal  sahne  solution.  ,  ^ 


ve       H.\\/- 


ec 


Fig.  353. — Leg  Muscles  of  the  Frog. 
Ventral    Surface. — (Ecker.) 


Fig.  354. — Leg  Muscles  of  the 
Frog.  Dorsal  Surface. — 
(Ecker.) 


Observe  on  the  outer  side  of  the  dorsal  surface  of  the  thigh  the  following 
muscles  (Figs.  353,  354).  The  triceps  femoris  (tr),  made  up  of  the  rectus 
anticus  (ra),  the  vastus  extemus  (ve),  and  the  vastus  intemus  (vi),  not 
seen  from  behind;  on  the  inner  side,  the  semi-membranosus  (sm)  and  the 
rectus  intemus  mmor  or  gracihs  (ri").  Between  these  two  groups,  note 
the  biceps  femoris  (b).  Above  the  thigh  observe  the  gluteus  (gl),  the  ileo- 
coccygeus  (ci),  and  the  pyriformis  (p). 

In  the  leg  observe  the  gastrocnemius  (g)  with  its  tendon  (the  tendo 
AchilUs),  the  tibiahs  anticus  (ta),  and  the  peroneus  (pe). 

Turn  the  frog  on  its  back  and  note  the  muscles  on  the  ventral  surface 
of  the  thigh,  the  rectus  intemus  major  (ri'),  and  minor  (ri"),  the  adductor 
magnus  (ad"),  the  sartorius  (s),  the  adductor  longus  (ad'),  and  the  vastus 


7o8  TEXT-BOOK  OF  PHYSIOLOGY. 

intemus  (vi).  In  the  leg,  in  addition  to  those  ahready  seen  from  behind, 
note  the  tibiahs  posticus  (tp)  and  the  extensor  cruris  (ec). 

Note  in  the  abdominal  cavity  the  three  large  spinal  nerves,  the  seventh, 
eighth,  and  ninth. 

Dissection  of  the  Sciatic  Nerve. — The  sciatic  nerve  is  composed  of 
the  seventh,  eighth,  and  ninth  spinal  nerves.  After  its  emergence  from 
the  pelvic  cavity,  it  passes  down  the  thigh  between  the  semi-membranosus 
and  the  biceps  muscles,  in  company  with  the  femoral  blood-vessels.  Below 
the  knee  it  divides  into  the  tibialis  and  peroneus  nerves;  the  former  sending 
branches  into  the  gastrocnemius.  In  its  course,  the  sciatic  sends  branches 
to  the  muscles  of  the  entire  leg. 

Carefully  separate  the  biceps  and  semi-membranosus  by  tearing  the 
connective  tissue  uniting  them.  The  sciatic  nerve  and  femoral  blood- 
vessels come  into  view;  with  a  bent  glass  rod  gently  separate  the  nerve 
from  its  surroundings  from  the  knee  to  the  thigh.  Begin  at  the  knee.  In 
order  to  expose  the  nerve  at  the  pelvis,  it  will  be  necessary  to  divide  the 
pyriformis  and  the  ileo-coccygeus  muscles.  Care  must  here  be  exercised, 
so  as  not  to  injure  the  nerve  which  Hes  immediately  beneath.  Lift  up 
the  uro-style  with  the  forceps  and  separate  it  from  the  last  vertebra. 
With  the  scissors  cut  off  the  vertebral  column  above  the  seventh  vertebra. 
Place  the  legs  on  the  dorsal  surface  and  then  divide  the  seventh,  eighth, 
and  ninth  vertebrae  lengthwise.  With  the  forceps  hft  up  one  lateral  half 
of  the  vertebrae  and  free  the  nerve  as  far  as  the  knee  by  dividing  con- 
nective tissue  and  nerve  branches.  Be  careful  not  to  injure  the  nerv^e 
with  scissors  or  forceps. 

The  Nerve-muscle  Preparation. — Divide  the  tendo  x\chillis  just 
below  its  fibro-cartilaginous  thickening  at  the  heel,  and  detach  the 
gastrocnemius  up  to  the  knee.  Cut  through  the  tibio-fibular  bone  just 
below  the  knee-joint.  Cut  the  femur  transversely  near  its  middle  and 
remove  the  muscles  from  the  lower  end,  carefully  avoiding  injury  to  the 
nerve.  The  completed  preparation  consists  of  the  gastrocnemius  muscle, 
the  sciatic  nerve,  with  half  of  the  seventh,  eighth,  and  ninth  vertebrae 
and  the  lower  half  of  the  femur. 


INDEX. 


A. 

Abducens  ner\'e,  558 
Aberration,  chromatic,  639 

spheric,  639 
Absorption,  221  • 

by  epitheHum  of  villi,  234 

of  foods,  231 

of  lymph,  231 

spectra  of  blood,  257 
Accommodation  of  the  eye,  630 

convergence  of  eyes  during,  635 

force  of,  634 

mechanism  of,  632 

range,  634 
Action  currents  of  muscles,  94 
of  nerves,  124 

reflex,  134 

of  medulla  oblongata,  497 
of  spinal  cord,  469 
Adrenal  bodies,  431 
Agraphia,  527 
Albuminoids,  36 
Alcohol,  effects  of,   143 
Alimentary  canal,   155 

principles,   139 
Allantois,  678 
Amnion,.  678^ 
Amylopsin,  205 
Amyloses,  25 
Animal  body,  structure  of,  19 

heat,  401 
Ankle  clonus,  475 

jerk,  475 
Aphasia,  526 

ataxic,   527 

amnesic,  527 
Apnea,  394 
Arterial  circulation,  301 

pressure,   316 
Arteries,  structure  and  properties  of,  309 
Articulate  speech,  588 
Asphyxia,  395 

Association  centers  of  cerebrum,  528 
Astigmatism,  638 
Auditory  area,  523 

nerve,  564 


B. 


Basal  ganglia,  490 
Bile,  209 


Bile,  composition  of,  211 
mode  of  secretion,  212 
physiologic  action,  213 
pigments,  212 
salts,  211 
Blastodermic  membranes,  676 
Blind  spot,  642 
Blood,  238 

changes  in,  during  respiration,  378 
circulation  of,  272 
coagulation  of,  240 

chemistry  of,  268 

extravascular,  269 

intravascular,  270 
constituents  of,  238 
corpuscles,  245,  263,  266 
defibrinated,  242 
general  composition  of,  267 
physical  properties  of,  239 
pressure,  314 

causes  of,  322 

methods  of  estimation,  316,  318 

variations  in,  326 
quantity  of,  267 
velocity  of,  in  arteries,  329 

of,  in  capillaries,  331 

of,  in  veins,  332 
Burdach,  column  of,  466 


Calorimeter,  406 
Capillary  blood-vessels,  312 
functions  of,  312 

circulation,  337 

electrometer,  689 
Capsule,  internal,  491 

functions  of,  501 
Carbohydrates,  25 
Carbon  monoxid  hemoglobin,  261 
Cardiac  cycle,  284 
Cardio-accelerator  center,  308 
Cardio-inhibitor  center,  307 
Cardio-pulmonary  vessels,  276 
Caseinogen,  416 
Caudate  nucleus,  491 
Cells,  structure  of,  43 

manifestations  of  hfe  by,  45 
Central  organs  of  the  nerve  system,  456 
Cerebellar  tract,  405 


709 


710 


INDEX. 


Cerebellum,  530 

functions  of,  532 

results  of  experimental  lesions,  534 
Cerebrum,  502 

convolutions  of,  504 

fissures  of,  502 

functions  of,  510 

localization  of  function  in,  513 

motor  area  of  the  chimpanzee  brain, 

motor  area  of  the  human  bram,  524 
motor  area  of  the  monkey's  brain, 

sensor  areas  of  the  human  bram,  521 
sensor  areas  of  the  monkey's  brain, 

515 
structure  of  the  gray  matter,  507 
structure  of  the  white  matter,  509 
Chemic  composition  of  the  body,  24 
Chimpanzee  brain,  motor  area  of,  521 
Cholesterin,  211 

Chorda  tympanum  nerve,  561,  563 
Chorion,  679 
Chyle,  236 

Ciliary  movement,  103 
muscle,  617 

function  of,  633 
Circulation  of  blood,  272 

forces  concerned,  340 
Clark's  vesicular  column,  455 
Cochlea,  656 

functions  of,  664 
Colostrum,  418 
Commutator,  693 
Complemental  air,  373 
Connective  tissues,  51 

conjugated  proteids,  35 
Corpora  quadrigemina,  489 

functions  of,  498 
striata,  490 

functions  of,  499 
Corpus  luteum,  670 
Cranial  nerves^  538 
Crura  cerebri,  488 

functions  of,  498 
Crystalline  lens,  622 


D. 

Daily  ration  of  U.  S.  soldier,  153 
Decidual  membrane,  675 
Defecation,  219 

nerve  mechanism  of,  219 
Deglutition,  172 

nerve  mechanism  of,  179 
Demarcation  current,  93 
Depressor  nerve,  309,  571 
Dextroses,  26 
Diabetes,  425 
Diaphragm,  357 


Dietaries,  153 

Digestion,   154 

Dilatator  pupillae  muscle,  616 

Direct  cerebellar  tract,  465 

pyramidal  tract,  464 
Ductless  glands,  427     , 
Ductus  arteriosus,  683 

venosus,  688 
Dyspnea,  395 


E. 

Electrodes,  non-polarizable,  690 
Electrotonic  alterations  in  excitabihty  of 
nerves,  126 

current,  126 
Electro  tonus,  125 
EncephaJon,  456 
Encephalo-spinal  fluid,  457 
Endocardium,  276 
Enterokinose,  208 
Epididymis,  672 
Epinephrin,  432 

Epithelial  tissues,  functions  of,  51 
Equilibration,  mechanism  of,  535 
Erepsin,  207 
Erythrocytes,  245 
Eupnea,  393 

Eustachian  tube,  654,  663 
Excretion,  436 

Expiratory  forces  and  muscles,  367 
Expired  air,  composition  of,  396 
Eye,  cardinal  points  of,  625 

dioptric  apparatus  of,  623 

hds  of,  652 

muscles  of,  648 

physiologic  anatomy  of,  614 

reduced,  62S 

schematic,  627 


F. 

Facial  nerve,  559 

paralysis  of,   562 
Fallopian  tube,  667 
Fat,  29 

absorption  of,  235 

digestion  of,  207 

emulsification  of,  31 

saponification  of,  30 
Feces,  218 
Fecundation,  674 
Fehling's  solution,  27 
Female  organs  of  reproduction,  666 
Fetal  circulation,  681 

membranes,  676,  678 

structures,  676 
Fibrin,  34 
Fibrinogen,  244 
Fillet,  484 


INDEX. 


Follicle,  Graafian,  666 
Food,  136 

animal,  149 

cereal,  150 

disposition  of,  140 

heat  value  of,  144 

percentage  composition  of,  148 

principles,   139 

quantities  required  daily,  137 

vegetable,  151 
Forces  aiding    the  movement  of   lymph 

and  chyle,  236 
Fovea,  61S,  620 
Funiculus  cuneatus,  4S4 

gracilis,  4S4 

G. 

Gall-bladder,  209 

Galvanic  current,  effect  of,  on  nerves,  125 

Galvanometer,   703 

Gangha,  cephalic,  586 

Gaseous  exchange  in  lungs,  386 

in  tissues,  386 
Gases  of  blood,  relation  of,  379 

tension  of,  383 
Gastric  digestion,  179 
glands,  182 
juice,  185 

mode  of  secretion,  187 

physiologic  action  of,  190 
Glossophan'ngeal  nerve,  566 
Glycogen,  26,  423 

Glycogenic  function  of  the  liver,  423 
Gmelin's  test  for  bile  pigments,  212 
GoU,  columns  of,  466 
Gowers'  antero-lateral  tract,  465 
Graafian  follicle,  666 
Graphic  method,  699 
Green  vegetables,  152 


H. 

Hairs,  453 

Hearing,  sense  of,  653 

Heart,  272 

action  of  sympathetic  nerve  on,  305 
of  vagus  nerve  on,  305 

blood  supply,  292 

beat,  causation,  294 
frequency  of,   284 

course  of  blood  through,  277 

cycle  of,  284 

inhibition  of,  305 

intracardiac  pressure,  287 

intraventricular  pressure  curve,   288 

mechanics  of,  282 

muscle-fibers  of,  280 

negative  pressure  of,  291 

nerve  mechanism  of,  301 

orifices  and  valves,  278,  285 


Heart,  physiologic  anatomy  of,  272 

sounds,  291 

synchronism  of  the  two  sides,  287 

work  done  by,  341 
Heart-muscle,  properties  of,  297 
Heat  dissipation,  407 

income,  404 

relation  to  work,  410 

rigor,  81 
Helmholtz's  theory  of  color  perception, 

650 
Hemianopsia,  546 
Hemoglobin,  253 

absorption  spectra,   257 

compounds  of,  260 
Hemoglobinometer,  Gowers',  256 
Hemometer,  v.  Fleischl's,  256 
Hering's  theory  of  color  perception,  650 
Horopter,  645 
Hypermetropia,  637 
Hyperpnea,  394 
Hypoglossal  nerve,  575 


Incus,  655 

Induced  currents,  696,  697 
Inductorium,  695 
Insalivation,  161 

nerve  mechanism  of,  168 
Inspiration,  364 

movements  of  thorax,  359 

muscles,  364 
Insula,  507 

Intercostal  muscles,  357,  358 
Internal  capsule,  491 

functions  of,  501 

secretion,  427 
Intestinal  digestion,  198 

juice,  201 

physiologic  action  of,  208 

movements,  215 

nerve  mechanism  of,  216 
Intracardiac  pressure,  287 
Intrapulmonary  pressure,  360 
Intrathoracic  pressure,  360 
Intravascular  coagulation,  270 
Invertin,  208 
Iris,  615 

functions  of,  635 

nerve  mechanism  of,  550,  636 
Iron  of  the  body,  41,  142 
Irritabihty  of  muscles,  72 

of  nerves,  117 
Island  of  Langerhans,  203 

of  Reil,  507 
Isometric  myogram,  83 
Isotonic  myogram,  79 
Isthmus  of  encephaian,  486 
functions  of,  492 


712 


INDEX. 


Jacobsen's  nerve,  567 
Joints,  60 

classification  of,  61 


K. 

Kidney,  440 

histology  of,  440 
Knee-jerk,  475 
Kymograph,  700 


L. 


Labyrinth  of  ear,  656 
Lacrimal  glands,  636 
Lactation,  416,  684 
Lacteals,  236 
Language,  525 
Large  intestine,  216 
Larynx,  588 

nerve  mechanism  of,  598 

structure  of,  589 
Lateral    columns    of    the    spinal    cord, 

465 
Law  of  contraction,  128 
Lemniscus,  484 
Lens,  crystalhne,  605 
Lenticular  nucleus,  491 
Leukocytes,  263 

classification  of,  265 
Levers,  97 
Limbic  lobe,  506 
Liver,  209,  419 

formation  of  urea  in,  426 

functions  of,  421 

production  of  glycogen,  423 

secretion  of  bile,  422 
Localization  of  functions  in   cerebrum, 

513 
Lungs,  structure  of  the,  352 
Lymph,  227 

absorption  of,  231 

composition  of,  228 

functions  of,  230 

movement  of,  236 

production  of,  228 

properties  of,  227 
Lymph-glands,  224 
Lymph-vessels,  222 
Lymphocytes,  227,  265 


M. 

Macula  lutea,  618 
Malleus,  655 
Mammary  gland,  415 
Mastication,   156 


Mastication,  muscles  of,  158 

nerve  mechanism  of,  160 
Meats,  composition  of,  149 
Medulla  oblongata,  483 

reflex  activities  of,  497 
Meibomian  glands,  672 
Membrana  tympani,  654 

functions  of,  661 
Menstruation,  670 
Metabohsm  on  proteid  diet,  148 

on  fat  and  carbohydrate  diet,  148 
Methemoglobin,   261 
Migration  of  leukocytes,  338 
Milk,  149,  416 

composition  of,  146,  416 

mechanism  of  secretion,  417 

modification   of  respiratory  rhythm, 

393 
Moist  chamber.  702 
Mosso's  plethysmograph,  336 

spygmomanometer,  318 
Motor   area   of   chimpanzee   brain,    521 
of  human  brain,  522 
of  monkey  brain,  515 
oculi  nerve,  531 
Mouth  digestion,  156 
Movements  of  the  eyeball,  646 
of  the  intestines,  215 
of  the  lungs,  369 
of  the  stomach,   194 
Muscle  action  currents,  94 
contraction,  77 

chemic  phenomena  of,  88 
electric  phenomena  of,  91 
graphic  record  of,  78 
modifying  influences  of,  80 
physical  phenomena  of,  75 
rigor  mortis,  89 
tetanus,  87 

thermic  phenomena  of,  90 
electric  currents  from,  91 
electric  currents,  negative  variation 

of,  93 
energy,  source  of,  89 
fatigue,  82 

groups,   special  action  of,   96 
sense,  608 
sound,  88 
spindle,  608 
stimuli,  73 
tissue,  65 

chemic  composition  of,  69 
elasticity,  70,  76 
histology  of,  66,  99 
irritability,  72 
physical  properties  of,  70 
physiologic  properties  of,  72 
tonicity,  71 
Myopia,  637 
Myosinogen,  33,  69 
Myxedema,  427 


INDEX. 


713 


N. 

Nerve,  abducens,  558 

auditory,  564 

facial,  559 

glossopharyngeal.  566 

hypoglossal,  575 

impulse,  119 

irritability,  117 

motor  oculi,  547 

olfactory,  541 

optic,  543 

patheticus,  552 

pneumogastric,  568 

spinal  accessory,  573 

stimuli,  118 

tissue,  histology  of,  105 

trigeminal,  553 
Nerve-muscle  preparation,  120 
Nerve  system,  functions  of,  45S 
Nerves,  chemic  composition  and    meta- 
bolism of.  III 

classification  of,  116 

degeneration  of,  115 

development  of,  114 

effects  of  galvanic  current  on,  125 

electric  currents  of,  121 

electric  currents  of,  negative  varia- 
tion of,  122 

electric  excitation  of,  121 

electric  phenomena  of,  121 

action  currents,  124 
diphasic  action  currents,  124 

peripheral  endings  of,  112 

physiologic  properties  of,  117 

pilo-motor,  454 

polar  stimulation  of,   128,  130 

relation  of,  to  central  nerve  system, 
III 

stimuli  of,  118 
Neuron,  105 

Nicotin,  actions  of,  216,  567 
Nucleus  caudatus,  491 

cuneatus,  484 

graciUs,  484 

lenticularis,  491 
Nutrition  of  the  embryo,  679 


O. 

Oculo-motor  nerve,  531 
Ohm's  law,  672 
Olein,  30 

Olfactory  nen^e,  525 
Oncograph,  447 
Oncometer,  447 
Operculum,  507 
Ophthalmic  ganglion,  585 
Optic  constants,  623 
thalamus,  491 

functions  of,  500 


Optogram,  645 
Organ  of  Corti,  658 
Osazones,   29 
Ossicles  of  ear,  656,  660 
Otic  ganglion,  586 
Ovary,  666 
Ovulation,  669 
Ovum,  667 
Oxygen  in  blood,  381 

in  tissues,  385 

quantity  absorbed  daily,  391 
Oxyhemoglobin,  261 


Pacinian  corpuscle,  603 
Palmitin,  30 
Pancreas,  201 
Pancreatic  juice,  203 

physiologic  action  of,  205 
Partial  pressure  of  gases,  380 
Parturition,  683 
Pathetic  nerve,  536 
Pepsin,  186 
Peptones,  191 
Perspiration,  450 

Peripheral  organs  of  the  nerve  system,  1 10 
Petrosal  nerves,  561,  562 
Pettenkofer-Voit    respiration    apparatus, 

388 
Pexin,  186 
Phagocytosis,   266 
Phloridzin  diabetes,  426 
Phonation,  588 

mechanism  of,  595 
Pilo-motor  nerves,  454 
Pituitary  body,  430 
Placenta,  680 

Plasma  of  blood,  composition  of,  242 
Pleura,  359 
Pneumatograph,  373 
Pneumogastric  nerve,  568 
Pneumograph,   371 
Polar  stimulation,  128 

of  human  nerves,  130 
Pons  varolii,  486 

functions  of,  492 
Portal  vein,  224 
Postures,  98 
Presbyopia,  636 
Prosecretion,  204 
Proteids,  31 

color  tests  for,  38 
Protoplasm,  properties  of,  46 
Ptyalin,  168 
Pulmonary  vascular  apparatus,  339 

ventilation,  378 
Pulse,  332 

frequency,  333 

wave,  velocity  of,  333 


714 


INDEX. 


Punctum  proximum,  63.). 

remotum,  634 
Pyramidal  tracts  of  spinal  cord,  464,  465 


R. 

Reaction  of  degeneration,   133 
Red  corpuscles,  245 

chemic  composition  of,  253 
function  of,  251 
life  history  of,  252 
number  of,  247 
of  vertebrated  animals,   250 
Reduced  hemoglobin,  261 
Reflex  action,   134,  473 

laws  of,  473 
Refractory  period  of  the  heart,  300 
Regnault's  and  Reisset's  respiration  ap- 
paratus, 390 
Relation  of  gases  in  the  blood,  379 
Rennin,   186 
Reproduction,  666 
Reserve  air,  375 
Residual  air,  375 
Respiration,  350 

changes  in  composition  of  air  during, 

376 
changes  in  composition  of  blood,  378 
changes  in  tissues,   384 
chemistry  of,  375 
complemental  air,  373 
frequency  of,  371 

mechanism  of  gaseous  exchange,  386 
nerve  mechanism  of,  396 
Respiration,  total    respiratory  exchange, 

387 
volumes  of  air  breathed,  372 
Respiratory  apparatus,  350 
movements,  359 

effects  of,  on  arterial  pressure, 

400 
effects  of,  on  the  flow  of  blood 
through   the    thoracic    vessel, 

399 
of  upper  air  passages,  370 

pressures,  360 

quotient,   377,    391 

rhythm,  371 

sounds,  374 

types,  370 
Retina,  617 

functions  of,   641 
Retinal  image,  623 

size  of,  629 
Rheocord,  692 
Rigor  mortis    89 
Rima  glottidis,  589 

respiratoria,  594 

vocalis,  594 
Routes  of  the  absorbed  food,  237 


Saccharose,  28 
Saliva,   164 

physiologic  action  of,   166 
Salivary  glands,  161 

histologic     changes    in     duiing 

secretion,  165 
nerve  mechanism  of,  170 
Sebaceous  glands,  438 
Sebum,  454 
Secretin,  204 
Secretion,  411 

internal,  in 
Semen,  673 

Semicircular  canals,  557 
Sensor  areas  of  human  brain,  521 

of  monkey  brain,  515 
Setchenow's  center,  477 
Sight,  sense  of,  614 
Skeleton,  physiology  of,  60 
Skin,  451 

nerve  endings  in,  603 
Smell,  sense  of,  612 
Spectroscope,   258 
Speech,  598 
Spermatozoa,  673 
Spheno-palatine  ganglion,  586 
Sphygmograph,  334 
Sphygmomanometer,  318 
Spinal  accessory  nerve,  573 

cord,  459 

encephalo-spinal        conduction, 

479 
functions  of,  468 

as  a  conductor,  477 
as   an  independent  center, 
468 
nerve  fibers  of,  463 

classification  of,  463 
reflex  actions  of,  470 
reflex  irritability  of,  475 
relation  of  spinal  nerves  to,  466 
spino-encephalic        conduction, 

478 
structure  of  gray  matter,  460 
structure  of  white  matter,  463 
tracts  of,  464 
Spirometer,  372 
Splanchnic  nerves,  584 
Spleen,  432 

functions  of,  433 
Stanton's  sphygmomanometer,  319 
Stapes,  655 

Starch,  digestion  of.   167 
Starvation,  145 
Stearin,  30 

Stereognostic  area,  524 
Stomach,  movements  of,  194 
nerve  mechanism  of,  197 
Suprarenal  capsules,  431 
Sweat-glands,  452 


INDEX. 


715 


Sympathetic  nerve  system,  577 

cephalic  ganglia  of,  585 
functions    of    the    cervical 

portion,  583 
functions    of    the    lumbo- 
sacral portions,  585 
functions    of    the    thoracic 
portion,  584 


Taste  buds,  610 

nerve  of,  610 

sense  of,  610 
Tears,  6^2 
Teeth,  156 
Tegmentum,  48S 
Temperature  of  body,  402 

sense,  606 
Tension  of  gases  in  blood,  383 

tissues,  386 
Tensor  tympani  muscle,  655 

functions  of,  652 
Testicles,  671 
Tetanus,  87 
Thoracic  duct,  227 
Thorax,  356 

dynamic  condition  of,  362 

mechanic  movements  of,  359 

static  condition  of,  360 
Thyroid  gland,  427 

functions  of,  428 
Tidal  air,  373 
Tongue,  610 
Total  carbon-dioxid  exhaled,  391 

oxygen  absorbed,  391 

respiratory  exchange,  387 
Touch,  sense  of,  602 
Trachea,  352 

Tracts  of  spinal  cord,  465 
Trigeminal  nerve,  553 
Trypsin,  206 
Tiirck,  column  of,  464 
Tympanum,  653 

U. 

Umbihcal  cord,  679 

Upper    air-passages,    respiratory    move- 
ments of,  370 
Urea,  437 

seat  of  formation,  426 
Uric  acid,  438 
Urine,  436 

composition  of,  437 
mechanism  of  secretion,  444 

influence  of  blood  composi- 
tion, 449 
influenceof  nerve  system,448 
relation    of    blood-pressure 
to,  445 
Urination,  449 


Urination,  nerve  mechanism  of,  450 
Uterus,  668 


Vagus  nerve,   568 

influence  on  heart,  305 
Valves  of  heart,  285 
Vasa  deferentia,  672 
Vascular  apparatus,  309 

nerve  mechanism  of,  342 

glands,  427 
Vaso-motor  center,  346 

nerves,  342 
Veins,  313 

Velocity  of  blood,  3  2  8,  332 
Venous  circulation,  339 
Vertebral  column,  63 
Vesiculae  seminales,  672 
Villi,  232 

functions  of,  234 
Visceral  muscle,  99 

functions  of,  102 

properties  of,  100 
Vision,  614 

accommodation,  630 

astigmatism,  638 

binocular,  644 

color  perception,  648 

functions  of  retina,  641 

hypermetropia,  637 

myopia,  637 

presbyopia,  636 
Visual  angle,  629 
Vital  capacity  of  lungs,  373 
Vocal  bands,  592 

sounds,  596 
Voice  and  speech,  596 
Volume  pulse,  336 

W. 

Walking,  99 

Wallerian  degeneration,  116 
Water,  amount  of,  in  the  body,  37 
Watery  vapor  in  breath,  377 
Wernicke's  pupillary  reaction,  55  i 
White  blood-corpuscles,  263 

classification  of,  265 

function  of,  266 

migration  of,  338 

origin  of,  266 
Wrisberg,  nerve  of,  561 


Yellow  spot,  618 

Z. 

Zona  pellucida,  676 
Zymogen,  168,  186 
pepsinogen,  186 
ptj'alogen,  168 
trv'psinogen,  208 


COLUMBIA   UNIVERSITY 

This  book  is  due  on  the  date  indicated  below,  or  at  the 
expiration  of  a  definite  period  after  the  date  of  borrowing, 
as  provided  by  the  rules  of  the  Library  or  by  special  ar- 
rangement with  the  Librarian  in  charge. 

DATE  BORROWED 

DATE  DUE 

DATE  BORROWED 

DATE  DUE 

C2S<63a)MSO 

n 


QP34 


383 

1905 


-.{ ^:  - 


